Yield of willow (Salix spp.) grown in short rotation coppice mixtures in a long‐term trial

Salix spp. genotypes were grown in random intimate mixtures comprising 5, 10, 15 and 20 components at 3 planting densities (10 000, 15 000 and 20 000 cuttings ha−1). Planted in 1994/95, plots were harvested every 3 years in 1998/99 (previously reported), 2001/02, 2004/05 and 2007/08. Each individual stool in both mono- and mixture plots was weighed. The total yield from mixture plots was consistently higher than the mean of the components in mono-plots and with only limited exceptions higher than the yield of any of the individual components grown in mono-plots. This occurred despite the loss of a number of disease-susceptible genotypes. When a stool died the remaining plants were able to colonise the vacant space and compensate for the loss. There was no clear benefit in increasing the number of components from 10 to 15 or 20 although host diversity is considered to be an important contributor to the effectiveness of a mixture both in disease reduction and yield enhancement. It is argued that the use of Salix spp. mixtures is highly advantageous, even in the absence of any disease pressure, and that mixtures increase the sustainability of a willow plantation.