Fine root trait-function relationships affected by mycorrhizal type and climate

Root respiration is a critical function of root carbon cycling and accounts for a major component of the global carbon budget. However, the insufficient understanding of the global scaling relationships between root traits and respiration rate (R_r) impedes the accurate prediction of carbon effluxes from roots. Here, we analyzed a dataset of 138 species to unravel and quantify the effects of mycorrhizal type, climate, soil, and phylogenetic structure of species on fine root trait-function relationships. Four commonly measured traits were included: root nitrogen content (RNC), specific root length (SRL), root diameter, and root tissue density. We found that mycorrhizal type and climate affected the relationships between fine root traits and R_r. Specifically, the slopes of SRL-R_r and RNC-R_r relationships for ectomycorrhizal (EM) plants were significantly greater than those for arbuscular mycorrhizal (AM) plants, irrespective of the phylogenetic conservatism or plant growth forms. This finding suggested that EM plants consumed more carbon than AM plants for constructing root tissue structures. In a warmer environment, the SRL-R_r relationship was stronger while the RNC-R_r relationship was weaker, possibly because R_r was limited by enzyme activity at low temperature and by substrate supply at high temperature. Our results highlighted the importance of mycorrhizas in influencing the root trait-function relationships. Thus, incorporating mycorrhizal types into trait-based terrestrial biosphere models may help to more accurately predict the future carbon-climate change feedback.