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).
Highlights
In many parts of the world, poplar (Populus spp.) plantations are established to meet the growing demand for bioenergy and wood production and to increase environmental protection [1]
Strong and highly significant relationships (p
This study provides evidence that site fertility or quality influences the trajectory, or shape, of the allometric relationship between diameter at breast height (DBH) and coarse root biomass of mature hybrid
Summary
In many parts of the world, poplar (Populus spp.) plantations are established to meet the growing demand for bioenergy and wood production and to increase environmental protection [1]. Planted poplars cover approximately 8.6 million ha worldwide, making poplar plantations and agroforestry systems a very important land use at the global scale [1]. Land area under poplar cultivation is expected to increase in the future because many countries have vast zones of marginal agricultural land that could be converted to bioenergy plantations to satisfy national energy demands [2, 3]. It was estimated that 8 % of the energy demand of Romania could be met with poplar biomass produced on the 1 million ha of recently abandoned arable land [4]
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