Robinia pseudoacacia

General info

Black locust is a medium-sized hardwood deciduous nitrogen-fixing tree. The tree is native to a small region in southern United States, but it has been transplanted across the globe in temperate and sub-tropical regions. Black Locust is one of the most commonly planted commercial hardwood trees with an estimated 2.5 million ha in lumber and energy production globally. In Hungry, black locust account for over 20 percent of forested land.

Black locust is a fast-growing tree, often growing to a height of between 12 to 30 meters high depending on the cultivar. It has pinnately compound leaves of 6 to 14 inches long that are alternately arranged along short branches. Each leaf includes 7 to 19 leaflets that are 1 to 2 inches long and has an open canopy structure. The leaves are dark blue-green in summer and turns yellow-green in fall. Depending on the cultivar, some trees have thorns at the base of the leaves.

The tree produces large white or rose-pink flowers which appear in large dense cluster on the tree and produces a powerful fragrance. The flowers attract honeybees in late spring providing a nectar resource for pollinators. The honeybees in return pollinate the tree.

Black locust produces pea-like fruit pods of 5 to 10 cm long and 1-2 cm wide which usually contains about 4 to 8 seeds. The seeds mature in autumn and persist through winter until early spring. The trunk is usually black and grey and has many deeply furrowed ridges and grooves that forms diamond shapes. The roots have nitrogen fixing nodules, meaning the tree has limited fertiliser requirements.

The wood from black locust is one of the hardest woods worldwide. It is pale yellowish brown, dense, hard, durable, and resistant to rot.

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Invasiveness of Black Locust

Black locust is considered an invasive species. And is classified as one of the top 100 woody invaders worldwide. It can quickly colonise, out competing other species of trees and shading out competitors. The suckering roots of black locust spread quickly and form many suckers resprouting into new shoots. Black locust shed its seeds and spread to establish new seedlings. The seeds can remain dormant for close to 10 years.

Farmers should consider monitoring the spread of the black locust seeds and suckers outside the planted location to minimize seedling establishment. The spread of black locust can to a limited extent on small scale be controlled by hand pulling, mowing, constantly removing the suckers from both the roots of the trees and from the trunk to minimize regrowth. However, these methods may not completely eradicate the establishment of seeds.

Site Suitability

Black locust is 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 600–700m above sea level.

Establishment and management

Black locust can be propagated by seed, root, or stem cuttings. When planting by seed, boiling water or sulfuric acid treatment will be required due to the hard seed coat. Plant black locust between November and March.

Black locust is fast growing producing large amounts of biomass which can be easily coppiced. The tree produces high-density wood which can be harvested and produced into pellets with high energy content.

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 4.9m 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.

The yield of black locust varies according to location, harvest timing, and agronomic practices. A study in Hungry reported average yield of 6.5 Mg ha^-1 yr^-1 at dense spacing of 22,000 plants per hectare on a 5-year for cultivars such as Üllői, Nyírségi, Jászkiséri, Kiscsalai.

Pests and diseases

The most common pest is the black locust borer (Megacyllene robiniae). The larvae burrow into and weaken the tree’s trunk and branches. Although black locust can survive locust borer attack, the damage causes stunted growth and internal decay which makes the tree more susceptible to breaking apart in very windy conditions.

The locust leaf miner (Odontota dorsalis) and the black locust gall midge (Obolodiplosis robiniae) are another main pest which affects black locust. The locust leaf miner causes the leaves to turn brown in the summer. The black locust gall midge causes gall formation and premature leaf drop. Other insect pest includes the sawfly larvae, bag worms, leaf and tree hoppers, aphids, and carpenter moths.

Although black locust has many pests and insects, many of these diseases seldomly kill the tree. Fortunately, the main pests – black locust borer and locust leaf miner – is not yet present in the UK.

Insecticide treatment may be required for treating pest and disease infestation, however, maintaining good growing conditions, and preventing insect attack is essential especially during the first one to three years of growth. Interplanting with other species may reduce susceptibility to pest and disease attacks.

Benefits and uses of Black Locust

• Soil improvement: Black locust is able to fix nitrogen which is ultimately returned to the soil. This increases the presence of nitrate, ammonium in the soil.
• Reclaiming/rehabilitating degraded soils: When grown on marginal lands with low soil fertility it has the potential to improve soil fertility due to its ability to fix atmospheric nitrogen. Black locust has been used in mine reclamation in Germany and in reclaiming degraded lands in Romania.
• Prevention of soil erosion: Its extensive root system and prolific suckering can prevent soil erosion, and on previously eroded soils increase soil porosity, soil organic matter, nitrogen, and phosphorus.
• Source of food for pollinators: The flowers are important source of food for honeybees, leading to the production of high-quality honey.
• Improve biodiversity: Black locust is fast growing tree and host to many species of insects, mammals, and birds.
• Windbreaks and shelterbelts: Black locust is used as windbreaks, shelterbelts, shade and shelters for animals in silvopasture grazing systems.
• Hard wood: The wood from black locust is one of the hardest wood and is highly rot resistant, making it an ideal material for fenceposts, poles, furniture, wooden panelling, flooring and firewood.
• Raw material for other products: Black locust is used as raw material in biotherapy, apiculture, food industry, landscaping, and many other products.

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

Based on an 8-10 year rotation, data from early UK trials identified yields in the range of 20-30 m³ ha/yr, on a bark-free basis, equivalent to 10-15 oven dry tonnes per hectare per year. Yields of up to 48 m³ ha/yr have been reported from a commercial plantation in Cornwall. Eucalyptus produces a high-density wood, with a net calorific value of 18 GJ t, which seasons quickly and is suitable for use as a biomass fuel or timber product. 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 (Populus spp.) belong to the family Salicaeae and are native to the Northern hemisphere. They are fast-growing deciduous flowering dioecious trees 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.

Poplar is among the fastest growing of temperate trees and is therefore of considerable interest as a bioenergy crop. The plant grows to a height of 15–50m with a trunk diameter of 2.5m. Once established, poplar can be harvested every 2-5 years over a life span of over 20 years. Poplar is a multipurpose crop with several environmental benefits such as reducing erosion, phytoremediation of contaminated soils, windbreaks, and biodiversity.

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 is suited to most soil types including clay, sand, loam, and humus soils. It grows particularly well on well-drained and fertile loam with a wide range pH of 5-8. Poplar can be grown as a Short Rotation Forestry (SRF) or Short Rotation Coppice (SRC). The plant can grow up to about 1.5m in height during the first year and from 3-5m for each of the following years depending on the cultivar, site conditions, spacing and management practices.

For large scale plantations, it is recommended to plant mixtures of poplar varieties with about 100 trees of a single cultivar in each block, and different cultivars in neighbouring blocks. If considering mechanised harvesting, plant at a spacing of 0.6m apart in twin rows with 0.75m between rows and a 1.5m alley between each twin-row. Poplar can be planted in densities of 1,500 up to 1,800 trees per hectare for bioenergy purposes.

Yields of 20-25m³ of wood/ha/yr is achievable in the UK.

Young growth is attractive to deer, hare, and rabbits. Appropriate fencing may be required in areas where these animals are prevalent. Common pests include the small poplar leaf beetle (Phratora vitellinae), and the large leaf beetle (Chrysomela populi).

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.