Plant Crops Bioenergy Research UK
Willow (Salix spp.)
The genus Salix includes approximately 300 to 450 species of willows and is a member of the Salicaceae family which also includes the genus Populus. Willows originate from wide areas of the Northern Hemisphere where they are primarily found inhabiting cooler moist zones. The species that comprise this genus are very heterogeneous exhibiting considerable variation in size, growth form and crown architecture. Most are compact shrubs with numerous thick branches and grow to a height of between 0.3 and 4 m, while others are true trees reaching in-excess of 40 m in height. They are deciduous and dioecious. The male and female flowers (catkins) appear early in the spring, either just before or at the same time as the first developing leaves. Sexual reproduction leads to frequent natural hybridization, creating great variation between clones and a huge relatively under-utilized gene pool. The haploid chromosome number is n = 19 (as for Populus spp.), but many species are tetraploid, and higher ploidy levels are common Barker et al, (1999). Vegetative propagation from dormant shoots allows selected hybrids to be immortalized as clonal planting stock material. Willows have been chosen as a dedicated bioenergy crop because of rapid growth and wood production in young shoots, producing high yields in a few years, ease of vegetative propagation, a broad gene pool and excellent ability to re-sprout after multiple harvests. First year growth after coppice can reach up to 4 m. Members of the subgenus Caprisalix (Vetrix) comprise over 125 species and consisting of middle-sized shrubs and are those primarily used for bioenergy production in short rotation coppice systems.
Management characteristics
Willows grown as a SRC plantation constitute densely planted high-yielding varieties harvested on a 2-5 year cycle (commonly 3 year) following coppice of the first establishment year's growth. This forms a multi-stem plantation, with each stool producing up to 20 shoots. The productivity of the stool that remains after coppice determines the life-span of the crop, but plantations are considered viable for at least 30 years. Willows can grow on a wide range of soil types with a pH optimum of 6.5 and a pH range of 5.5 to 7.0. Water consumption of this crop is high and can reach 4.8 mm m-2 in mid-summer (El Bassam, 1998) an annual rainfall of 600 - 1000 mm is considered ideal for willow growth (DEFRA, 2002).
In the UK cuttings should be taken when buds are fully dormant (between December and March). Freshly cut material should be planted between February and March if soil conditions allow. If not immediately planted cuttings should be stored in sealed plastic bags (to avoid desiccation) and kept between -2 C to -4 C. Following an overnight submersion in water freshly cut material, or cold-stored cuttings, should be planted in early spring (February - March), although cold-stored material has been successfully planted as late as June.
Planting of willow can be fully mechanized, unlike planting of poplar. One approach a Salix Maskiner 'Step Planter' cuts 2 to 3 m long willow rods to 20 cm lengths immediately before planting these cuttings (Tubby & Armstrong, 2002).
For the best economic return a planting density of 15, 625 stools ha-1 has been suggested from a study with both Salix viminalis and S. dasyclados (Bullard et al. 2002). The recommended planting design for SRC willow is planting in double rows with row spacing of 0.75 m and 1.5 m between double rows. Within each row the plants should be spaced 0.59m apart (Tubby & Armstrong, 2002), giving a planting density of 15, 000 cuttings ha-1.
During the establishment year willows are intolerant to weed competition. A broad spectrum contact herbicide should be used to remove all perennial weeds prior to cultivation, and residual soil-acting herbicides can be applied post planting (Tubby and Armstrong, 2002). Following cut-back at the end of the establishment year the vigorous dense re-growth of up to 4 m per year is sufficient to suppress weed competition. In relation to biomass production willow has a low demand for nitrogen and no fertilization should be applied during the establishment year.
As for poplar Melampsora rust is the most important fungal pathogen in the UK and Chrysomelids (willow beetles) are the most important insect pest of willow SRC, both having a detrimental effect on yield, (DEFRA, 2002).
Harvest and yield
The dry matter content of winter harvested material is generally in the range 40-55 % at harvest but end-users may generally want 70 % dry matter. As for poplar, willow SRC can be harvested by direct insitu cut and chip, whole shots or billets may also be harvested (DEFRA, 2002).
Average UK maximum commercial yields can be above 10 ODT ha-1 y-1 depending on planting density and genotype (Wilkinson et al., 2007). However experimental yields have reached more than 18 ODT ha-1 y-1 using new genotypes, some of which are now commercially available (DEFRA, 2002).
Broad chemical composition and energetics
The broad chemical characteristics of SRC willow on a percentage basis are: Cellulose 40 %, Hemicellulose 30 % and lignin 30 %. SRC willow has a net calorific value of around 18.5 GJ t-1 dry weight.
Production
Between 2001 and 2006 the cumulative planting of SRC had reached 1180 ha. A further 1420 ha of plantation have been approved for 2007 making a cumulative total of 2600 hectares producing SRC under the energy crops scheme (DTI, 2006). Including the planting applications obtained under a number of different grant schemes across the UK, the total area of SRC planted under all grant schemes may be considered approximately 6000 hectares. The vast majority of SRC plantation under such schemes may be assume to comprise of willow.
References
Bassam N. El. 1998. Energy plant species. Their use and impact on environment and development. James and James (Science Publishers) Ltd. London, UK
Bullard MJ, Mustill SJ, McMillan SD, Nixon PMI, Carver P and Britt CP.(2002). Yield improvements through modication of planting density and harvest frequency in short rotation coppice Salix spp.-1.Yield response in two morphologically diverse varieties. Biomass and Bioenergy, 22, 15-25.
Wilkinson JM, Evans EJ, Bilsborrow PE, Wright C, Hewison WO and Pilbeam DJ. (2007). Yield of willow cultivars at different planting densities in a commercial short rotation coppice in the north of England. Biomass and Bioenergy, 31, 469-474.
