Controls on fine-root trait variation and implications for defining Root Functional Types

Linking plants and the soil, fine roots (i.e., roots < 2 mm in diameter) play a central role in below-ground processes. While general bivariate trait trade-offs emerge from leaf economics, there is currently a lack of consensus on the empirical patterns among fine-root traits, likely reflecting multi-dimensional environmental constraints on fine roots and their interactions with mycorrhizal fungi. In particular, when factors such as species, climate, and mycorrhizal association are not properly accounted for, contrasting conclusions can be reached. Here we implemented a hierarchical Bayesian model to simultaneously quantify the major controls of fine-root trait variation: phylogeny, mycorrhizal association, environmental conditions, and root order. Using the Fine-Root Ecology Database (FRED) v2.3, we conducted a global assessment of the variation in morphological (root diameter, specific root length [SRL], and root-tissue density) and chemical traits (nitrogen, phosphorous, and calcium contents).

When root order is accounted for, fine-root traits show clear phylogenetic, environmental, and mycorrhizal signals. Diameter and SRL varied less within species than across species, suggesting that these traits are phylogenetically conserved. Within individual species, the variations in root diameter, nitrogen content, and phosphorous content are significantly associated with environmental conditions. Although mycorrhizal types are generally conserved within plant families or genera, variation in the mean value of individual fine-root traits is not linked to mycorrhizal type. Instead, mycorrhizal type modifies the multivariate trade-offs among traits across species, which may reflect different strategies for growth and competition belowground. We are currently expanding the analysis to include traits of fine-root anatomy, architecture, dynamics (e.g. lifespan), and physiology. Based on the current findings, we suggest a two-step representation of Root Functional Types (RFTs) in ecosystem models: first, RFTs should distinguish major phylogenetically-constrained groups and different trade-off relationships; then, trait variation within RFTs should be determined based on environmental conditions.