Changes in trait and phylogenetic diversity of leaves and absorptive roots from tropical to boreal forests

Abstract

The stress-dominance hypothesis (SDH) predicts the dominance of environmental filtering under harsh conditions and the dominance of competition in favourable habitats. Here, we aimed to assess the generality of the SDH using both leaf and absorptive root traits and phylogenetic diversity at a large scale. We examined the changes in the trait and phylogenetic diversity of six leaf and absorptive root traits along a soil fertility gradient from tropical to boreal forests. Trait and phylogenetic convergence and divergence were tested by the null model approach. Leaf economic traits (i.e. specific leaf area and leaf nitrogen concentration) and root tissue density (RTD) exhibited a coordinated response to soil fertility gradient, shifting from convergence toward species with conservative leaf traits and low RTD under infertile condition to high trait divergence under fertile soil. Similarly, community phylogenetic structure varied from convergence to divergence along the soil fertility gradient. However, variation in other traits was not consistent with the SDH prediction. The SDH depends on the trait’s ecological role, and RTD is the most consistent root trait with leaf economic traits that reflect community assembly along soil fertility gradient. These results offer a new perspective for understanding complex integration of above- and belowground assembly processes, and emphasize the importance of incorporating belowground traits and phylogenetic information into community ecology.