Global effects of soil and climate on leaf photosynthetic traits and rates

Aim: The influence of soil properties on photosynthetic traits in higher plants is poorly quantified in comparison with that of climate. We address this situation by quantifying the unique and joint contributions to global leaf-trait variation from soils and climate. Location: Terrestrial ecosystems world-wide. Methods: Using a trait dataset comprising 1509 species from 288 sites, with climate and soil data derived from global datasets, we quantified the effects of 20 soil and 26 climate variables on light-saturated photosynthetic rate (A_area), stomatal conductance (g_s), leaf nitrogen and phosphorus (N_area and P_area) and specific leaf area (SLA) using mixed regression models and multivariate analyses. Results: Soil variables were stronger predictors of leaf traits than climatic variables, except for SLA. On average, N_area, P_area and A_area increased and SLA decreased with increasing soil pH and with increasing site aridity. g_s declined and P_area increased with soil available P (P_avail). N_area was unrelated to total soil N. Joint effects of soil and climate dominated over their unique effects on N_area and P_area, while unique effects of soils dominated for A_area and g_s. Path analysis indicated that variation in A_area reflected the combined independent influences of N_area and g_s, the former promoted by high pH and aridity and the latter by low P_avail. Main conclusions: Three environmental variables were key for explaining variation in leaf traits: soil pH and P_avail, and the climatic moisture index (the ratio of precipitation to potential evapotranspiration). Although the reliability of global soil datasets lags behind that of climate datasets, our results nonetheless provide compelling evidence that both can be jointly used in broad-scale analyses, and that effects uniquely attributable to soil properties are important determinants of leaf photosynthetic traits and rates. A significant future challenge is to better disentangle the covarying physiological, ecological and evolutionary mechanisms that underpin trait-environment relationships.