Key messages:

• Buffer strips provide an effective land management option to reduce flood risk, soil erosion, and groundwater pollution from agricultural land.
• Buffer strips consist of planting strips of either grasses, other herbaceous perennials or tree species in 6-20 m wide strips along field margins or beside watercourses.
• Perennial biomass crops such as energy grasses (e.g. miscanthus, switchgrass, reed canary grass) and short-rotation trees (e.g. willow coppice and poplar) can all be used to create buffer strips. Their suitability needs to be matched to the buffer functions required, the site conditions, and planting location.
• For harvesting biomass, site logistics such as access and operating space for harvesting machinery needs to be considered in planning the locations of biomass buffer strips.

Introduction

A buffer strip is an area of land which is either left uncultivated or planted with perennial grasses, shrubs and trees. Planting strips or alleys of perennial biomass crops on existing agricultural land can provide a number of important benefits in terms of flood management, soil recovery and improvements in biodiversity, in addition to providing a harvestable resource. Wider adoption could help better manage landscapes to be more resilient to the effects of climate change, mitigate flood risk, and reduce the environmental impacts of intensive agriculture.

How are buffer strips used?

Buffer strips have been proven to be effective at reducing soil erosion, leaching and run-off of agricultural chemicals which is known to result in pollution of watercourses.

Perennial buffer strips planted adjacent to arable land have been shown to reduce soil sediment, nutrient loss, and pesticide drift while improving soil health and farmland biodiversity. Agroforestry systems have also been shown to provide effective windbreaks, provide wildlife corridors that can connect fragmented habitats and can also enhance the natural regulation of pests.  Provided the biomass crops can be safely and effectively harvested, their inclusion in buffer zones adds value by providing an additional harvestable resource which requires little or no maintenance. The biomass crops can make use of nutrient run-off, providing higher biomass yields while simultaneously reducing the nutrient load entering water courses which is a major source of pollution.

Broadly speaking, based on their location, buffer strips fall into one of two categories:

• In-field strips – across the slope of the field, along land contours and field margins.
• Riparian strips – flanking the banks of watercourses.

The current Environmental Land Management (ELM) scheme in England supports the adoption of buffer strips and there are woodland grants and incentives available for establishing and maintaining woodland for nature recovery, riparian buffers, improving water quality and reducing flood risk but biomass crops and short-rotation forestry (<8 yr rotation) are not currently eligible under current rules.

Use of buffer strips was piloted under the Catchment Sensitive Farming partnership, co-ordinated by Natural England and the Environment Agency, 3836 riparian buffer strips were established on participating farms in England between 2006-2018 and proved a popular measure, with 1 in 6 farms adopting their use. In-field and riparian buffer strips were both within the top-five most impactful catchment-sensitive farming measures identified. Use of biomass crops in buffer strips has great potential, but their adoption has so far been limited.

The main functions of buffer strips.

The main functions of buffer strips can be summarised as: Natural flood management, nutrient and pollution control, protection of soils and nature conservation.

Natural flood management – Buffer strips containing tall grasses, trees or shrubs act as a physical barrier which slows the run-off of surface water following heavy rains making them effective natural flood management strategies which are likely to be of growing importance. Buffer strips can help achieve this by acting in the following ways:

• Increasing the interception of rainfall – leaves catch rain before it reaches the ground slowing the rate of flow and allowing a proportion of the rainfall to evaporate from the leaf surface.
• Slowing the flow of surface water – above-ground vegetation provides a physical barrier which slows the flow of water running off the soil surface.
• Increasing absorption of water into the soil – Established buffer strips can improve soil structure allowing water to penetrate deeper into the soil, increasing the volume that can be stored in the soil and reduce the likelihood of saturation which leads to greater run-off.
• Larger perennial crops and tree species take up significant quantities of water from the soil, can help reduce soil saturation and ameliorate areas prone to waterlogging. Conversely, in drier conditions, buffers can also help to conserve soil moisture and reduce effects of drought.

Pollution control – In areas where agricultural land is more intensively managed, nitrate-N (NO₃^–) and phosphorous (P) leaching and run-off is known to be a main source of pollution resulting in eutrophication of lakes and waterways. Deep rooting perennial plants and tree species can act as a biological filter which take up and utilise nutrients before they enter watercourses. Studies have shown that riparian buffers can remove between 30-99% of nitrate and 20-100% of phosphorous depending on site conditions and the buffer composition, with combinations of grasses and trees tending to be most effective. In situations where frequent incidences of fertiliser run-off are common, some riparian buffers have been shown to reach saturation after a few years if the biomass is not regularly removed. For this reason, using biomass crops in buffers provides an added benefit as the leaching fertiliser can result in higher harvestable yields of biomass and also help maintain the effectiveness of the buffer strip in terms of N and P uptake. In addition to nutrient run-off, buffer strips can also be effective for wastewater management. Biomass crops such as willow short-rotation coppice have been shown to be particularly effective, permanent nutrient removal of 55 kg PO₄³⁻eq ha yr has been reported based on field trials conducted in Northern Ireland.

Pesticides are another potential source of pollution from agricultural practice. The pesticides applied in the UK have a wide range of different properties and modes of action, but in general most pesticides will preferentially adsorb to dead organic material such as leaf litter. Incorporation of tall, dense and fast-growing species in the buffer strip can also act as a windbreak or shelter belt which can create a physical barrier to prevent pesticide drift. Biomass crops which create deep leaf litter layers before the biomass is harvested, such as miscanthus and willow, may also be particularly effective if protection from fertiliser run-off and pesticide drift are both required. Poplar and willow have been shown to take up and immobilise a wide range of pollutants, including pesticides.

Source: Environment Agency (2020). 3D buffer strips: Designed to deliver more for the environment. Environment Agency, Bristol.

Soil health – The above ground vegetation of the buffer strip helps catch and retain soil that has been eroded by wind or water. On degraded arable soils, uncultivated buffer strips will also allow the soil to recover over time. Deep rooting plants can help stabilise soils at higher risk of erosion, such as riverbanks and areas around water bodies, often collectively referred to as riparian zones.

Buffer strips have also been shown to enhance soil carbon through sequestration, root turnover, leaf litter, and simply by leaving the area unploughed for a prolonged period which allows the soil time to recover and reduces incidences of soil compaction.  The total carbon stocks under short rotation forestry have been reported as 1.5 times higher than alternative land uses when planted on agricultural land.

 Nature conservation – Biodiversity and wildlife corridors – Buffer strips provide important habitats and can provide cover for wildlife to allow them to move freely between different habitats which are often fragmented by field boundaries. Tall grass and tree species incorporated into buffer strips also provide shade which can be of benefit to aquatic life in water courses. Perennial biomass crops also provide important winter cover. As they are harvested in late winter, they provide vegetative cover when adjacent agricultural land is often bare which also poses even greater risk of flooding and erosion.

Perennial biomass crops best suited to use in buffer strips

A number of different biomass crops may be suitable for use in buffer strips. Perennial grasses bred specifically for high-yield and energy content are one option. Stiff-stemmed grassland species that are resistant to high run-off rates are most suitable and can be selected as a feedstock for multiple purposes: Combustion, anaerobic digestion, forage/grazing or animal bedding. Wildflower mixtures are also an option for inclusion. Some energy grasses, such as miscanthus, may require a minimum planting area and an established local market in order to be economically viable to harvest on smaller scales, unless used on-farm as animal bedding.

Short-rotation coppice (SRC), e.g. tree species such as willow and poplar, which are cut to a low stump (stool) and produce multiple stems that are harvested every 2-5 years, are also an option where land is not too steep. Alternatively, short-rotation forestry (SRF) may be suitable. SRF uses high-yielding tree species grown at high stocking densities for 8-20 year rotations. Wood is harvested when stems reach approximately 20cm diameter. If space permits, conventional woodland management of wooded buffer zones is a long-term option.

Types of buffer strip

In terms of composition, buffers can be classed as: Vegetated, incorporating grasses and wildflower species; Wooded, incorporating tree and shrub species; or integrated/engineered, incorporating a mixture of species and/or earthworks. Integrated or engineered buffer strips are recommended as the best designs for incorporating biomass crop species. With reference to riparian buffer strips, a three-zone structure has been recommended as most effective for use with biomass crops:

1. Grass strip – This strip reduces the flow rate of surface run-off, filters sediment and reduces erosion. Nutrients are taken up by the plants during the growing season. The strip may be composed of any perennial grasses, including energy grasses. Depending on the grass used, the strip can also be used for extensive grazing, silage/hay production, animal bedding, or for bioenergy.
2. Woodland strip – This strip improves interception of rainfall, deep rooting tree species help stabilise banks and filter nutrient and pollutant run-off via uptake and immobilisation in the biomass. Taller species also act as windbreak and prevent erosion and pesticide drift. This strip can be composed of mixtures of short-rotation coppice (SRC) or short-rotation forestry species (SRF). The harvested biomass can be chipped or cut as billets for firewood.
3. Undisturbed strip – helps to protect planted strips during flooding events, reduces bank erosion and provides a buffer between planted biomass crops and water courses to allow space for harvesting and maintenance operations.

Slope can be used as one of the key considerations for planning a biomass buffer

Other considerations

Depending on where buffers are located, existing land drains can negate the effects of buffer strips due to providing a bypass for surface water run-off. In cases where extensive networks of land drains are already present, redirecting flow to a drainage ditch between the field and the buffer could provide a solution. Where this is not possible, a constructed wetland area could be another option for environmental protection and conservation.

The greater the level of engineering that goes into buffer strips, the greater the cost, but also the greater the benefit. Planning needs to take account of biomass crop establishment and harvesting requirements both in terms of cost and management considerations.

Where livestock are also present, or in areas where browsing damage from rabbits/deer is a problem, fencing may be required to prevent grazing of establishing buffer strips and to keep livestock away from water courses.

Biomass Connect Media:

Dr Chris Johnston of the Agri-Food and Biosciences Institute (AFBI), a Biomass Connect partner, gave talk at the UK Low Carbon Agriculture Show, February 2023, showing results of field trials into the effectiveness of short-rotation coppice willow for environmental protection and wastewater management.

Current UK policy

The current Environmental Land Management (ELM) scheme in England supports the adoption of buffer strips and there are woodland grants and incentives available for establishing and maintaining woodland for nature recovery, riparian buffers, improving water quality and reducing flood risk. Biomass crops and short-rotation forestry, with rotation length less than 8 years, are not yet eligible for ELMs or woodland creation (EWCO) incentives.

Conclusion – Main considerations when establishing biomass buffer strips.

The optimum design, composition and width of a buffer strip depends on a wide range of factors which include:

• The environmental and conservation issues being addressed.
• The previous and adjacent land use.
• The soil type and condition.
• The slope, topography and hydrology of the site.
• The desired biomass products and yields.
• Harvesting technology required and local markets available for the biomass.
• Cost of measures required and government payments available to support their use.

Further information:

Environment Agency (2020). 3D buffer strips: Designed to deliver more for the environment. Environment Agency, Bristol.

Related Biomass Connect Articles:

How does Short Rotation Coppice (SRC) willow affect biodiversity?

What effect does planting biomass crops have on soil carbon?

Other resources:

Funding for landowners – Nature based solutions to reduce flood risk

Examples and case studies – How nature-based solutions can mitigate flood risk.

Riparian forest management on freshwater environment – Best management practice guide.

Natural Flood Management Handbook – A practical guide for farmers.

Take home messages:

• Biomass pelletisation is the process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill to produce small cylindrical pellets.
• Biomass pellets have uniform size and shape, high bulk and energy density, and low moisture content, all contributing positively to the supply chain management and logistics cost.
• The quality and characteristics of biomass pellets are determined based on key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature.
• A high-quality pellet should be dry, hard, durable, and with low ash content. The ENplus certification scheme outlines pellet properties and related threshold values.

Introduction

The global demand for biomass is increasing in response to the need to reduce dependency on fossil fuels, concerns about environmental problems related to its use, and the increasing desire to mitigate climate change. Biomass raw materials are widely regarded as a significant renewable energy source and an alternative to fossil fuels. However, there are limitations with the use of biomass feedstock in its natural form for energy conversion systems. Biomass feedstocks are bulky, have low bulk density, non-uniform properties, low energy density, and high storage, handling, and transport costs. One way to overcome the limitations is using densification to convert biomass feedstock into pellets before use for energy purposes.

Biomass Pellets

Biomass pellets are biomass materials compressed into high energy dense pellets, characterised by; homogenous shape and size, low moisture content, higher bulk density, thus reducing the storage, handling and transportation cost and lower environmental impacts. Biomass pellets could be used directly as fuel for residential heating stoves, heating boilers and large-scale power plants. Biomass use in the form of pellets generates a biofuel that is more cost-effective than the direct use of non-modified biomass residues for energy production. This article explores the critical issues in the biomass pelletisation process, biomass pellets characteristics and standardisation of high-quality biomass pellets for energy production.

Pellet production process

Biomass pelletisation is the process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill consisting of a die with cylindrical press channels and rollers, to produce small cylindrical pellets. The friction between the biomass and the press channel generates a force compressing the biomass into pellets that are dense and cut at a uniform size. The biomass pelletisation process consists of multiple steps including pre-treatment (particle size reduction, drying, and conditioning), pelletising, and post-treatment (cooling, screening, packaging).

Pre-treatment involves reducing the biomass particle size and drying to reduce the moisture content. The harvested biomass is processed into chips and fed into a mill, further reducing the particle size in order to avoid blockage in the pellet mill. Studies have found that biomass materials with small particle sizes and a large surface area during pelletisation result in a higher density, and stronger pellets.

Following the particle size reduction, the biomass material goes through a drying phase where the material is dried in a dryer. The moisture content affects the overall quality of the final pellet produced. Depending on the biomass material and the lignin content, additives or binding agents may be added to improve the quality of the final product. Studies have shown the need to incorporate additives to biomass feedstock when pelleting biomass materials with zero or low lignin content. Lignin is one of three major components of lignocellulosic biomass with binding attributes. Additives such as starch, corn flour, potato flour, vegetable oils, etc., which are materials intentionally added into the pellet production or added after production should not exceed two percent of the total pellet biomass. Additives have the benefit of improving the quality of pellets, reducing emissions, increasing production efficiency, or marking the pellets.

The pelletisation process takes place in a pellet mill. The pellet mill has basic components of a die and cylindrical press channel and rollers. The friction between the biomass and the press channels causes pressure to build and increases the temperature of the biomass material which softens the lignin content, binding the biomass material and compressing the biomass into small cylindrical pellets.

After pelletisation, the biomass pellets are allowed to cool to solidify into more durable forms. Once cooled, the biomass pellets are stored or packaged, ready for use. The fine materials that are not formed into pellets are recycled back into the pelletisation process.

Pellitisation

Pellet quality requirement

Pellets produced from biomass should meet international pellet standards. The UK Pellet Council (UKPC) manages the license for ENplus pellet quality certification in the UK, ensuring adherence to the standards and the production and delivery of high-quality wood pellets for heating. The ENplus certification scheme defines three pellet quality classes (ENplus A1, A2 & B) which are based on the ISO 17225-2. The ENplus certification scheme outlines pellet properties and related threshold values.

Biomass feedstock has an influence on the quality of the pellets produced. A high-quality pellet should be dry, hard, durable, and with a low ash content of less than two percent. According to the ENplus standards, the length of the pellet varies ranging from 3-40mm and a diameter of 6-8mm. Biomass pellets should have a moisture content of less than 10 percent. Dust produced during the processing stages of biomass collection, size reduction and pelletisation should be minimal (no more than 1 percent). Dust is highly combustible, leading to explosions and imposing health hazards.

Furthermore, quality pellets should be durable with high bulk density. Particle bonding by the introduction of additives is considered a vital event in biomass pelleting. The production of durable pellets is a function of the lasting attraction between individual particles during biomass pelleting. Where additives are added into the pellet production or added after production, it should not exceed two percent of the total pellet biomass.

Parameters affecting the quality of produced biomass pellets

The quality and characteristics of biomass pellets are determined based on key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature.

The type of biomass feedstock influences the quality of the pellet.  Different feedstocks have different characteristics and energy requirements for pelletisation, which directly impacts production cost and production capabilities. Biomass materials often used for pelleting are wood from forestry, followed by agricultural residues. The limited availability of wood resources and increasing global demand for biomass pellets has resulted in efforts to broaden the raw material base used for pellet production, including short rotation coppice biomass crops and herbaceous perennial biomass such as Poplar (Poplar spp.), Miscanthus (Miscanthus giganteus), and switch grass (Panicum vigatum). For instance, a study found poplar as a suitable material for producing pellets that meet many of the pellet quality standards (see Table 1) required on the market. Studies on Miscanthus pelletisation have however often found the use of Miscanthus biomass producing pellets of low durability and high ash content and requiring high energy inputs. The lower lignin content and poor bonding between particles have been identified to account for the low pellet quality. The recommendation to improve the pellet quality of biomass with such characteristics is the addition of binders and mixtures of different raw materials as they affect the quality of the pellets.

Biomass feedstock with optimal moisture content is essential for producing quality pellets with better mechanical durability, strength, and thermal conversion performance. Recommended moisture content ranges from 10-15%. Moisture content affects the quality of pellets especially if pellets are to be stored before use. Research has shown that decreasing moisture content will increase the density and durability of pellets. Increasing the moisture content above the recommended optimum values however, has negative influence on the pellets mechanical durability and reduces the pellets density.

The applied pressure during the pelletisation process affects the natural binders such as lignin, starch, protein, and water-soluble carbohydrates present in the biomass material. As pressure increases up to a certain point, these binding components are pressed out causing the biomass particles to bond together. Studies have shown a positive effect of increasing pressure on various biomass feedstocks quality by increasing the durability and hardness of the produced pellets. Also, research has shown optimal die temperature for pelletising biomass feedstock is close to 100 °C.

Furthermore, the particle size of the biomass feedstock depends on the biomass characteristics, particle size reduction methods during pre-treatment and the pelletising equipment. Decreasing the biomass particle size increases friction in the press channel of a pellet mill resulting in increasing pellet density.

Conclusion

The global demand for pellets is increasing in response to the need to reduce dependency on fossil fuels and concerns about environmental problems related to their use. Biomass pellets are produced from woody and non-woody biomass feedstocks through a process of condensing biomass material into energy-dense pellets by forcing the biomass material under high pressure and temperature through a pellet mill to produce small cylindrical pellets. Biomass pellets have uniform size and shape, high bulk and energy density, and low moisture content, all contributing positively to the supply chain management and logistics cost. The quality and characteristics of biomass pellets are determined by key parameters such as the type of biomass feedstock, moisture content, particle size, binding material, pressure, and temperature. A high-quality pellet should be dry, hard, durable, and with low ash content. The ENplus certification scheme outlines pellet properties and related threshold values.

Key messages:

• Bioenergy with carbon capture and storage (BECCS) involves generating heat and power from biomass fuels, capturing the emissions released and storing them.
• BECCS represents one of only a few technological options currently available to remove historic CO₂ emissions from the atmosphere.
• The UK government has stated that future large-scale biomass power facilities will not be supported without the inclusion of CCS.
• To achieve net-negative emissions reduction via BECCS, significant quantities of land will be required to grow dedicated energy crops. This must be managed sustainably to prevent negative impacts on biodiversity and competition with food production.

Introduction

Bioenergy from plant biomass coupled with carbon capture and storage (BECCS), is regarded by the UK government as one of the key greenhouse gas removal (GGR) mechanisms currently available to avoid global warming passing beyond 1.5°C. BECCS involves using plant biomass to remove historic CO₂ emissions from the atmosphere, generate power, and subsequently capture the carbon emissions released and store them long-term. The UK Climate Change Committee have estimated BECCS could sequester between 20 – 65 Mt CO₂ equivalent per year by 2050, equivalent to 15% of the current UK CO₂ emissions.

What are Carbon Capture and Storage technologies?

The overall purpose of carbon capture utilisation and storage (CCUS) is to stop carbon dioxide (CO₂) emissions from combustion reaching the atmosphere and adding to global warming. Instead, the CO₂ produced is captured at source and is either utilised in another industrial process (CCU) or stored deep underground in geological formations (CCS).

The British geological survey has described CCS as ‘the oil extraction industry in reverse’: replacing natural gas and oil removed from the earth with CO₂ produced from the combustion of it. The process is already in use in existing oil recovery operations where it used by oil companies to extract more oil from existing wells.

While use of CCUS to capture CO₂ released from the burning of fossil fuels would undoubtedly help reduce future emissions, it does nothing to help reduce the levels of CO₂ that have already been released into the atmosphere and already driving climate change. While other ‘direct-air’ carbon capture (DAC) technologies are also being developed, BECCS has the potential to be deployed more swiftly because it utilises the existing biology of plants to capture historic CO₂ from the atmosphere and store it. The technology this requires can be retrofitted to many existing facilities. This provides options for decarbonising industries that are already emitting significant quantities of CO₂, such as power stations, cement production and steel manufacture. If sufficient plant material can be grown, harvested and transported sustainably, BECCS could offer a viable means of reducing global carbon emissions.

Carbon Capture and Storage

How does BECCS work?

While plants are actively growing, they are removing CO₂ from the atmosphere and storing it in their bodies – essentially nature’s own version of carbon capture and storage. Plants capture CO₂ from the atmosphere during photosynthesis; where CO₂ combined with energy from the sun, water and nutrients from the soil, is converted into biomass that is used to build their above and below-ground structures. When the plant dies and rots or is burned, most of the CO₂ captured during its lifetime is released back into the atmosphere. However, if the carbon emitted from burning biomass is captured and stored long-term, the process potentially becomes ‘carbon-negative’. Put simply, the BECCS process involves carbon being actively removed from the atmosphere by plants to produce a biomass fuel, that fuel is used to generate heat and power, and the emissions produced in the process are captured and stored.

How is the carbon captured?

There are three main methods of carbon capture:

1. Post-combustion – Capturing GHGs as they exit the flue after combustion.
2. Oxyfuel combustion – Combustion of fuel in an excess of oxygen so it is completely converted to a mixture of CO₂ and steam.
3. Pre-combustion – Converting fuel to a hydrocarbon gas (gasification) and removing CO₂ formed during the process prior to combustion.

The 3 main methods of Carbon Capture – Adapted from Global CCS Institute

Post-combustion and Oxyfuel combustion are most applicable for use by existing power and heat generators as it is possible to retrofit these systems. Pre-combustion processes are more applicable to gasification processes which can be used for heat and power but also for generation of liquid transport or aviation fuels. In this instance the process captures CO₂ released during the gasification step of the process, but CO₂ will still be released when the hydrocarbon fuel produced is subsequently burned.

The gasses that are captured during this process undergo a process called ‘scrubbing’ where they are passed/bubbled through a solvent media which causes particulates in the gas stream to be removed and the pure CO₂ is separated, pressurised to a liquid state, and can then be bottled, piped or transported in the same way as other commercially used gasses. Once in a clean, transportable state the CO₂ can either be:

• Utilised (CCU) for other industrial processes: manufacturing, generation of hydrogen or use in plant or aquaculture growth chambers.
• Stored (CCS) by pumping the CO₂ into suitable geological formations (>1km) deep below the land or sea.

A comprehensive review of the BECCS processes has been written by Shahbaz et al. (2021).

Where is BECCS currently being used?

Carbon Capture Utilisation and Storage (CCUS) technologies are already being developed in many parts of the world. According to a recent report by the International Energy Agency (IEA), in September 2022, there were 35 commercial facilities in operation collectively capturing approximately 45 Mt CO₂ globally, with many more either planned or under development. CCUS is also one of the main elements of the UK governments Clean Growth industrial strategy. According to the IEA, globally over 2 Mt CO₂ are currently being captured annually from biogenic (biomass derived) sources, half of which was geologically stored.

• BECCS coupled with bioethanol production currently represents 90% of operational carbon capture facilities. The Decatur Industrial CCS project in Illinois, USA is one of the largest CCS facilities in operation since 2018. The Illinois project captures CO₂ from bioethanol production and geologically stores 1 Mt CO₂ per year from the process. Another 40 bioethanol facilities are planned, the majority based in the USA, which are estimated to collectively capture 15 Mt of biogenic CO₂ by 2030.
• BECCS coupled with heat and power facilities are also under development with 15 Mt planned predominantly from dedicated biomass power facilities and a third from waste-to-energy facilities. In 2020 a large-scale solid biomass plant with CCS was commissioned in Japan but is still identifying suitable geological storage. Europe’s first BECCS Pilot facilities are currently being trialled and developed at Drax power station in Yorkshire, with larger-scale deployment of the technology expected to be operational by 2027.
• BECCS coupled with other industrial processes are also in development including five cement plants in Europe, several pulp and paper production facilities and two hydrogen generation facilities.

CCUS is considered a key technology in the UK governments Clean Growth industrial strategy and the first operational CCUS facilities are planned to be operation within the next five years and deployed at scale over the next decade. The UK governments 2021 Biomass policy statement stipulated that future large-scale biomass power facilities would not be supported without the inclusion of CCS.

The map below shows key UK locations for CCS clusters identified by BEIS (2018) The UK Carbon Capture Usage and Storage deployment pathway: an action plan.

Key UK locations for CCS clusters.

Significant challenges remain

While technical aspects of the CCS process have been in use for some time, much of the infrastructure required for CCS does not yet exist, and there are only a few large-scale demonstrators that have been built or monitored in long term trials. Questions have been raised whether BECCS would be cost effective compared with money spent on other forms of renewables in energy generation, but there are currently few other options that would also help with historic greenhouse gas removal. Concerns have also been raised regarding how safe and secure the carbon dioxide stored below ground would be, how this can be monitored, and how potential leaks could be managed. The EU commission are currently developing a CCS directive to help address these concerns and provide regulation.

The CCC acknowledge that there are other pathways with a better than 50% chance of limiting global warming to below 1.5°C without large-scale deployment of BECCS, but these pathways would require significant and rapid changes in energy use efficiency, shifts in diet, low population growth and large-scale afforestation.

Sourcing biomass sustainably is critical

Global adoption of BECCS will inevitably require large quantities of biomass, and large areas of land will be required to produce it. According to the CCC, land used for energy crops will have a lower carbon content than afforested land, but energy crops produce higher yields of biomass which can be harvested annually (energy grasses) or every 3-5 years (short rotation coppice). For this reason, the CCC have suggested that energy crops used with BECCS can provide greater cumulative long-term carbon sequestration than afforestation provided their supply chain emissions are low. A recent study on use of BECCS in the UK identified that to meet carbon sequestration of 50 Mt CO₂ per year by 2050, an estimated 1 million Ha of UK agricultural land would be required, in addition to either importing 6-8 million tonnes of biomass per year or relying on an additional 0.59 or 0.49 million Ha of domestic forest resources.

There are concerns as to whether this biomass can be grown and sourced sustainably without causing other negative impacts on the environment; especially in terms of impacts on ecosystem services, biodiversity, water use and other factors associated with land use change such as trade-offs with food production. The CCC also acknowledge the risk that unsustainable biomass harvesting could pose, both domestically and internationally. Sustainability criteria are applied in the UK to biomass used in heat, power and transport sectors (RED II) in attempt to mitigate negative consequences of biomass use. The EU has begun instituting a law aimed at preventing global deforestation and helping to establish deforestation-free supply chains.

BECCS could provide an important mechanism to reduce emissions and remove greenhouse gasses from the atmosphere. However, full life-cycle assessment of biomass production and supply will be critical to ensure BECCS is truly net-negative and does not result in other deleterious effects on our natural environment or food production systems.

Take home messages:

• SRC willow plantation supports high plant and animal species diversity in comparison with arable lands.
• The abundance and richness of biodiversity in SRC willow is associated with the crop and its management, the plantation size and layout design, and interaction with surrounding landscapes.
• Planting different small multi-genotype plots and different age classes of SRC willow, with different coppice rotation enhances biodiversity.
• SRC Willow stand edges have high plant and animal species diversity compared to stand edges on arable agricultural land.
• The low input use and low soil disturbance after establishment of SRC willow plantation provides a conducive environment for many plant and animal species to survive.

Introduction

The production of biomass crops such as short rotation coppice (SRC) willow, is an important part of the UK’s Net Zero strategy to generate more homegrown power and electricity. SRC willow biomass has the potential to be converted into renewable energy. SRC willow is a short rotation woody crop that rapidly produces large amounts of renewable biomass when harvested every two to five years. It provides environmental benefits of minimizing soil erosion and also increases wildlife habitat and biodiversity for birds, pollinators, and small mammals. Despite the expansion potential of the SRC willow, the large-scale deployment of willow is hampered by growers and public concerns that the extensive commercial production of SRC willow could have negative effects on biodiversity.  In this article, we provide insight on the dynamics of interaction of SRC willow and its effect on plant and animal biodiversity.

SRC willow and plant diversity

SRC willow contributes to plant diversity when integrated into existing landscapes and farming systems. Studies have shown that SRC willow plantation supports high plant species richness and abundance in comparison with arable lands. For example,  a study found 27% greater plant species richness (133 flora species) and greater weed cover on SRC willow plantation when compared to the neighbouring arable agricultural land. The low input use and low soil disturbance after establishment of SRC willow plantations provides a conducive environment for many plant species to survive.

Plant species obtain all their habitat needs from the place where they establish on the SRC willow plantation. Plant species colonization on the SRC willow plantation takes place from the surrounding plantation, seed bed bank, and through living vegetation tissues like rhizomes, tillers, or living roots in the soil. In the initial years of establishment, the plant diversity is like that of arable agricultural land, and as the SRC willow plantation ages, the plant diversity evolves to deciduous forest, if the willow plants are not regularly coppiced.

The layout and size of the SRC willow plantation and its interaction with surrounding landscapes affect plant biodiversity. Effective site preparation in the establishment of SRC willow plantation requires well laid-out planting design and staging area to facilitate the use of farm machinery. Studies suggest having small multi-genotype SRC willow plots within a plantation with different coppice rotation times to enhance biodiversity. A study showed that plant species richness increased with increasing size of a SRC plantation but only until a size of 0.1–0.3 ha was reached. Different genotypes of the willow plant differ in the morphology of shoot, branches, and leaves. This creates niches for diverse plant and animal habitats.

Furthermore, the plant density of the SRC willow influences the dynamics of the canopy closure which influences the plant species richness and abundance. In the initial years of establishment of SRC willow plantation, more light demanding plant species, usually annual species with shrub-like herbaceous structure, grow on the field. As the canopy further closes with increasing age of the SRC willow, more shade tolerant perennial plants typical of forest grow on the field. Coppicing rotation length as well affects the canopy structure, thus, evolving from a bare field initially colonized by herbaceous plants initially after coppicing and as the canopy closes, more shade loving forest types of plants colonize the plantation.

Willow Plantation in February

The stand edge and pathways in SRC willow plantation used as open spaces to facilitate use of farm machinery and to serve as physical barrier on largescale plantation, provide a habitat for diverse plant species. SRC Willow stand edges have high plant diversity and species richness when the stand edge is allowed to develop with naturally occurring vegetation, or when purposefully sown with grasses or herbaceous seeds. Studies report higher plant population and species richness density at the edge of the SRC willow plantation than in arable stand edges. Biodiversity in the willows can also be enhanced by planting and managing a diverse mixture of herbaceous plants in the stand edge and pathways that immediately surround and allow access to the willow crops. Wider stand edges of six metres or more will be less shaded and support more plant species.

Other studies have shown interactions of the SRC willow plantation with other surrounding landscapes influences plant biodiversity. The more diverse the surrounding landscape, the more species can establish in the plantation. Establishing smaller plantations with longer stand edges facilitates species from the surrounding landscape more than larger plantations.

SRC willow and animal diversity

Bird and mammal species found in SRC willow plantation are typical of open field and woodlands. Young plantation attracts birds and animals associated with open fields, where over time as the willow ages and increases in height, it attracts forest associated animals. The dynamic habitat characteristic of willow provides different function to different species over time. Unlike plant species where the place of establishment on the willow field provides all the nutrients needed by the plant, animals seldom rely completely on SRC willow for fulfilling all their habitat needs. They use the willow for one or more functions of shelter, nesting, and foraging.

Different genotypes of willow grow in different ways and create different habitat attracting different animals. For instance, branching attracts certain types of bird species, and the herbaceous structure of the willow during the initial years of establishment attracts small mammals and insects. The surrounding landscape and stand edge of the SRC willow plantation support higher species richness abundance of invertebrates and birds. Willow plantation provides wildlife corridors connecting adjacent and fragmented landscapes which is beneficial for creating habitat for certain species and a conduit for movement for biodiversity.

Insects

Honey Bee on Willow

SRC willow plantations provide habitat and sustenance for a large diversity of insects such as bees, spiders, butterflies and ground beetles. Studies have shown that more insects live on willow than other tree species. Willow provides pollen early in spring (March to end May) for bees to build their colony.  The willow male catkins produce pollen, and the female catkins produces pollen and nectar that bees and other pollinators forage for food. Willow sex is important factor in bees visitation differences. A study found male willow supporting greater abundance, more richness and greater diversity of bees. Careful selection of both male and female willow is important to provide pollen at different flowering times for pollinators over an extended period.

Furthermore, the less disturbance to the soil after establishment, favours earthworm populations.  Other studies have found arthropod activity densities significantly higher, sometimes almost double, in SRC willow plantations. Species richness and abundance of butterflies as well increased in SRC willow plantation when compared to arable lands. Likewise, SRC willow has been found to have a positive effect on ground beetles and arachnids biodiversity quality.

Use of pesticide is a known factors for decline in insect abundance and diversity. The low input use in SRC willow management allows for many insect species to flourish. Landowners and land managers should consider biological control of insects over the use of insecticides to enhance insect diversity on willow plantation.

Mammals

Mouse

Willow plantations have a high abundance and richness of mammals such as wood mice (Apodemus sylvaticus), common shew (Sorex araneus), field vole (microtus agrestis) and many others. The herbaceous nature of willow plant structure between planting and the first coppicing, and thereafter the plant growth after each coppicing cycle provides a semi-long term habitat for mammal species associated with open fields and meadows. Studies have shown that more small mammal species used willow in the year following coppicing because of the herbaceous undergrowth. Coppicing allows undergrowth abundance of weeds flora which provides food for small mammals. For example, a UK study indicated that willow is a good habitat for small mammals when the willow plantation is allowed to become weedy.

Willow provides small mammals with all their habitat needs as they have a small spatial niche which willow plantations size can provide. Large mammals on the other hand have a wider spatial niche which the willow plantation may not necessarily be able to cover and must depend on surrounding landscape for their habitat demands to survive. Studies have shown that dominant small mammals from adjacent forest survive in willow.

Birds

Bullfinch on Willow

Willow plantation attracts numerous woodland and other types of bird species such as  Chaffinch, long-tailed tit, Robin, Blackbird, Sparrowhawk and many others including some endangered bird species. A survey of SRC plantation in Wales recorded 25 bird species both in the breeding season (April – September) and winter, sighting birds such as Blackbird, Goldfinch, Redpoll, and Song thrush, nesting in SRC willow plantation.  Birds are more abundant in the edges of the SRC willow than the stand edges around arable agricultural lands. Studies have reported fewer bird populations in the interior of large SRC willow plantation than the hedgerows.

The abundance of bird species is associated with the age, coppice stem, planting density, and weed cover in the SRC willow plantation. Different bird species select different age classes of SRC willow for their habitat needs. The age of SRC willow affects the height, canopy cover and ground cover. While more small mammal species tend to use willow crops in the year following coppicing because of their herbaceous undergrowth, more birds tended to use older willow crops. Planting different age classes of willow within a plantation will provide diverse habitat for various birds, thus maximizing the abundance and species diversity of birds.

Studies have shown that bird nesting habitat can be improved by including varieties of SRC willow in which birds preferentially nest. Also, planting different genotypes of SRC willow and a greater density has a positive effect on bird population. The surrounding landscape of SRC willow plantation influences the bird composition found in the SRC willow plantation. Studies suggest SRC willow next to different habitat types such as forests to maximize the bird species richness and abundance.

SRC willow management considerations

SRC willow plantation should be planned and managed to create a positive impact on biodiversity. Landowners and managers should consider the following;

• Wider stand edges of at least 6m in width should be incorporated in the design of the SRC willow plantation. There should be active management of the SRC willow stand edges by allowing naturally occurring plants to colonize the stand edges or purposively planting seeds to maximize plant biodiversity and support diverse animal diversity such as pollinators and small mammals.
• Establish small multi-genotype stands of SRC willow within a plantation with different coppice rotation times to enhance biodiversity.
• Encourage growth of weed species in SRC willow plantation after the establishment phase of the plants. The weed cover attracts diverse animal and plant species.
• Plant different age classes of SRC willow and increase the planting density on the field to enhance the structural and functional diversity and create diverse niche habitat for various species of plants and animals.
• Minimize or avoid insecticide use as the insecticides affect the biodiversity and the environment adversely.
• Minimize or avoid pesticide use on the nontarget invertebrate population and preferably use biological control.

Conclusion

SRC willow has a positive effect on plant and animal biodiversity. The abundance and richness of biodiversity in SRC willow is associated the crop and its management, the plantation size and layout design, and interaction with surrounding landscapes. The low input use and low soil disturbance after establishment of SRC willow plantations provides a conducive environment for many plant and animal species to survive. Planting different small multi-genotypes plots and different age classes of SRC willow with different coppice rotation enhances biodiversity. SRC Willow stand edges has high plant and animal species richness and abundance. There should be active management of the SRC willow stand edges by allowing naturally occurring plants to colonize the stand edges or purposively planting seeds to maximize plant biodiversity and support diverse animal species such as pollinators and small mammals.

Take home messages:

• Biomass crops accumulate relatively large quantities of below and above ground biomass which is made of carbon sequestered from the atmosphere.
• The frequent harvest of above ground biomass enhances vigorous shoot regeneration and root turnover, which enhances soil carbon stock.
• Biomass crops after establishment, requires no tillage during the lifetime of the plant, which enhances soil carbon stock when compared with annual crop soils which are regularly ploughed.

Introduction

Biomass crops play a key role in the UKs Net Zero Strategy which sets out how the UK will deliver on its commitment to reach net zero emissions by 2050. Biomass crops have the potential to contribute to this target by producing renewable energy, reducing greenhouse gas emissions, and supporting soil carbon (SOC) sequestration; the long-term storage of carbon (C) in soil. SOC provides the benefit of enhancing soil quality, increase supply and quality of water, improve biodiversity, and reduce atmospheric CO₂. Unfortunately, our soils are depleted of SOC and degraded, largely affected by land use, soil management and farming systems. Intensive agriculture has caused UK’s arable soils to lose about 40 to 60% of organic carbon. There is the need to restore soil carbon and increase SOC stock in the soil by adopting management practices that enhances soil carbon concentration in the soil. Planting biomass crops provide a mechanism to enhance soil carbon. In this article we focus on the effect of planting biomass crops on soil carbon.

Fundamentals of soil carbon

Soils contain two to three times more carbon than the atmosphere, and a relatively small increase in the stocks could exert a significant role in mitigating green-house gas emissions. The challenge in the management of soil carbon is to conserve existing soil carbon stocks and to remove carbon from the atmosphere by adding to stocks retained in the soil. Soil carbon sequestration is one way to reduce CO₂ loss into the atmosphere and increase the pool of carbon stored in the soil. Soil carbon sequestration is the transfer of atmospheric CO₂ into soil via plants. Soil carbon sequestration is considered a key measure to mitigate climate change and improve soil health.

Soil carbon is the solid carbon stored in soils. There are inorganic and organic soil carbon forms. Inorganic soil carbon is predominantly found in carbonate minerals derived from weathering of rocks and minerals, whereas soil organic carbon (SOC) is the carbon found in soil organic matter (SOM). SOM is organic material (e.g., plant tissues, microorganisms, animals) in various stages of decomposition in the soil. SOC is an important component of the functioning of the ecosystem. It helps to improve soil structure and provides resilience to physical degradation. This reduces risks of soil erosion and nutrient leaching from the soil. Keeping SOC in the soil helps to reduce emissions of atmospheric CO₂, increases microbial activity, improves soil aeration and increases water storage and availability to plants.

Figure 1. shows a simplified diagram on how carbon is transferred between the atmosphere, plants and soil of our ecosystem. SOC is formed from the interaction of ecosystem processes such as photosynthesis, respiration and decomposition of SOM.

Fig. 1 A simplified diagram on carbon transfer

• During photosynthesis, light energy is captured by plants and used to convert CO₂ absorbed by the plants from the air, and water from the soil, to build carbohydrates which acts as a source of food for plant growth.
• Atmospheric carbon fixed in the plant leaves and branches are transferred down through the roots to the soil.
• Plants exude carbon through their roots to feed soil microorganisms.
• The microorganism in the soil decomposes organic residue such as fallen leaves, branches and roots in the soil releasing carbon deep into the soil. During this process, the soil microorganisms release CO₂ into the atmosphere through respiration.

Thus, there is an inflow of CO₂ into the soil (carbon inflow) and CO₂ moving out of the soil (carbon outflow). Soil therefore acts as a source or as a sink for atmospheric CO2. The amount of carbon present in soil depends largely on the rate of decomposition of soil organic carbon to CO₂ by microorganisms and the rate of SOM input into the soil. The more SOM input, the greater the residence time, and the more carbon that is locked out of the atmosphere.

Current state of soil carbon in UK soils

A study reports average densities of soil organic carbon of 133 t C ha⁻¹ and 164 t C ha⁻¹ in England and Wales. The study further reports that to increase these stocks by 0.4% to this depth each year would require annual sequestration of 0.534 t C ha⁻¹ in England and 0.656 t C ha⁻¹ in Wales. A report on the state of soils in UK, indicated that, UK soils currently store about 10 billion tonnes of carbon which is roughly equal to 80 years of annual UK greenhouse gas emissions. Soil degradation leads to increased carbon emissions and could speed up climate change. Land use, soil management and farming systems, influences the SOM causing a net addition or depletion of soil carbon stocks. Intensive agriculture has caused UK’s arable soils to lose about 40 to 60% of organic carbon.

Restoring soil carbon is essential to enhance soil quality necessary to sustain and improve food production, increase supply and quality of water, enhance biodiversity, reduce atmospheric CO₂ among other benefits. Restoring soil carbon in degraded lands requires increasing SOC concentration in the soil by adopting best management practices. One option could be to plant perennial biomass crops that live for 10-20 years and accumulate biomass which is made of carbon. Several UK government policies have called for actions to sustainably manage UK soils to prevent degradation, advocating for measures to protect and enhance soil carbon stocks. In managing UK soils, biomass has a key role to play in improving soil carbon and resilience to climate change.

Biomass crops and Soil Carbon

Biomass crops are non-food crops grown for energy production. They are usually perennial woody or herbaceous plants. Some commonly grown woody biomass crops are short rotation crop willow (salix spp), poplar (populus spp) and short rotation forestry. Examples of herbaceous biomass crops are Miscanthus (Miscanthus giganteus) and switch grass (Panicum spp). The advantages of using these crops for biomass production include rapid growth rate and high biomass production, low nutrient requirement, and ability to re-sprout after multiple harvests. They are considered to have the potential to sequester large volumes of CO₂ captured from the atmosphere during plant growth through photosynthesis and store carbon in the soil. The benefit of biomass crops sequestering carbon makes it an important crop in contributing to reducing greenhouse gas (GHG) emissions.

The effect of biomass crops on soil carbon

Biomass crops accumulate relatively large quantities of below and above ground biomass which is made of carbon sequestered from the atmosphere. Biomass crops produce high amounts of biomass above and below ground. The fallen leaves and branches also add organic matter to the soil. Studies have shown that leaving Miscanthus crop standing over winter increases litter fall at around 30-45% leading to the accumulation of biomass on the soil surface. Miscanthus organic material is shown to have a slow decomposition rate and is predicted to have the potential to store between 2-3 metric tonnes depending on the crop yield and the initial organic carbon level.

Biomass crops are harvested every 1-3 years during the lifetime of the crop. A process termed coppicing. Coppicing involves cutting down the tree or above ground biomass to its base and allowing it to reshoot multiple stems. After a few years, the stem or above ground biomass can be harvested, and the cycle begins again. The frequent harvest of above ground biomass enhances vigorous shoot regeneration and root turnover, which enhances soil carbon stock.

The root structure of biomass crop after establishment continuously grows throughout the life cycle of the plant, storing and transferring carbon to the soil. After coppicing the above ground biomass, the roots are left in the soil and continue growing. Perennial rhizomatous grasses such as Miscanthus allocates a large proportion of the aboveground carbon to the roots and rhizomes, further increasing soil organic carbon stocks. Miscanthus contributes 0.98 ± 0.14 Mg C4-C ha⁻¹ yr⁻¹ through litter drop and root turnover.

Biomass crops after establishment, require no tillage during the lifetime of the plant, which enhances soil carbon stock by minimising disturbance to the soil. Tilling (ploughing) soils reduces SOC stocks in the soil through enhancing soil aeration and reducing the physical protection of SOM, leading to increased decomposition rates and release of CO₂ into the atmosphere. After establishment of biomass crops on the soil, the soil is not tilled, thus, there is less disruption of soil aggregates and exposure of SOM to microbial activities. For example, a study found that, SRC willow planted in South England had lower soil respiration (912 ± 42 g C m^-2 yr^-1 ) and a net sink for carbon (221 ± 66 g C m² yr¹). Substantial amount of carbon can be stored in the soil when the land is less ploughed.

The amount of carbon stored in the soil largely depends on the initial soil carbon content and prior land use. Planting biomass crops on lower carbon soils, such as arable lands, minimizes soil carbon losses and promote soil carbon sequestration in the long term. Studies have shown, that soils with high carbon stock such as grasslands, forest and peatlands have high carbon stock and conversion to planting of biomass crops is likely to result in soil carbon loss. It is recommended that Landowners and managers should measure the carbon stock prior to planting biomass crops to better predict soil carbon stock change after planting biomass crops.

Conclusion

Soil Organic Carbon (SOC) is formed from the interaction of ecosystem processes such as photosynthesis, respiration and decomposition of SOM. Soil carbon provides the benefit of enhancing soil quality which is essential to sustain and improve food production, increase supply and quality of water, enhance biodiversity, and reduce atmospheric CO2. Planting biomass crops could provide a mechanism to enhance soil carbon. Biomass crops have high above ground and below ground biomass which stores a significant amount of carbon in the plant. Coppicing above ground biomass enhances vigorous shoot regeneration and root turnover which enhances soil carbon stocks. Furthermore, biomass crops after establishment, requires no tillage during the lifetime of the plant, which facilitates better accumulation of soil carbon. For these reasons, planting biomass crops help to improve soil quality, provide resilience to physical soil degradation and help mitigate climate change.

• Demand for solid biomass has risen dramatically over the last decade, principally driven by the power sector.
• In 2021 biomass power became the third largest contributor to UK electricity generation
• The majority of plant biomass has derived from imported material, predominantly in the form of wood pellets, but domestic biomass production has also seen a dramatic increase.

The utilisation of solid biomass as a fuel in generation of heat and power has seen a significant increase over the last ten years as many coal-fired power stations converted to using biomass. This article provides a summary of the data reported by BEIS and OFGEM specific to solid biomass use in the UK heat and power sector over the last 5-10 year period.

The role of bioenergy in UK energy generation

The Department for Business Energy and Industrial Strategy (BEIS) publish an annual digest of UK Energy Statistics (DUKES). Data is reported in terms of million tonnes of oil equivalent (Mtoe). The report published in 2022 stated that bioenergy and waste on average contributed 10% of the total energy generated in the UK in 2020 and 2021. Increase in the output from bioenergy and waste was in part due to reductions in output from fossil fuels, nuclear power and outputs from wind, solar and hydro power which were affected by less favourable conditions during this period.

Energy from bioenergy and waste was the fourth largest contributor to UK energy generation in 2021. Predominantly through large-scale electricity generation.

Biomass also contributed to generation via a network of around 2,000 smaller combined heat and power (CHP) facilities which are designed to enable use of both electricity and heat from fuel conversion. Co-generation can improve efficiency by up to 20% compared with electricity generated without heat recovery. CHP schemes accounted for 7% of the total electricity generated.

In 2021, CHP schemes were predominantly fuelled by natural gas with solid biomass, biogas and waste contributing (6.3%, 8.2% and 5.6% respectively).

Source: DUKES, 2022

Solid biomass (wood, waste wood, animal and plant biomass) accounted for 36% of the renewable energy output in 2021. Two thirds of the energy generated from solid biomass was used for electricity and a third for generation of heat.

Solid Biomass in UK Energy Generation – The overall trend 2010 – 2021

The rise in the use of solid biomass has principally been driven by electricity generation. The data reported in DUKES shows the steep rise in use of plant biomass from 2010- 2021 in terms of thousand tonnes of oil-equivalent. Plant biomass is differentiated from wood in this data as the plant biomass category includes a wide range of solid biomass materials. The majority has been composed of forestry or timber processing residues which are differentiated from wood as they have been further processed in some way, such as by milling, pelletisation or chipping.

Solid Biomass used for energy generation 2010 – 2021 in thousand tonnes of oil equivalent (DUKES, 2022)

The majority of plant biomass used in electricity generation has derived from imported material, predominantly in the form of wood pellets. UK plant biomass supply has also increased significantly, predominantly in the form of wood chip from forest residues.

Solid Biomass used for energy generation 2020 – 2021 in thousand tonnes of oil equivalent (DUKES, 2022)

The wood supply quoted in the DUKES data refers to the use of traditional firewood, principally for domestic heating applications, which has also seen a small increase in uptake over the last ten-year period. Waste wood e.g. wood derived from previous industrial uses or recycled material, has also made a small but consistent contribution to energy generation.

Of the total solid biomass used for energy generation in 2020-21, the use of plant biomass for electricity generation was the largest single contributor, accounting for 69% of the total use. Plant biomass used in combined heat and power systems (CHP or autogenerators) was the second largest contributor (18%) followed by the domestic use of wood for heating which contributed 11% of the total biomass converted.

Location of Major Power Producers in the UK (operational May 2022)

Source: DUKES, 2022

Type, form and origin of Biomass used in UK energy generation

The Renewables Obligation (RO) was designed to incentivise large-scale renewable energy generation in the UK and is administrated by OFGEM. Data regarding what biomass is being used, the quantities consumed, and its’ origin is collected and made available by OFGEM annually. Accredited power generators receive payments from the government in return for certificates (ROCs) which are obtained for roughly every megawatt (MW) of energy generated from renewable sources. In response to concerns regarding sustainability, RO Sustainability criteria were introduced in 2015/16 which must be reported against in order for generators to qualify for ROC payments. Current guidance requires generators with outputs exceeding 50 kW to submit monthly reports to verify compliance with RO sustainability criteria except for generators using sewage or landfill gas, municipal waste, or biomass with a biogenic content <90%.

Forms of biomass being used in UK Energy generation 2020-2021 as reported in Renewables Obligation Biomass Sustainability profile datasets.

Based on the data from the OFGEM biomass sustainability dataset profile data for 2020-2021; includes all reported consignments, facilities with outputs, grouped by feedstock type and form. The data presented refers solely to biomass used in combustion applications and does not include anaerobic digestion. Data is presented in thousand metric tonnes (kt).

Wood pellets, produced from a mixture of virgin timber, forest and process residues represent the largest share of the biomass consumed in the heat and power sector. Wood chip from timber, residues and waste-wood are the second major contributor of biomass material. Several power stations are designed to run on bailed material, principally cereal straws, but can also process bales of other biomass types (miscanthus & agricultural residues).

A full breakdown of the data is provided in the accompanying PDF document. Click here to download the Biomass Connect Technical Article: “Data Summary – Solid Biomass consumption trends in the UK energy sector 2016-2021” [PDF – 452 KB]

• Pre-preparation and drying of timber before harvesting is essential for reducing drying costs and improving profits for biomass timber and woodchip
• The carbon benefit from planting woodland for biomass production helps to meet UK renewables targets, and offsets carbon emissions from burning coal or other high carbon outputting fuel sources
• Incentives exists for both homeowners and farmers toward increasing the utilisation of biomass produced clean energy across the UK

In this article we will explore the further potential for trees and hedgerows in bioenergy production. Parts 1 and 2 covered potential tree species and management techniques, whereas here, part 3 will discuss forestry in terms of biomass.

Drying and processing harvests for biomass production

While healthier wood may bring greater yields and prices based on quality, timber collected for biomass does not share similar requirements to timber collected for architecture, furniture, or other specific uses, and therefore can consist of poor-quality trees, including dead or dying wood. Therefore, timber collected from thinning exercises, from woodlands for all uses may be developed as biomass.

Potentially the biggest challenge in producing timber for biomass, particularly within the UK, is the drying process. This directly impacts timber price and quality and can lead to expensive infrastructure costs.

Trees can be pre-prepared to decrease moisture content before harvesting which can significantly reduce the time and costs required in post-harvest drying sessions. This can involve organic and chemical methods on live trees, or simply harvesting dead or dying trees. This will have reduced moisture content due to ill health and transpiration rates.

Ring barking, also called girdling, is where a complete strip of bark around the circumference of the tree is removed, preventing transpiration of water from the roots to the top growth. This helps to dry out stem tissue above the cut line and leads to to a reduction to <35% moisture content. The upper stem of the tree usually dries faster, leading to a possibility of drying the sparser upper tissues of the tree for biomass, and retaining better quality, thicker lower stem tissues for traditional timber. The slow thinning of limbs and leaves also enables other surrounding trees to gradually adapt to the changing conditions, reducing the risk of wind throw.

The benefits for ring barking include reduced storage space required for drying timber and providing better quality biomass timber with potential for increased calorific values. Drying at the source also shortens the supply chain, reducing the need for transportation and improving final profit margins.

Due to significantly reduced moisture content, final harvest weights are likely to be much lower than fresh weights, resulting in improved efficiency during harvest and better soil protection during extraction.

Chemical thinning involves injection of a chemical, such as glyphosate herbicide into the stem. This is often combined with ring barking, where glyphosate is injected upwards, or sprayed into the exposed phloem tissues.

Sour felling/Transpirational drying, is a more traditional method, where entire trees are harvested and left intact on the forest floor, or in stacks by easy access roadsides. This has similar benefits to ring barking, although generally takes up greater space, and requires more invasive harvesting techniques and therefore sour felling is commonly used with clear felling management schemes than in continuous stands. Leaves and limbs are often left intact, to increase moisture loss through transpiration.

Ring barking is generally the preferred method of pre-preparation, as it results in less inconsistency within the drying effect, compared to the horizontal stacking methods, and as sour felling involves multiple operations, ring barking is likely to be a more economical solution. Ring barking also avoids the unsightliness and fire risk of felled trees within a woodland.

Continuous cover forestry (see part II) is the recommended approach to harvesting woodland stands, by retaining irregularly aged and sizes of individual to maintain local environments.

Post-harvest, trees should be left to dry completely. This can be carried out on-site, either within a solar kiln on the property for 6-11 weeks, or by letting the timber rest as rods (up to 8m in length) or billets (5-15cm lengths) outside for up to 2 years. Timber may also be sold for downstream processing off-site, although generally higher returns would be achievable through on-site processing. Drier timber, with moisture contents below 10% are likely to achieve the highest gross profit both for traditional timber and timber for biomass, although net profit should be calculated to include costs of processing.

After timber has been successfully dried to 10%-35% moisture content, the timber can be processed to the intended fuel type; log or woodchip. Again, this can be achieved on-site with a wood screw chipper, drum or by simply cutting logs to length. Pelleting requires more complex processing, where pellets are formed in a pellet plant from already processed and ground woodchip or wood shavings, dried to 8-12% moisture content. Alternatively, the timber can be transported or sold to an off-site plant, which can continue any downstream processing, although this is likely to have an impact on final profits. As before, any potential increases in final prices for processing your own stock should be balanced against the need for manual labour, the expenses incurred for machinery and storage of final chip.

When chipping for biomass, timber should be processed to set specifications, to avoid clogging fuel feeds and damaging machinery, incurring costly repairs and downtime. Woodchip and pellet quality is ranked, with regards to moisture content, ash contaminants and size, and superior biofuel quality can result in greatly improved profits.

Figure 1: A structured approach to forestry planning, management and processing for biomass production

Carbon benefit from planting new woodland

Correctly managed woodland for biomass production can have significant impacts as a sustainable source of renewable energy, provided that harvested forests are replenished and sustainably managed. Some of the carbon benefits provided by woodland include provisioning services, including timber and biomass production as a substitute for more carbon intensive materials, regulating services such as carbon sequestration, and supporting services, including improving soil health and nutrient and oxygen production, which will have significant impacts on carbon capture.

Land management methods can have substantial impact on environmental and carbon benefits within a forest. For example, continuous cover forestry (CCF), offers improved carbon benefits compared to clear-felling, due to maintaining a structured woodland stand throughout multiple harvests, and thus maintain high soil quality and health, reducing the carbon release from overly tilled, or dried out soils. Coppicing also offers carbon benefit compared to regular harvesting as it avoids the need for regular replanting, and silvopastoral agroforestry, involving planting trees with livestock helps capture carbon emissions from animal excrement and help alleviate carbon pollution from high carbon areas.

Currently, much of Europe’s biomass comes from aged European and American forests. Harvesting from ancient and developed forests can have negative effects on carbon emissions, by releasing carbon dioxide through burning significant carbon stores within natural forests, and not replenishing the sites. Where forests have been storing carbon for centuries, it may take upwards of 100 years to redevelop carbon stores to the same magnitude. By utilising managed, sustainable woodland domestically through both existing sites and new woodland creation, the UK will be able to meet renewable targets without affecting the critical carbon stores held within ancient forestry. Forestry specifically developed for timber and biomass production are often cultivated for a maximum of 40 years, and therefore will not lead to the release of old carbon stores, and furthermore, will only require 40 years to replenish to similar levels.

Latest modelling and improved genetics have increased yield class and reduced rotation scenarios by up to 10 years if aimed specifically for biomass production; growth is far too rapid to be of structural grade and value in that reduced time period. By planting new woodlands for biomass, global production has the potential to reduce the need for biomass from more culturally important aged stands. The UK has targets to add 40,000 hectares of woodland a year by 2030 and reach 19% coverage by 2050. These changes would move us closer in comparison with Europe who have 39% current tree cover. These increases in woodlands will help to create a range of green job opportunities within the forestry and environmental sectors spreading economic growth across the country. Furthermore, woodland developed specifically for bioenergy will also have significant impacts on UK carbon footprints and help to meet net zero targets. Calorific values for dry wood can be as high as 5.3 MWh/tonne, although realistic values for wood of a moisture content between 10-30% is 3.5 – 4.7 MWh/tonne. Average household energy consumption for the UK in 2018 was 3.6 MWh, suggesting that each tonne of timber could sustain a house for a year. In comparison, coal has approximately twice the calorific value of wood, but is a less sustainable fuel and agroforestry offers a more carbon neutral solution to growing energy demands.

Furthermore, in 2019 the UK had an estimated average 5.2 tonnes of carbon emissions per capita, with a trend towards improved rates. The calculated benefit of Eskdalemuir forest, Scotland, is 7.3 tonnes CO2 per hectare per annum. If a similar trend can be observed across all countries in the UK, woodland creation is likely to have a significant impact on the nation’s carbon emissions footprint.

Willow and poplar are ideal species for biomass production as fast growing trees, ideal for coppicing and able to grow on marginal land. Harvesting for biomass in short rotation coppicing should be performed every 4-5 years or longer, to reduce the bark:stem ratio and corresponding high ash contents, although mature timber production will take longer.

Summary

Woodland creation for bioenergy production is a critically important role for current forestry, as it not only provides the carbon benefit through sequestering carbons within the developing woodland, but also offsets coal consumption as a sustainable alternative fuel. Currently global forests are being decimated to meet Europe’s renewables targets, but by creating new woodland domestically within the UK and worldwide we can provide a more sustainable future crop, without risking long-established forestry.

• Weed Control is paramount to a strong early establishment and development of forestry woodlands.
• Short rotation coppicing is a great method for producing biomass in short cycles
• Continuous cover forestry matches timber production with environmental sustainability and should protect soil health, forest biodiversity and long-term soil carbon storage within the woodland even through continuous harvesting.
• Agroforestry is a low-invasive method for including silviculture on agricultural land, to complement current livestock and crops within the field.

In this article we will explore the further potential for trees and hedgerows, and planting and management methods that could be utilised, when considering woodland creation for bioenergy production. Forests cover approximately 31% of the global land area around 4.06 billion hectares, with 3.9 billion m3 of timber harvested annually, of which approximately 50% is used as fuel. Within the UK, 90% of forests are managed plantations.

Management methodologies

Planning for woodland creation can take time, and it is important to plan time to get the correct advice and plan accordingly before starting work. Landowners can submit expressions of interest when the window for planting schemes opens, and if approved, will have a contract stating work should be completed by a set date (usually specified by the scheme before application).

When establishing woodland to meet different objectives the said objective has a predominant influence over how to manage the woodland. Undermanaged woodlands on farms very often contain poor-quality timber with many farmers believing that their woodlands are unproductive. In reality, the quality and quantity of timber produced will strongly depend on the level of management and type of intervention whilst many other factors can also contribute to farm woodland performance such as soil types, elevation, exposure, and planting the right tree in the right place. To reap the greatest potential product yields, tree plantations should be matched to the land available, with different species preferring different soil types, light intensities and climates.

Netting and Livestock fencing is important to avoid grazing animals damaging new early growth, and fortunately most funding opportunities offer grants specifically for fencing production and maintenance.

Ground preparation and establishment of new woodland

Forestry companies and agents may be recruited to design and implement woodland creation schemes, including grant applications and site surveys. Since heavy machinery is required for harvesting, it’s recommended to plant on land with suitable road access for larger vehicles. Though we have discussed the opportunities for mitigation of soil damage in forestry applications in another article, it’s prudent to incorporate planning for access to manage the woodland by allowing for travel throughout the woodlands, including footpaths, at the time of establishment.

Site preparation is essential, as forestry plantations are likely to be in the ground for over 20 years, and therefore getting thorough initial grounding will have direct impacts on long-term financial returns.

Weed control is one of the most important aspects of ground control before woodland creation, as weeds such as dock, brambles and weed grasses, will become tangled up in root systems and compete for soil nutrients, and may have significant impacts on early tree development and mortality rates. Herbicide application, or alternative weed control methods, should be applied several months before planting, and shortly after planting. Weed control methods can be selected similarly to weed control in cropland, including herbicides such as glyphosate, and mulching using sheeting. As the plantation develops, and the woodland canopy reduces light penetration to ground level, weed control methods can be reduced.

Ploughing may not be necessary, although grass should be mown short, and the area should be weeded to reduce competition for water and resources. Any ploughing is likely to release more ground carbon and can be detrimental to final figures where the woodland carbon code is to be applied for. In cases where ploughing is needed, the type of ploughing will depend on tree species and soil type, and advice should be taken for each plot. Ploughing is generally performed in the Autumn to enable soils to weather over winter.

It is recommended to plant trees while they’re dormant and least likely to get damaged. Planting is usually performed after the last frost of the year, between late February to April, while still ensuring planting as early in the year as possible to provide a greater first-year biomass development and maximising root development before risking mortality from a hot summer, or over the following winter.

Planting densities should be based upon tree species being utilised (see Trees for Bioenergy part I), although densities should be above 2,000 per hectare to provide better weed control at canopy establishment, and a maximum of 15,000 to avoid competition between individuals leading to weaker structure, thinner trees and greater need for remedial thinning.

Trees can be planted by:

• Pit planting: digging a specific hole for each shoot, recommended for most soil types
• Slit planting: forming a thin slit with your spade for inserting the root plug, recommended for stony soils
• T-notch planting: creating a T-shape with your spade and levering the cross-section open with your spade, recommended for grassy soils susceptible to drought (but not clay soils)

Short rotation forestry and coppicing

Short rotation coppices and forestry usually consist of a mix of tree species, often including high-yielding willow species, (such as common osier and basket willow), poplar and alder, as fast-growing, sturdy plants that are relatively easy to propagate. A mixture of tree species helps to provide disease and pest resistance to the woodland as a whole, in addition to greater variation being able to cope with climate changes. Monoculture plantations, formed of one tree species, are easier to harvest, however, generally have lower yield densities due to low resilience and direct competition. Where a greater variety of crops may be established, competition is generally reduced due to a wider range of rooting depths and canopy heights.

A frequent planting scheme for coppice forestry, particularly willow plantations, involves 10,000-15,000 straight cuttings (18-30cm tall) per hectare, planted 2/3 of the rod’s length deep, in a double row design, with 0.75m distance between the double rows and 1.5m to the next double row. Within rows, the distance between trees ranges from 0.4m to 1m depending on species and density. Planting is usually in March, following the last winter frost. Weed control is vital at this stage, with both pre-and post-planting herbicide treatment recommended, or similar organic methods, such as mulching.

Willow is cut back the following winter, before bud-break, to encourage multi-stemmed coppicing and within 3 months, canopy closure will help provide weed control to substitute herbicide applications. Trees with fewer stems, such as poplar, should be cut back only after the first harvest. Harvesting is then carried out on a 2-5 yearly cycle, between October-March, across a 20-30 year lifespan.

Alder is another tree species particularly suited to short rotation coppicing, and ideal for a polyculture plantation.

Coppiced timber is relatively easy to store, and is generally stored as whole stems, and dried for combustion for 12 months outdoors until the moisture content has decreased to approximately 30%. The stems are then cut into rods (8m long), billets (10-15cm long) or chipped, depending on preference for end use. Chipped biomass often has the larger market demand, although is harder to store and more likely to decompose long-term.

The widest commercial use for short rotation coppice is generally for biofuel to feed combined heat and power generation units producing direct heating and electricity.

Further information

Further information on many aspects of SRC for growers, researchers and the bio-energy industry can be obtained from:

• Forest Research – Short rotation coppice
• Defra – 2020 Report on ‘Crops Grown For Bioenergy in the UK: 2019

Continuous Cover Forestry (CCF)

Clearfelling is currently the most common method of forestry practice, where trees are uniformly harvested to promote select species and establishment in a monoculture ecosystem. The method is economically favourable, however, has drawbacks in that it has significant effects on natural habitats, and subsequent loss of topsoil has further repercussions on carbon release from the soils. In the current global environment, more sustainable methods are being developed and advocated to reduce the detrimental effects of logging and preserve natural ecosystems.

Continuous Cover Forestry (CCF) is a flexible forest management system to form resilient yet diverse environments. CCF solicits sustaining the forest ecosystem and habitats there within, wherever possible. The forest canopy is maintained at least one level at all times, by utilising a range of different tree species at different ages forming a more irregular structure:

• Irregular species mixtures
• Irregular tree sizes
• Irregular horizontal structure
• Irregular vertical structure

Through natural forest succession, natural regeneration of trees, and selective harvest of single trees or small samples, production and regeneration occur simultaneously. Another major benefit of CCF forest management is the high-quality timber management within a permanent forest structure and enabled selection of the optimum individuals specifically at their target size and value. CCF enables a steady source of income, rather than a larger scale income after 25-40 years, and facilitates the ability to harvest without the effects on environment health and “landscape scars” that may be controversial when clear-felling.

CCF practices lead to improved carbon storage in soils and developing stands. By avoiding clear feeling, the forest structure is maintained, reducing the risk of soil run-off and depletion after harvesting, by maintaining a forest canopy and structure. This results in a lower risk of loss of soil fertility and higher humidity levels. Soil water yields are also improved within a continuous forest stand, with reduced siltation risks and nitrate flushes that are sometimes a concern downstream from clear-felling sites.

In the UK, most CCF stands include coniferous and broadleaf stands, on sheltered sites with favourable conditions for natural regeneration and good browse control.

Clocaenog Forest is a large-scale operational trial by Natural Resource Wales, in west Denbighshire and east Conwy, demonstrating CCF methods of forestry in increasing intensity, across over 25 hectares. It has undergone twenty years of assessment with the last felling cycle performed in 2019 and acts as a strong demonstration of how CCF can be successful in the UK climate.

The CCF principles cover:

1. Adaption of the forest to the site, to include a natural variation of tree species, mixed aged stands and spacing between individuals, and replicate a natural woodland.
2. Adopting a holistic approach, where the ecosystem is considered the production capital (including soil, carbon, water, fungi, flora, fauna and the trees), with timber as a complimentary output
3. Maintenance of forest habitats, to support a self-sustaining woodland with a constant forest canopy. This involves strictly no clear-felling of all individual trees within an area, and instead, harvest selected high-quality trees that have reached a specified target size.
4. Developing preferred individuals through the selection of good quality smaller trees across a broad range of diameters, to be maintained as future crops and inferior quality trees (such as thin or weak trees) are removed to enable the maximum potential of the selected crop. At any single intervention, a maximum of 20% of the stand increment can be removed, determined either basally or by volume. Crown thinning is also encouraged to enable greater light intensity to reach ground level, and support new growth of younger generations.
5. Development of forest and stand structure involves the development of an irregular stand structure at the compartment level, over the continuous development of the forest and its canopy. It stems from the idea that by concentrating on maintaining prime individual trees rather than uniform stems, overall timber production is likely to be more sustainable and more varied across the woodland. When transforming an even-aged stand to a CCF stand, irregular structures are likely to develop in response to higher rates of disturbance, including regeneration gaps of significant size.
6. Thinning cycles should be carried out every 3-5 years to maintain the structure and system of the forest. Specimens to be removed should be controlled based on stem diameter and increment, rather than age and area as seen in more traditional forestry methods. Trees should be removed at target stem diameter, unless damaged, and if not contributing to stand structure. For broadleaves, heavy thinning is encouraged when trees reach a height of 9-11m, with over 6-7m of branch-free bole, to release potential final crop trees from the competition and enhance crown and diameter growth.

CCF also offers commercial benefits compared to clear-felling methods, including bringing forward cash flow due to heavier thinning in development years, leading to a more stable cash yield, less affected by fluctuating timber prices, and maintaining the capital value of the forest. Replanting costs are also avoided due to natural regeneration within the established woodland

The Continuous Cover Forestry Group (CCFG) promote the transformation of even-aged plantations to structurally, visually and biologically diverse woodlands and are a member of Pro Silva, a federation of forestry organisations that work together to advance silvicultural systems to support the environment. The group offers an annual programme of UK site visits, practical workshops and networking within the UK, to provide sources of practical experiences, to those interested in CCF silviculture.

Agroforestry

Silvo-arable agroforestry and silvopastoral agroforestry involve the planting of trees and hedgerows within fields currently utilised for crops and livestock, without impacting the production yields of the main target product. Agroforestry in this way can balance carbon capture and potential mitigation of climate change with the need for food production. Weed control, as standard, is essential during the pre-planting and post-planting phases, and plastic mulches are often used to provide weed control without the use of herbicides that may affect developing crops or livestock.

In silvoarable systems, crops are grown simultaneously with long-term tree crops and provide an annual income while the tree crop matures. Trees are planted in rows a minimum of 10m apart to enable heavy machinery and tractors to tend to the crops without undue disruption, and generally aligned North-South. Timber, biomass, fruit, pollard or coppiced trees are all potential choices for tree cultivation, depending on the end product or management style preferred, with poplar and walnut, popular choices for silviculture systems based on equivalent annual values per hectare. Timber trees can be planted in single, double rows, or even triple rows, where they can be sandwiched between nurse trees, such as alder or coniferous trees, which help to encourage straight growth. Nurse trees, and lower-quality timber trees, may then be thinned at a later date when trees begin to reach maturity. Shrubs and hedgerows may offer wind protection during early development. Crop quality and yields, particularly in winter-hardy and heat-sensitive crops including oats, can be improved by the benefits generated by the nearby silviculture. The trees provide improved microclimates as they offer shelter from winds and direct sunlight and improvements in soil carbon capture. This can offset any crop yield reduction from the removal of tree strips from cultivation.

Rows of trees and shrubs also provide wildlife corridors and habitats to fauna, particularly during harvesting season, and attract natural predators and competing invertebrates to dispel crop pests.

Silvopastoral systems are where livestock are grazed under trees, which can provide improved shelter and fodder for livestock. Pastoral woodland offers a wood source while improving the performance of livestock, including sequestering carbon into heavily grazed and often nutrient-poor soils. During the summer livestock often suffer in hot dry environments, leading to significant impacts on milk production and performance. Trees can offer substantial shade and cooler, damper areas, which will benefit animals and can help prevent overheating during heat waves, although livestock should still be monitored during periods of high temperatures. Integrated woodland may also shelter water sources and prevent siltation. During winters, trees may offer shelter to livestock from cold winds, rain and storms, and reduce flooding risks of local streams and rivers.

By planting trees near livestock and poultry, trees have the additional benefit of capturing carbon at the source and can sequester greater amounts of carbon before it enters the atmosphere. Planting trees within livestock fields also may offer shade and protection from the elements, in addition to offsetting some of the carbon emissions at the source.

Planting trees on agricultural land also encourages increased biodiversity of wildlife and helps to recreate historical landscapes, similar to traditional forests where animals grazed.

It is advised to plant trees at an even spacing of 400 per hectare, to facilitate little impact on light levels, while providing adequate shelter for livestock and preventing excessive trampling around woodland patches. Trees may be thinned to maintain levels of pasture production, and improve timber quality

Care should be taken while selecting trees, to ensure species are not toxic to the surrounding livestock, such as yew, which can be fatal if ingested. Other species, such as fast-growing specimens or those with a spreading canopy, for example, larch and ash may have potential impacts on grass ley production. Care should also be taken to protect the trees from browsing livestock, particularly during early development, and broadleaf trees should be protected with plastic tree shelters.

When planning to introduce silviculture to pastoral land, the silvopastoral agroforestry toolbox as well as advice from the innovative farmers.org may be helpful sources of experienced advice.

Summary

There are recommendations for landowners and farmers involved with woodland management to start evolving from traditional forestry methods and concentrate more on providing an improved ecosystem as the primary output from managed woodland, with sustainable timber production as a huge benefit. They suggest that methods such as continuous cover forestry mechanisms will improve carbon storage, soil quality and forest biodiversity of the woodland, leading to improved environmental effects and a more sustainable forest long-term. The financial benefits can include a yearly income from woodlands after a few years, rather than 25-40, and every year forthwith. By mixing short rotation coppicing with hardwood timber production, forests may maintain high levels of biodiversity alongside mixed timber productions, and maturity at a range of ages. Furthermore, by including silvoarable and silvopastoral systems on farmland, carbon capture and ecosystems can be improved without impacting entire productive fields or changing the use of profitable ventures.

• Planting areas of woodlands is a great method to increase carbon capture, supported by a number of grants and incentive schemes
• Mixed tree species plantations result in improved disease and pest resistance, increased biodiversity of flora and fauna, and more resilient economic returns than monoculture plantations.
• There’s a wide choice of both broadleaf and conifer tree species to consider, each with a range of end-markets and specifications, producing a timber supply for biomass, paper pulp, furniture and building construction.

Previous ‘Farming Connect’ articles have explored the benefits of planting trees and hedgerows with regard to biodiversity, flooding, livestock and environmental impact. Here we explore the further potential for trees and hedgerows concerning bioenergy production. This article discusses a selection from the wide range of tree species often cultivated in agroforestry for biomass.

The importance of tree plantations for biomass

Producing bioenergy and bio-products from commercially cultivated biomass crops is just one part of a critical solution to reduce pressure on fossil fuels and reduce carbon emissions. The production of forestry on both a large or small scale is one of the strongest methods to tackle carbon emissions. Woodland creation reduces the carbon footprint in a number of ways:

• sustainable management, including timber farming, accelerates carbon sequestration in the soils;
• carbon in timber products, such as architecture and furniture, is locked up for longer;
• timber substitutes carbon-intensive alternatives;
• home-grown timber relieves pressure on global forests;
• end-of-life wood products can be recycled (biomass, biochemical and chipboard).

Forestry sectors across the UK are under demand to expand due to growing markets for home-grown timber, amidst requirements to enhance forestry ecosystems and habitats. The UK forms the world’s second-largest importer of forest products, with £8.5 billion worth of wood imported from overseas in 2021. With 3.24 million hectares of woodland, only 13% of the total land area of the UK is under forest cover.

Historical and future target woodland creation rates from 1971-2030. Graph from confor.org.uk

Where is our timber material going to come from?

Although most of the UK timber resources are currently imported, there is increasing interest and demand in home-grown and environmentally sustainable timber, for both downstream production and increasing biomass demand. The percentage of UK woodlands has largely decreased in the past 30 years, as although more cultivated areas are added each year this is often balanced by woodland area being permanently removed for more appropriate habitat restoration to specific land type and approved development. To cope with increased demand, more woodland is required and the UK government wish to support development. An improved trend towards increased woodland creation can be observed, with approximately 2,300 hectares of new woodland development established in England alone between 2022-2023 absorbing around 600,000 tonnes of CO2 by 2050.

Multiple tree species may be cultivated for timber and bioenergy production, and for habitat creation. There is something to be said for utilising native tree species, rather than imported species, as they are often better suited to the surrounding environment and climate, and it reduces concerns about invasion and competition with the local ecosystem. Several potential tree species for consideration are listed below:

Willow

Willow is a popular consideration for woodland plantation, and confers a high tolerance of wet or marginal lands. There are approximately 400 different willow species, of which the most commonly used for managed woodland systems are the common osier (Salix viminalis), and its hybrids with S. burjatica and S. schwerinii, white willow (S. alba) and crack willow (S. fragilis). Ideally a mix of willow species should be planted to impede spread of disease or pests. Planting as a monoculture may lead to improved harvesting, however inclusion as part of a mixture of tree species will improve pest and disease resistance and local biodiversity.

Willow also had roles within wastewater treatment; in Estonia, 1995, a S, viminalis plantation was cultivated with wastewater from a residential plot for 25 people. Results showed a significant improvement in oxygen demand and nitrogen emissions in treating water, and a first year yield of 1.6 tonnes per hectare.

For specific harvesting regimes such as short rotation coppice, Willow has a high planting density at 15,000 trees per hectare, enabling high overall yields for woodchip production to supply the biomass industry. Willow should be planted early after the last frost to enable a long first growing season without risk of exposure to sub-zero temperatures. Rods should be planted 0.75m apart with 1.5m between rows and the site should be rolled immediately after planting, with pre-emergence herbicide applied within 2-5 days of planting. Mineral soils, with a pH between 5.5 and 7.5 are recommended for planting willow plantations. Browsing animals can be a risk during establishment, but can be prevented using adequate fencing.

Shoots are generally cut back during the first winter to encourage greater shoot density the following season. The first harvest is generally taken between years 4-5 after planting, with subsequent harvests taken every three years. Yields are approximately 10-12 tonnes per hectare.

Prices for willow wood are relatively high, although prices will differ depending on quality and end use. Tree plantations may be economically improved by utilising species with improved suitability for your land type, and inclusion of a variety of tree species, rather than a singular species, may increase potential end markets (See Short Rotation Crops). However, mature willow has high moisture content and lots of bark, which may make downstream processing difficult. Drying on site will increase final prices for timber and biomass, and can be achieved by letting cut billets rest for 1-2 years outside, or for 6-15 weeks in a solar kiln.

Alder

Common alder is a common timber species throughout Europe, able to adapt to a range of climates from Finland and Siberia to North Africa. It also can thrive on marginal lands, including lake shores, wet, sandy soils and rocky gravels, although it prefers moist, nutrient rich sites, and has a high tolerance to frost and salt spray. Alder is particularly sturdy in nutrient-poor soils compared to other species, due to its nitrogen fixing ability. This makes alder an important crop to consider regarding forestry establishment on reclamation sites where soils are low in nitrogen and organic matter. It frequently grows naturally within mixtures with ash, hazel, birch and oak, and is recommended for mixtures particularly for its use as a ‘nurse tree’ due to its ability to fix nitrogen in soils.

Alder may be planted at very high densities (10,000-100,000 stems per hectare for short rotation coppicing or at a woodland density of 2,500 stems per hectare) although they will compete at higher densities, leading to self-thinning and slow growth. 750-1,500 stems per hectare will substantially increase diameter growth rates during the first 10-15 years. The recommended planting density is approximately 4,000 per hectare and higher, (2m between rows, 1.25m within rows), to allow for thinning of poorer quality stems during development. Due to rigorous early growth, alder should not require vegetation control at establishment, and is usually planted at 2 years old, when approximately 50-80cm tall. Early alder development is rapid, and they often grow up to a metre per year for the first 15-20 years and tend to reach full development within 30-40 years, although they don’t tend to extend past 20m, with 40cm diameter. Continued thinning will favour higher quality trees and maintain diameter growth rates up to 20% higher than unmanaged stands.

Alder is mostly free from pest and disease problems, except for woodworm and Phytophthora alni disease – a specific alder species disease, that causes tarry deposits, poor foliage and death, generally spread from nurseries. The pathogen is also commonly carried by water, affecting riverside and streamside corridor Alder.

Alder produces a fine-grained timber, and is widely used for plywood, particularly as a veneer and may also be chipped for biomass. It also used for clog making and produces a top quality charcoal product. Alder and willow are both well-suited to water-logged soils, and are often found on land with poor drainage, creating ‘wet woodland’. New woodland creation is not allowed on peat and planting schemes are rejected by NRW, as peatland is inherently better at carbon sequestration when left in its natural state.

Poplar

Poplar is a highly popular species for tree farms within the US and Europe, as one of the fastest growing trees utilisable within the climate. Rapid growth enables high yields within a few short years, and trees can grow to 5m tall by 3 years old.

Quaking aspen, cottonwood, balsam poplar and lombardy poplar are popular poplar species, and may be bred to produce rapid growing hybrids. Hybrids have the benefit of improved disease resistance, high-yields and improved timber quality, while breeding against certain limitations of their parents; for example, lombardy poplar has a poor quality timber, despite particularly high growth rates, but some of its hybrids have retained the high yields, with high quality timber production.

Hybrid poplar species can grow at approximately 6 times the growth rate of similar species, resulting in an economic return within 10-12 years. They require little maintenance compared to similar biomass crops. Poplar is ideally planted at 10,000 -20,000 trees per hectares (approximately 2m between rows, and 1m between plants within rows with harvesting gaps of 3m). For higher densities, smaller cuttings of 20-25cm, with a minimum diameter of 10mm and to include a prime bud, are advised. Poplar may be grown on marginal soils, and is often grown alongside willow and alder in mixed plantations. Weed control will be required for the first few seasons, until the canopy is mature. The plantation are harvested every 5-7 years where they’re cut back to stumps to enable coppiced development with little additional planting costs.

Poplar is often used for pulp and paper industries, as a utility wood (for pallets, crates and upholstered furniture frames) and as a biomass fuel. It was a favoured timber to produce the matchstick and woodland owners planted small areas to supply timber to the match making companies such as Bryant & May. However, with the advent of the “lighter” matchstick production dwindled drastically and the poplar plantations lost their value and were left to mature and many can be seen today in areas of north east Wales.

Conifers

Over half of the UK forest land area is conifer woodlands (1.63 million hectares. Conifer wood is rapid-growing with high quality timber, and a wide range of potential uses, from building, to paper pulp, to bioenergy. Conifers are generally cold-tolerant, and wind-firm, commonly known as “evergreens” sue to their ability to withstand UK winters.

Scot’s pine, yew and juniper are all native conifer species, however most conifer forestry in the UK is introduced species, including Douglas fir, Sitka spruce, Corsican pine and larch.

Conifers are generally advised to be planted at a density 2,000 – 3,000 per hectare (approx. 2m x 2m or 1.5m x 2m apart), depending on species, although, as with all plantations, a mix of species is preferential.

Scot’s pine are long-lived trees, with a natural lifespan of 100-150 years and grow to approximately 36m. They are the only timber-producing conifer native to Scotland. Scot’s pine are planted approximately 1.4m apart, at a density of 2,500 – 3,000 per hectare. They thrive in poor soil and support a variety of wildlife, including insects, birds and multiple mammal species, which may browse on bark, foliage and seeds. The timber is strong, albeit not naturally durable, but takes preservatives well and is commonly used for building, furniture, chipboard, telegraph poles and paper pulp. In years gone by it was planted at intervals and specific locations, for example on the brow of hills to mark and show the way for the livestock drovers along drover roads. Some of these individual trees can still be seen in the landscape today.

Douglas fir, originating from Northern America, can grow up to 100m tall, and a height of 60m is possible in British forestry. It is adapted to a range of soils, however grows best on deep, moist and well-drained clay and silt loams, and may struggle on poorly drained soils. Under suitable conditions, as Douglas fir plantation can produce up to 10-12 tonnes per hectare a year and timber is usually used for sawmill timber, paper pulp, plywood, veneers, furniture and panelling.

Sitka spruce grow to 50-60m tall, and can flourish in upland, wet or acidic soils. It is the most common tree involved with forestry in the UK, accounting for approximately 50% of all commercial plantations. A plantation of 25-40 year old Sitka should provide 350-500 tonnes per hectare with prices up to £50 a tonne. Their high-density timber is generally used for paper (smaller trees), boats, pallets and packing boxes. They are susceptible to pests such as the green spruce aphid and spruce bark beetle, and other issues such as root rot. Maelor nurseries and Tilhill forestry have produced an ‘improved sitka spruce’ species, with 20-30% more volume at rotation and increased yield class up to YC30.

Corsican pine are more productive than Scot’s pine, with faster growth and straighter trunks, however are susceptible to red band needle blight, and are a less valuable resource to wildlife. They grow best on acidic, freely draining sandy loams, including sand dunes, and in warmer climates. They tolerate heat and drought well, but are susceptible to winter frost damage, and thus suited particularly to drier lowland areas of Britain. Corsican pine is also a light demanding pioneer species, it may not be so suited to continuous cover forestry management and requires more open conditions, particularly during early development.

A mix of conifer trees, potentially mixed with broadleaf species, is ideal, for improved resistance against pests and diseases including species specific blight, and in regards to proving a natural habitat for a richer and more diverse ecosystem.

Eucalyptus

The use of eucalyptus may be seen as controversial, as they’re not a native species to the UK, and therefore may have invasion risks. However, as a fast-growing and high-quality hardwood, with significant annual yields, high pest resistance and adapted to virtually all climatic conditions, eucalyptus is a species of interest to many involved with silviculture.

From over 700 species of Eucalyptus, several have been identified as suitable for UK climates, generally sourced from Australian regions of more temperate climates, including colder winters, such as the mountains in Tasmania and parts of the great dividing range in New South Wales and Victoria. Eucalyptus denticulata, E. nitens, E. glaucescens, E. gunii and E. globulus are popular species for UK cultivation and E. glaucescens has been successfully established throughout Wales, the Midlands and Scotland, due to its adaptability to site conditions, cold tolerance and unpalatability to grazing deer.

Eucalyptus plantations of species particularly suited to the British climate, such as E. gunnii can produce 16-22 tonnes of dry matter per hectare, each year, making eucalyptus one of the most high yielding trees currently used in forestry in the UK.

Eucalyptus has multiple uses, not limited to timber from the wood, which may be used for wood products and bioenergy, but also the leaves provide an antiseptic oil in addition to many traditional uses utilised by indigenous populations.

Fruit orchards

Fruit producing orchards will also increase the overall benefit across the farm, particularly when planted in silvo-pastoral sites, within livestock fields. Fruit trees also confer the benefits from carbon emissions and by improving biodiversity in the surrounding habitats, although are not suitable for planting in high-yielding larger forestry situations, such as conifer or broadleaf forests. The unharvested fruit produced can feed a range of species, supporting a range of local fauna, including as complimentary fodder for livestock, with the harvested fruit proving a yearly income source.

While an extensive fruit plantation may be able to provide another commercial crop on a major scale, even a small orchard will help improve self-sufficiency on the farm and reduce the carbon footprint of the domestic grower. Planting an orchard within an Agroforestry regime can be part of a multi crop regime with the trees complementing additional crop production from the soil.

Conifer and broadleaf forestry

When selecting species for cultivation, suggestions should be co-ordinated to the specific site and climatic conditions, as different species will benefit most from certain environments and soils. Consideration should also be taken with regards to the purpose of the scheme, and desired outcome of product. For timber, conifers and hardwoods should make up the majority of the plantation, while for bioenergy, only the highest yielding, rapid-growing species should be considered. If planting for carbon capture, or for fresh habitat, a range of species would be ideal, with contemplation about providing a native woodland using UK native species only. Local knowledge and Ecological Site Classification are the best methods for assessing which species a best suited to specific sites.

Ideally, a mixture of conifer and broadleaf tree species would be planted on a site, to provide maximum biodiversity, and a range of uses and resilience against pests, diseases and the duplicity of markets. ‘Nurse species’, such as nitrogen-fixing alder, are ideal companion plants in forestry plantations, and are able to support and benefit other surrounding tree species.

• ​New legislation targets require petrol to be blended with 9.75% bioethanol by 2020, requiring increased bioenergy crop production while not impacting food crop production
• Miscanthus can thrive on marginal land and low quality soils, reducing pressure over land use for crops and conflicts over food versus fuel production.
• An estimated range of net profits from £183-£211/ha per annum (minus haulage) can be predicted, when taking into account planting and harvesting costs

What benefits can miscanthus bring me?

Miscanthus is a hardy perennial grass crop originating from South East Asia, grown horticulturally and en masse for bioenergy production.

Crops cultivated for bioenergy must be high energy, with large, fast-growing biomass. Miscanthus species are perennial grasses with potential for very high rates of growth, and some species, such as the sterile hybrid M. x giganteus, can reach up to 4m each year, with aboveground dry matter biomass yields up to 15-25 t ha-1 across Europe. This offers a higher biomass yield than other bioenergy crops, such as Short Rotation Coppice (SRC) e.g. willow or poplar, and cereal straw, including barley, wheat, oats and rape.

Miscanthus is ideal for marginal land use, where soil quality may be lower or land steep. It can flourish on virtually any soil types, and thus offers the opportunity to utilise unprofitable fields. Currently Miscanthus has few known pests or diseases, leading to a highly resilient crop in the field, with little requirement for pesticide or fungicide treatments. With minimal input required post planting, Miscanthus is an ideal crop for the busy farmer.

Due to the clonal nature of most commercial miscanthus species through asexual rhizome propagation, the crop is fairly uniform, leading to improved harvests and crop maintenance. The grass is an ideal crop for buffer zones, promoting soil microbial activities and efficiently removing NO3-N and nitrate from groundwater and soil through the rhizosphere surrounding the rhizome and fine roots.

The habitat provided by the miscanthus crop can provide shelter for small mammals, and birds throughout the season. As a crop that doesn’t generally get harvested until post-senescence, the crop can provide shelter over the usual harvesting periods and through winter. The Welsh Government aim to improve biodiversity through the Public Goods Scheme, and suggest creating new habitats across the country is a priority; planting more crops with varying harvest times is likely to help improve habitats for small fauna year round.

A low mineral content is desirable for biomass intended for thermal conversion, and therefore minerals are re-mobilised into rhizomes over the winter, enabling nutrient sequestration for the following growing season. Furthermore, a late harvest should lead to reduced contractor rates, as prices are likely to be lower outside of the typical harvesting season. Miscanthus can also be planted late in the year, with an ideal planting window until the end of May, again avoiding unnecessary conflict with planting times of other crops.

Currently miscanthus is predominantly used for co-firing in coal furnaces, as a high energy and highly lignocellulosic species, for which the crop may be baled similarly to straw, or processed into pellets. There are also many alternative markets, including but not limited to, domestic fuel alternatives, biocomposites and animal bedding. Welsh water park, Blue Lagoon, is also heated through Miscanthus and woodchip biomass from a local energy centre.

Miscanthus may also be converted into ethanol through a variety of pre-treatment options, such as chemical (e.g. NaOH), physical (e.g. hammer milling) or biological (e.g. enzymatic hydrolysis), before fermentation with Saccharomyces cerevisiae (yeast).

For animal bedding, the grass is finely chopped and spread under wheat straw, and offers benefits including improved absorbency, grip, and on a minor scale, darkling beetle numbers were reduced in miscanthus replicates compared to wheat straw replicates. Producing animal bedding in-house offers the farmer an opportunity to reduce the need to import excess products.

The importance of miscanthus as a bioenergy crop

Globally, energy demands are increasing. As strain is being placed on limited energy supplies, pressure is being pushed on politicians and consumers to consider more sustainable alternatives. As yet, no clear single source has been identified that could wholly replace current carbon energy sources, however novel technologies are being designed across physics, chemical and bioenergy sectors to reduce pressure on current limited fuel supplies.

Bioenergy crops offer a carbon neutral solution to this ever-growing problem, where the carbon sequestered by the plant during its lifespan may be utilised as an alternative carbon fuel after harvest. Bioenergy crops are being utilised around the globe for biofuel, such as bioethanol and biodiesel, bio-products, including bio-plastics and biopolymers, and as an alternative for coal in coal burning factories.

The EU Renewable Energy Directive includes a statutory target that 10% of transport fuel should be sourced from renewable sources, such as electricity, hydrogen or biofuels. Fuels of 10% renewable sources (E10 fuels) are used across mainland Europe, particularly Germany, France and Finland, although as yet are not widely available across the UK.

In 2017, European industry ePURE estimated that the UK had the third largest renewable ethanol production capacity in Europe, with an installed production capacity of 985 million litres. Defra estimated 132,000 hectares of agricultural land (>2% of all arable land) were cultivated with bioenergy crops (53% of this for the UK road transport market). This suggests that the UK should have little issue with engaging with directives to increase biofuel production and consumption on a national level. By cultivating higher yielding crop species, biomass production is likely to increase while not having significant increases in land use.

The UK’s Renewable Transport Fuel Obligation (RTFO) guidance for fuel suppliers requires suppliers to produce fuel blended with renewable ethanol biofuel sources. Petrol in the UK is currently blended with 4.75% renewable fuel, (0.5% of transport fuel from sustainably produced bioethanol), with biofuel percentage targets of 9.75% by 2020. Such figures suggest that the demand for bioenergy crops is likely to increase over the next decade, potentially leading to greater incentives developed and improved profit margins.

Incentives

The price of imported wood chip is likely to rise as a result of leaving the EU in 2019. With the export tariff for goods from the EU between 2-4%, in addition to rising biofuel costs, and the expected increase in complexity of supply chains outside the single market, the costs of importing goods including bio-products are predicted to rise further. The UK is currently expected to retain ambitious environmental targets set by the EU regarding 2020 and 2030 renewable energy targets, which will require a solution. By cultivating more of our own renewable crops on unused arable land, the UK may still be able to meet targets in a cost-effective manner. It is hoped that the UK government will offer greater incentives for planting renewable crops over the next few years.

Whereas the UK has limited land available for long-term forestry crops compared to much of Europe, fast-growing bioenergy crops such as miscanthus offer an alternative biofuel source that may be able to help alleviate reliance on imported fuel.

In the long-term, the UK government have claimed to support UK businesses in development of new markets in the “bio-economy” and wish to play “a leading role in providing the technologies, innovations, goods and services of this future”. £162m is to be invested in innovation for low carbon industry and the bio-economy and there are plans to replace the Common Agricultural Policy with increased incentives for investment in sustainable agriculture. The government has also announced a ‘25 Year Environment Plan’ from 2018, that has been largely welcomed by Farming Unions and will provide incentives to farmers to deliver a range of public goods. This includes new approaches to incentivise more landowners and farmers to plant trees for agroforestry and bio-energy, and hopefully will extend to other high-throughput bioenergy crops in the future.

The Welsh Government have announced a Public Goods Scheme, following policy changes after Brexit, to provide a “new, meaningful income stream for farmers able to supply those environmental services not supported by the market”, and suggest for some producers, public goods payments will provide a large proportion of their future income. This has garnered interest from many bioenergy technologies, including Confor, a promotor for sustainable forestry and woodland practice. A press release on the 4th June, 2019, following responses from several interested sectors, proposed annual payments to farmers in return for environmental outcomes, including hitting carbon targets. The Welsh Government further convey an involvement in the development of renewable energy from biomass during the transition to a low carbon economy, with plans to ensure Wales’ communities have access to advice, expertise and funding to harness proven renewable technologies.

Estimated annual income (minus grants)

As a long-lived plant, sustainable over 15-20 years of annual harvests, miscanthus may bring in an annual profit without yearly establishment costs. Initial costs of miscanthus establishment has decreased over recent years, and is estimated at £1500–£1700 a hectare in the UK, depending on desired density, with costs expected to decrease further as technologies and cultivars are developed. Harvesting costs are relatively cheap at an estimated £170 ha-1, assuming 14 t/ha-1 harvests (this price will be further reduced if the equipment is already on site). If a conservative lifetime estimate of 15 years is used, the estimated cost per year, to include establishment costs divided across the expected lifetime and yearly harvesting is only £280. Revenues are estimated using current costs of harvested miscanthus for fuel at £31-£40/tonne, leading to an estimated income of £183-£211 per hectare, minus haulage costs.

Overall, including establishment and crop care a net margin of £900 per ha may be expected. The first full yield may be as late as the third harvest, and profits achieved over the first few harvests are likely to improve further.

Summary

Overall, miscanthus appears to offer a sustainable form of renewable energy for multiple industries, which are only likely to increase in demand over the coming decades. With a substantial lifetime, over 15 years, the crop is likely to become a key player in the renewable energy market and it should be expected that as the market demand increases, the potential value of the crop, particularly for early starters, should increase proportionally.

With the government planning to hit ambitious renewable energy targets over the next couple of decades, the demand for sustainable bioenergy sources that conflict little with food production will skyrocket. As a crop that will thrive on underused, marginal land with poor quality soils, miscanthus is one solution to an almost impossible problem.