Legacy effects control root elemental composition and stoichiometry in subtropical forests: Empirical support for the biogeochemical niche hypothesis

Abstract Under biogeochemical niche (BN) theory, plant allocation of elements to organs to maintain fundamental biological processes varies with species, leading to the formation of species‐specific BNs. However, empirical support for the BN theory is largely restricted to plant leaf elemental composition and stoichiometry, with a lack of clarity about the contribution of fine root element content. Here, we analysed fine root concentrations and stoichiometry of 9 elements including carbon (C), nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), aluminium (Al) and manganese (Mn) in 137 tree species of a subtropical forest to quantify BN and test the theoretical framework of the BN hypothesis. Our study showed phylogenetic signals of fine root elemental composition and stoichiometry. Fine root elemental composition of the 21 most abundant co‐existing species tends to be unique and primarily driven by root N content, as indicated by canonical discriminant analysis. Legacy effects (phylogeny and species) explained 23.3%–70.7% of the variation across the different variables used to characterize fine root elemental composition, stoichiometry and BNs, whereas combined effects of soil property, mycorrhizal association type and topography factors explained 3.9%–17.7%. Synthesis : These results indicate that phylogenetic and taxonomic distance (calculated as distance metrics) represent a proxy for species‐specific evolution and achievement of optimal function linked to bio‐element use. Thus, our study provides new empirical evidence in support of the BN hypothesis, based on plant fine root elemental composition and stoichiometry, and improves our mechanistic understanding of species coexistence dynamics.