Priorities of woody species trait-climate associations at continental scale

Plant functional traits underpin ecological strategies and determine ecosystem responses to climate change, yet the key climate drivers of traits remain unresolved at continental scale. Key questions are whether across large gradients species traits are driven more strongly by temperature or precipitation—and by annual or by growing season mean climates. Across 16 ecosystems across the continental USA and northern Mexico, we measured 328 species-site combinations, including 246 unique woody species, for 59 traits relating to hydraulic and photosynthetic physiology, leaf and wood structure and anatomy, and nutrient and isotope concentrations using standard protocols, and tested associations with the macroclimate of both the species’ natural distributions and of the sampling sites, i.e., growing season and mean annual temperature, precipitation, potential evapotranspiration, and aridity index. Across species-site combinations, ninety-eight percent of traits were associated with one or more climate variables. More traits were correlated, and showed stronger correlations, with mean and growing season precipitation than temperature, and more trait variation was explained by precipitation than temperature in multiple regressions. Potential evapotranspiration and aridity index were also strong trait predictors. Seasonal and annual means did not differ in the strength of trait correlations for precipitation, whereas growing season temperature was a stronger predictor of traits than mean annual temperature. Our findings highlight the importance of mean annual precipitation a driver of the distribution of plant traits, indicating the strong sensitivity of species to ongoing shifts in moisture availability at continental scale.