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Plastic Allometry in Coarse Root Biomass of Mature Hybrid Poplar Plantations

Fortier, Julien; Truax, Benoit; Gagnon, Daniel; Lambert, France. 2015. BioEnergy Research

Abstract

In this study, we sampled coarse root biomass of three poplar clones in four 13-year-old hybrid poplar (Populus spp.) plantations, with the objective of developing an allometric relationship between diameter at breast height (DBH) and coarse root biomass. A second objective was to test significance of site, clone and ×site clone interaction effects on coarse root biomass, using analysis of covariance (ANCOVA), with DBH as a covariate. Across the four sites, the general allometric relationship between DBH and coarse root biomass was highly significant (R 2=0.78, p<0.001). However, given the high significance of the site effect (p<0.001) in the ANCOVA and the differences in data distribution between sites, two allometric relationships were developed based on the fertility class of sites (high and moderate). Environment-specific allometric relationships developed for two site fertility classes had a better fit (R 2=0.81-0.90, p< 0.001). ANCOVA, with DBH as a covariate and site fertility class as a main effect, also showed that both of these variables significantly affected (p< 0.001) coarse root biomass allocation. Environment-specific equations showed that higher coarse root biomass was allocated in harsher environments for a given DBH, probably to improve access to growth limiting soil nutrients or to build-up larger storage sites for amino acids and non-structural carbohydrates. Consequently, poplar coarse root biomass growth is both driven by ontogeny (size) and environment, reflecting the plasticity of the root system of mature poplars. Implications of using general vs. environment-specific equations in estimating stand-level root biomass and shoot to root ratios are discussed. Shoot to root ratios calculated using environment-specific equations were more strongly correlated to key environmental variables than ratios calculated using the general equation, with soil NO3 supply rate being the strongest predictor of the ratio (R 2=0.90, p <0.001).

Key Words

Plasticity, Belowground biomass equation, Site fertility, Large woody roots, Bioenergy, Carbon partitioning and allocation