Willows (Salix spp.) grown as short rotation coppice (SRC) are viewed as a sustainable source of biomass with a positive greenhouse gas (GHG) balance due to their potential to fix and accumulate carbon (C) into below ground biomass and soil. However, exploiting this potential has been limited by the paucity of data available on below ground biomass allocation and the extent to which it varies between genotypes. Furthermore, it is likely that allocation can be altered considerably by environment. To investigate the role of genotype and environment on allocation, four high yielding willow genotypes were grown at two replicated field sites in southeast England and west Wales, UK. Above and below ground biomass was intensively measured over two two-year rotations (2010-2014). Significant genotypic differences in biomass allocation were identified, with below ground allocation differing by up to 10% between genotypes. Importantly, the genotype with the highest below ground biomass also had the highest above ground yield. Furthermore, leaf area was found to be a good predictor of root biomass which offers the opportunity of an easier and non-destructive method for estimating root biomass Growth environment was found to significantly impact allocation; the willow genotypes grown at the west Wales site had up to 94% more biomass below ground by the end of the second rotation, A single investigation into fine roots (July 2013) showed the same pattern with double the volume of fine roots present. This greater below ground allocation may be attributed primarily to higher wind speeds experienced at this site, plus differences in humidity and soil characteristics. These results demonstrate that the capacity exists to breed plants with both high yields and high potential for C accumulation
