Fine root function responds to small-scale soil heterogeneity in a tropical tree
C. DALLSTREAM, L. MILDER, J.S. POWERS, F.M. SOPER
Department of Biology, McGill University, 1205 Dr Penfield Ave, Montreal, Quebec, H3A 1B1, Canada

Fine roots exhibit enormous variation in functional traits. Currently, it is unclear whether this variation is driven primarily by small-scale heterogeneity or broad environmental gradients. Understanding such trait-environment responses could improve predictions of ecosystem feedbacks to global change. We quantified trait variation with nested, spatially coupled sampling of fine roots and adjacent soils in a tropical dry forest tree (Handroanthus ochraceus) from subindividual to regional scales. Even within a species, principal coordinate analysis revealed coordination between traits, e.g. phosphomonoesterase activity (PME), arbuscular mycorrhizal colonization (M%), and nitrogen concentration (N) covaried (PC2, 23%). Root trait variation was greatest sub-individually for specific root length and respiration rate, and regionally for PME and M%. This pattern may reflect soil heterogeneity; soil inorganic N varied mostly within sites whereas inorganic P and cations varied mostly across sites. In fact, redundancy analysis showed that PME, MYC, and N responded significantly to inorganic P, exchangeable magnesium, and bulk density (RDA1, 24%). Overall, we explained 32% of fine-root trait variation with the chemical and physical characteristics of soils that roots directly encountered. Root trait-environment responses may be strongest when adjacent soils are sampled, and when sampling schemes capture the dominant spatial scales of soil heterogeneity.