Robinia pseudoacacia

General info

Black Locust is a hardwood deciduous medium size tree with leguminous properties associated with root nodules formations beneficial to strong bacterium interactions. It is the second most planted hardwood tree globally after eucalyptus spp, accounting for significant proportions of both Hungary’s and the Mediterranean region’s tree species. It is native to a small region of the South East of the United states but has been naturalised across temperate regions including Europe, and Asia and in several instances has been considered an invasive species.

Black locust is often considered weedlike as it can spread through underground suckers and shoots and, as a pioneer species, is often first to establish in areas of damage or canopy openings in environments. It can outcross or undergo selfing to provide adaptability for a successful pioneer species alongside asexual sucker strategies. When it undergoes asexual reproduction, mother and daughter plants are integrated sharing resources through a common root system. It is commonly utilised in plantations for short rotation coppicing alongside reclamation of polluted soils such as surrounding mines. They produce large white flowers attractive to honeybees.

The species has undergone a significant amount of cultivation enhancement through breeding, largely for wood timber property enhancement, though Kiscsalai and Ulloi varieties have focused on energy plantation benefits and other co-benefits such as frost resistance. Its wood is well known for being heavily resistant to fungal decay.

Cultivation and agronomy

Generally, a good pioneer species which is shade intolerant and requires a lot of sun. Black locust thrives in moist loamy well drained soils but can tolerate dry nutrient poor soils also. It can tolerate a range of pH’s across European plantations (3.2–8.8) with an optimum range of 5.5–7.0. Optimum conditions ensure the productivity of root-associated nitrogen fixing bacteria, which can help to fix between 75–150 kg of N ha-1 yr-1. It can tolerate a range of altitudes, though it grows best between 600–700 m above sea level.

Whilst it does have some pest and pathogens of note, it is largely considered to be a heavily resistant species with its most damaging native pathogen (the locust borer Megacyllene robiniae) having not made its way to Europe. It can achieve up to 14 Mg ha-1 yr-1 according to certain studies with variability in output depending on rotational strategies employed and successive years of production and maturity.

Utilised normally on a 1–3-year rotation period, it can grow to heights of 10m within 5 years with coppice growth from stool shoots having been demonstrated maximums of 4.9 m a year in certain studies. The volume growth from coppice stands has been shown to range between 10 – 16 m3 ha-1 y-1 as early as 5–10 years into cycles. However, over time, if regeneration of stands is performed from stool shoots, a decreasing trend of wood production is observed, necessitating the need for regenerating plantations via seed or root sucker alternatives periodically. This leads to many strategies currently employing 2–3 coppice cycles, or at most 5 cycles in some studies, before stools are removed and replanted from other sources.

Alnus glutinosa
Other short rotation forestry species include ‘Italian’, ‘Green’ & ‘Red’ alders

General info

Native to the UK, common alder is a water loving tree that grows well in swampy areas or close to rivers, lakes and ponds, with root structures that provide support toward prevention of soil erosion in these areas. The wood produced from this species isn’t as susceptible to rotting when waterlogged and has historically played roles in underwater construction and boat building. Alder forms associations with bacterium Frankia alni, allowing it to thrive in lower nutrient soils, and within the genus Alnus there are 30 species. Common alder is a monoecious species with both male and female catkins found on the same tree. They undergo wind-pollination and seeds are distributed by wind and water on maturation.

A notable current risk for this species is the discovery of a hybrid strain of Phytophthora, which can cause alder dieback, when previously it was assumed that alder was not impacted by these fungi significantly. Alder is also suggested to have medicinal properties within the bark and seeds with possible interesting antioxidant and antimicrobial activities. It may be suitable for both short rotation forestry due to its relatively significant growth levels in the first 10-15 years and is amenable to coppicing regrowth with the coppice quality being most suitable for bioenergy biomass utilisation.

Cultivation and agronomy

Common alder grows well in all soil types but requires significant moisture levels and does not grow well at higher elevations. The leaves lack mechanisms for controlling transpiration and this in part is a large factor relating to their water-demanding nature. They are tolerant of a range of temperatures, growing well across both northern and southern Europe.

They can reproduce via seed which following maturation (age 3–30 depending on ecotype) they produce every 3–4 years but new seed germination requires light levels that are generally unachievable under a mature stands canopy. The seeds of the alder species can float on water and remain viable in this situation for around a year. Successful germination is reliant on higher humidities.

Other alder species have different tolerances with ‘Italian’ and ‘Green’ alders needing less water whilst ‘Red’ alder does not do well on dry or sandy sites. They have yields averaging 4–14 m3 ha-1 y-1 with a normal rotation length of 35-50 years across species when not harvested on a SRF system. They grow at a rate of up to 1.5 m a year between peak ages of 4 to 10 years old before slowing and max heights fall around 28.5 m tall (between ages 50-80).

Where yield is concerned the annual volume increment reaches maximum around the 20-year age mark achieving 13–18 m3 ha-1 y-1. Alder is also noted as having potential as a coppicing species particularly in mixed systems where alder can offer the co-benefits associated with its nitrogen fixation abilities to boost co-localised species growth patterns.

Eucalyptus nitens, Eucalyptus gunnii, Eucalyptus urnigera, Eucalyptus dalrympleana, Eucalyptus glaucescens, Eucalyptus rodwayi, Eucalyptus globulus

General info

Eucalyptus are a flowering tree species known for their strong aromas and rich oils which make them highly flammable. They can grow as shorter shrubs or as taller evergreen trees and sometimes release sticky resins through breaks in the bark, hence being known as the “gum tree”. Mostly native to Australia, these fast-growing trees are present more in the southeast and lowland areas of England in the UK with two of the most common species being Eucalyptus gunnii and Eucalyptus globulus.

Flowers of the species have several stamens with a fluffy appearance which are enclosed in a cap (operculum) made of fused sepals and petals. The woody fruit capsule that is formed on fertilisation is called a gumnut and releases seeds into the environment. Flowers produce an abundance of nectar making them attractive to many pollinator species.

Cultivation and agronomy

Eucalyptus spp. are highly productive, demonstrating yields up to 30 m³ ha^-1 y^-1 in some small plot trials for E. nitens, with predictions from UK experiments across species ranging from 16 m³ ha^-1 y^-1 to 31.7 m³ ha^-1 y^-1 in a mixed Eucalyptus planted study. Some species are more or less cold tolerant than others with E. nitens being among the less tolerant and undergoing damage and death from cold winters in the UK. On the other hand E. urnigera and E. glaucescens are two of the more cold tolerant species under consideration.

One notable aspect of Eucalyptus species in the UK is that E. nitens appears to lack, or have very limited, coppicing ability so despite being one of the highest relative yield producing SRF species (under best conditions) it is recommended that E. glaucescens, E. gunnii and E. rodwayi be utilised as they can coppice.

As Eucalyptus species are not widespread in the UK, they are a species of SRF which have very low risk of damage from pests and diseases in the UK. The colder tolerant species are also not known to be particularly susceptible to phytophthora spp. or foliar pathogens. However, Irish plantations of Eucalyptus have been impacted by psyllid pest Ctenarytaina eucalypti and leaf beetle Paropsisterna selmani which could require consideration.

Paulowniaceae spp

General info

A hardwood tree species encompassing 17 species native to China and East Asia. Noted as one of the fastest growing trees in the world, it is unsurprising that its potential as a bioenergy crop is of interest.

Wild type species tend to utilise the C4 photosynthetic enzyme cycle being a key element towards their ability to gain biomass quickly, however, many more modern hybrids employed utilise the C3 enzyme cycle. It produces a lightweight but strong timber which has been heavily used by Japanese craftsmen for decades for the construction of furniture and instruments. Paulownia leaves and flowers are also known to be good sources of fat and sugar for fodder, and nitrogen for green manure usage, suggesting potential roles in agroforestry incorporation strategies.

Cultivation and agronomy

Species require permeable soil and don’t tolerate strongly acidic soils (pH 5 – 8.9) being optimum and tend to grow best in temperatures between 15 – 16 °C though they have a range of tolerance between -25 and 47 °C depending on the species. In a growth review of this species in Europe, different species and hybrids demonstrated highly variable yields and wood qualities.

Yields in the second year of cultivation studies ranged from as low as 1.5 oven dried tonnes ha^-1 y^-1 to as much as 14 oven dried tonnes ha^-1 y^-1. Despite showing approximately equivalent energy profiles to both willow and poplar (~19 Mj kg^-1), there is debate over the quality of fuel pellets produced due to low density of woods and potential requirements for processes, such as biomass torrefaction to produce better fuel quality results.

Paulowniaceae spp. are at risk during juvenile growth, due to the very large leaf size and lack of sufficiently woody stems early on. Plantations of these tree species are susceptible to being destroyed by high winds causing mechanical damage if windbreaks and protection are not put in place. Paulowniaceae spp. have shown comparable short rotation coppice results to willow in certain studies but could also have potential as a short rotation forestry consideration crop.

It has been noted that over just three growing seasons P. elongata varieties could achieve 92 kg/tree and in plantation strategies using 2000 trees per hectare, 150 – 300 tonnes y­^-1 was produced 5 – 7 years after planting.

Populus spp
P. nigra, P. deltoides, P. maximowiczii and P. trichocarpa, P. tremula

General info

Poplar is a fast-growing deciduous flowering dioecious tree from the northern hemisphere with around 30 species present in the genus with many able to naturally hybridise. The species most under investigation for breeding of commercial varieties include P. nigra, P. deltoides, P. maximowiczii and P. trichocarpa.

Whilst poplar species are of interest as bioenergy crops due to their fast-growing status, work with commercial varieties in Europe is looking to: improve species resistance to common pests (including poplar mosaic virus); improve the growth rate further; and improve climate/soil condition adaptations of these species.

Cultivation and agronomy

Poplar tends to yield 4–14 m3 ha-1 y-1 and grows on a wide range of soils but some species are best avoided on peaty soils whilst others benefit from ex-agricultural fields for best growth rates. It can be harvested via modified forage harvesters and harvesting via traditional timber methods can be performed on 6–10-year cycles in an SRF system. They can be harvested on 3-year coppice rotation for use as a SRC methodology but is noted as performing better on a slightly longer 4–5-year rotation.

Outside SRF situations, these species have a normal rotation length of 40-55 years and as such can be considered as having a relatively short lifespan. They can grow around 1.5 to 3m a year depending on variety and locations. Many species can reproduce from dormant hardwood stem cuttings like willow, which can act as a beneficial propagation methodology. They can be planted at around 1,500 up to 12,000 trees per hectare for bioenergy purposes, with higher and lower density plantings having different considerations with regards to biomass produced vs costs and time before ROI is observed for example.

During the first few years, they need considerate weed control to avoid competition as well as pest control to avoid the impact of diseases such as leaf rusts and stem cankers including Septoria musiva and plant damage from cottonwood leaf beetles.

Salix spp.

General info

Willow is one of the most popular short rotation coppicing species for energy crop utilisation, it is a pioneer species that establishes well in new environments with rigorous juvenile growth. It comprises over 400 species worldwide with around 10% of these being deciduous tree species and most being multiple stemmed trees and shrubs.

Due to popularity and success, there are >50 willow cultivars registered with the Community Plant Variety Office (CPVO) with more the 25 commercially available in the UK. Commercial varieties provide benefits including increased rust resistances and increased yields compared to wild varieties due to targeted enhancements. Key willow species for bio-energy production in the UK tend to originate from northern temperate species as they can largely deal with the range of temperatures faced in the UK.

Cultivation and agronomy

Willow can be harvested on 2–5-year rotations at densities up to 15,000 plants per hectare depending on the productivity of the site and the environmental conditions. Planted willow can remain viable for 15–30 years and has a rapid growth rate achieving 6–8 metre heights between each rotation of cutting (depending on rotation year strategy employed).

Generally, willow is adaptable to a range of soils and conditions, however dry sandy soils may struggle to supply sufficient moisture for efficient growth, and organic peat heavy soils may cause too much competitive weed impacts to make the growth strategy efficient. Willow coppices need more water than any other conventional agricultural crop therefore areas of high rainfall are extremely beneficial.

Despite the temperate adaptability of most willow species for higher productivity rates it is recommended that plantation sites be below 100m above sea level. It has been shown that Willow harvesting yields equated to 5–14 oven dried tonnes ha-1 y-1 across several studies, with 1 hectare supply of willow woodchip providing energy roughly equivalent 4,500 litres of home heating oil (depending on moisture levels of willow biomass and productivity of a given hectare per year etc).

Commercially, willow propagation involves the planting of winter-dormant stem cuttings in the spring. Mechanical planting is performed whereby cut stem sections from willow whips are inserted into prepared soils. Following the first growing season cutbacks are performed to around 10cm above ground level to encourage the formation of multi stemmed stools for increased productivity benefits.