Coordination among root exudation C, mycorrhizal colonization, and functional traits and their responses to drought in five temperate tree species

Root exudation and mycorrhizal colonization are two critical “outsourcing” strategies for tree nutrients exploitation. Quantifying the plasticity of these two processes under water stress and their coordination with root morphological and chemical traits is important for understanding of the tree adaptations to drought. Here, we carried out a short-term rainfall exclusion experiment in a common garden with five temperate tree species in northeast China, and quantified the co-variation of root exudation carbon (C), mycorrhizal colonization, and four morphological and four chemical traits in response to drought. Root exudation C was positively correlated with resource-acquisitive root morphological traits (specific root length and area), but negatively correlated with resource-conservative ones (root diameter and tissue density). Percentage of mycorrhizal colonization decreased as increasing root non-structural C concentration but was weakly correlated with the root morphological traits. Drought increased root diameter, tissue density, nitrogen concentration, exudation C, and mycorrhizal colonization, but decreased non-structural C concentration, and specific root length and area. However, tree species and mycorrhizal types had weak effects on the responses of fine root traits to drought. Overall, our findings suggest that drought promotes resource-conservative root traits for hydraulic safety but increases root resource acquisition by “outsourcing” processes, i.e., increased C investments in rhizosphere microbes (through root exudation C) and mycorrhizal collaboration.