Small-scale forestry for bioenergy consumption – Part III: Biomass production and incentives

  • 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.

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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.

Latest Technical Articles

Small-scale forestry for bioenergy consumption – Part II: Forestry management

  • 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.
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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:

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.

Latest Technical Articles

Small-scale forestry for bioenergy consumption – Part I: Important tree species for consideration

  • 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.

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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.

Tree Species Broadleaf/ Conifer Planting density per hectare Approximate yield per hectare (t/ha/year) Other information
Willow Broadleaf 15,000 10-12 Short rotation coppicing (SRC)
Alder Broadleaf 10-20,000 16 Nitrogen-fixing, SRC
Poplar Broadleaf 9,000 4-20 Yields inconsistent, SRC
Sitka Spruce Conifer 2,500 10-20
Douglas fir Conifer 2,000 10-12
Scot’s Pine Conifer 2.5 – 3,000 8-12* *Based on 0.98m3 per 1 tonne
Corsican Pine Conifer 2.5 – 3,000 79 *Based on 0.98m3 per 1 tonne
Eucalyptus Broadleaf 1 – 2,000 16-22

 

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.

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Miscanthus as an alternative crop for farmers

  • ​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
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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.

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