Abstract
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) below ground. 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 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. 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 below ground biomass. Growth environment significantly impacted allocation; the willow genotypes grown in west Wales had up to 94% more biomass below ground by the end of the second rotation. A single investigation into fine roots 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, 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.
Highlights
Willows (Salix spp.) grown as short rotation coppice (SRC), are being developed as sources of biomass for the production of bioenergy, biofuels and high value products for the chemical industries especially across Europe, north-eastern and midwestern USA and Canada [1e4]
Estimates of below ground biomass production and the allocation of total plant biomass to above and below ground pools in willow are limited as roots are notoriously difficult and time consuming to sample
We found significant differences in biomass allocation between the four willow genotypes included in this study, Table 3 e Linear mixed effects model for root length density (RLD) for the two willow varieties, Endurance and Resolution, testing for the effects of genotype, site and depth and any interactions therein
Summary
Willows (Salix spp.) grown as short rotation coppice (SRC), are being developed as sources of biomass for the production of bioenergy, biofuels and high value products for the chemical industries especially across Europe, north-eastern and midwestern USA and Canada [1e4]. Annual net primary productivity of root biomass increased from 1 to 3 years, but the actual allocation below ground declined each year from 25e30% to 10e12% of total biomass in 1- and 3-year old plants, respectively. The age of the individual S. viminalis stools varied from 4 to 22 years old and the shoots were of 1e3 years in age dependent on the date of the last coppice From these limited data it was estimated that investment below ground increased from 10 to 25 odt haÀ1 over 25 years, and in contrast to Rytter [15] the allocation of total plant biomass to below ground increased marginally. The allocation of total plant biomass to the roots showed no difference with age
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