Articles | Volume 14, issue 2
Biogeosciences, 14, 481–495, 2017
Biogeosciences, 14, 481–495, 2017

Research article 30 Jan 2017

Research article | 30 Jan 2017

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

Ning Dong1,3,4, Iain Colin Prentice1,2, Bradley J. Evans1,3,4, Stefan Caddy-Retalic5,6, Andrew J. Lowe5,6,7, and Ian J. Wright1 Ning Dong et al.
  • 1Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
  • 2AXA Chair of Biosphere and Climate Impacts, Department of Life Sciences, Imperial College London, Silwood Park Campus, Buckhurst Road, Ascot SL5 7PY, UK
  • 3Terrestrial Ecosystem Research Network: Ecosystem Modelling and Scaling Infrastructure, University of Sydney, NSW 2006, Australia
  • 4Faculty of Agriculture and Environment, Department of Environmental Sciences, University of Sydney, NSW 2006, Australia
  • 5Terrestrial Ecosystem Research Network: Australian Transect Network, University of Adelaide, North Terrace, Adelaide, SA 5005, Australia
  • 6School of Biological Sciences and Environment Institute, University of Adelaide, North Terrace, Adelaide, SA 5005, Australia
  • 7Science, Monitoring and Knowledge Branch, Department of Environment, Water and Natural Resources, Hackney Road, Kent Town, SA 5005, Australia

Abstract. 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.

Short summary
The nitrogen content of leaves is a key quantity for understanding ecosystem function. We analysed variations in nitrogen per unit leaf area among species at sites along a transect across Australia including many climates and ecosystem types. The data could be explained by the idea that leaf nitrogen comprises two parts, one proportional to leaf mass, the other (metabolic) part proportional to light intensity and declining with CO2 drawdown and temperature, as optimal allocation theory predicts.
Final-revised paper