Leaf nitrogen from first principles: field evidence for adaptive variation with climate

Nitrogen content per unit leaf area (N_area) is a key variable in plant functional ecology and biogeochemistry. N_area comprises a structural component, which scales with leaf mass per area (LMA), and a metabolic component, which scales with Rubisco capacity. The co-ordination hypothesis, as implemented in LPJ and related global vegetation models, predicts that Rubisco capacity should be directly proportional to irradiance but should decrease with increases in c_i:c_a and temperature because the amount of Rubisco required to achieve a given assimilation rate declines with increases in both. We tested these predictions using LMA, leaf δ¹³C, and leaf N measurements on complete species assemblages sampled at sites on a north-south transect from tropical to temperate Australia. Partial effects of mean canopy irradiance, mean annual temperature, and c_i:c_a (from δ¹³C) on N_area were all significant and their directions and magnitudes were in line with predictions. Over 80% of the variance in community-mean (ln) N_area was accounted for by these predictors plus LMA. Moreover, N_area could be decomposed into two components, one proportional to LMA (slightly steeper in N-fixers), and the other to Rubisco capacity as predicted by the co-ordination hypothesis. Trait gradient analysis revealed c_i:c_a to be perfectly plastic, while species turnover contributed about half the variation in LMA and N_area. Interest has surged in methods to predict continuous leaf-trait variation from environmental factors, in order to improve ecosystem models. Coupled carbon-nitrogen models require a method to predict N_area that is more realistic than the widespread assumptions that N_area is proportional to photosynthetic capacity, and/or that N_area (and photosynthetic capacity) are determined by N supply from the soil. Our results indicate that N_area has a useful degree of predictability, from a combination of LMA and c_i:c_a - themselves in part environmentally determined - with Rubisco activity, as predicted from local growing conditions. This finding is consistent with a "plant-centred" approach to modelling, emphasizing the adaptive regulation of traits. Models that account for biodiversity will also need to partition community-level trait variation into components due to phenotypic plasticity and/or genotypic differentiation within species vs. progressive species replacement, along environmental gradients. Our analysis suggests that variation in N_area is about evenly split between these two modes.