A review of Willow (Salix spp.) as an integrated biorefinery feedstock

Throughout history, the genus Salix (willow) has been an incredibly useful temperate plant for humans, with widespread global distribution and species indigenous to all continents except Antarctica. Estimations of the number of species range from 450 to 520 worldwide, and there are still more natural hybrids and multi-hybrid combinations. Several biomass willow breeding programmes have been established across the globe. All of these attempt to produce fast-growing, high-yielding stems with a straight habit and minimal side branching that are highly adaptable to different sites and are also disease and pest resistant. Short rotation coppice (SRC) cultivation involves growing willow at close spacings with a stocking rate of around 15,000 per hectare with harvests every 2–4 years. The crop is mechanically harvested, typically using a forager, and material has recently been used for bioenergy applications. Trial plots have achieved yields of up to 20 odt/ha/yr, whilst well-tended commercial crops have yielded up to 14 odt/ha/yr. Global willow breeding programmes have produced a wide variety of commercial genotypes that have suitable properties for easy planting and harvesting and have the added benefit of elevated levels of bioactive compounds, including salicin, present in the bark, which can be used in medical and veterinary applications. These high-yielding willow varieties grow well in the wetter regions of the globe, including NW Europe, and afford multiple harvests before re-planting. Salix's versatility and adaptability and the SRC cultivation process make them an ideal candidate feedstock for use in an integrated biorefinery to produce a range of biobased materials, including pharmaceuticals, and biocomposites, fuels, energy and fertiliser.