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

• Published in: Biogeosciences Vol. 14; no. 2; pp. 481 - 495
• Main Authors: Dong, Ning, Prentice, Iain Colin, Evans, Bradley J, Caddy-Retalic, Stefan, Lowe, Andrew J, Wright, Ian J
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 30.01.2017; Copernicus Publications
• Subjects: Annual temperatures; Biodiversity; Biogeochemistry; Capacity; Climate change; Components; Ecology; Ecosystem models; Ecosystems; Environment models; Environmental aspects; Environmental factors; Environmental gradient; First principles; Hypotheses; Irradiance; Leaf area; Leaves; Metabolism; Nitrogen; Nitrogen content; Nitrogen cycle; Observations; Ordination; Phenotypic plasticity; Photosynthesis; Plant cover; Precipitation; Ribulose-bisphosphate carboxylase; Soil; Species; Temperature; Temperature requirements; Tropical climate; Variance analysis; Variation; Vegetation; Australia
• ISSN: 1726-4189; 1726-4170; 1726-4189
• DOI: 10.5194/bg-14-481-2017

Nitrogen content per unit leaf area (Narea) is a key variable in plant functional ecology and biogeochemistry. Narea 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 ci : ca 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 ci : ca (from δ13C) on Narea were all significant and their directions and magnitudes were in line with predictions. Over 80 % of the variance in community-mean (ln) Narea was accounted for by these predictors plus LMA. Moreover, Narea 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 ci : ca to be perfectly plastic, while species turnover contributed about half the variation in LMA and Narea. 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 Narea that is more realistic than the widespread assumptions that Narea is proportional to photosynthetic capacity, and/or that Narea (and photosynthetic capacity) are determined by N supply from the soil. Our results indicate that Narea has a useful degree of predictability, from a combination of LMA and ci : ca – 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 Narea is about evenly split between these two modes.