Tropical carbon cycle implications of multiple competing representations of Vcmax
The tropical carbon cycle is a key component of the Earth System and multiple streams of evidence suggest a large tropical carbon sink that balances emissions from tropical deforestation. The primary flux of carbon into terrestrial ecosystems is via photosynthesis and photosynthetic rates are strongly affected by the maximum capacities of the photosynthetic machinery—known as Vcmax and Jmax. These functional leaf traits are sensitive to leaf nitrogen content and, in some tropical ecosystems, leaf phosphorus—an important limiting nutrient in the tropics. To integrate the limited data on leaf phosphorus as a driver of photosynthesis we performed a meta-analysis of Vcmax and Jmaxin relation to leaf nitrogen and phosphorus.
Accurate carbon cycle modelling requires accurate parameterisation of Vcmax and Jmax and our aims were to evaluate a number of alternative modelling assumptions used to simulate Vcmax and Jmax: Trait covariance (Vcmax and Jmaxrelationships with leaf nitrogen, or nitrogen and phosphorus), constant values (variable by PFT), trait-environment relationships (variable by PFT), and the co-ordination hypothesis. Variability in tropical carbon cycle projections caused by these competing photosynthetic trait assumptions was simulated with a modified version of the Sheffield Dynamic Global Vegetation Model.
The meta-analysis yielded trait regressions suitable for use in terrestrial carbon cycle models, and demonstrated that low leaf phosphorus substantially reduced the sensitivity of Vcmax to leaf nitrogen. Using leaf phosphorus and nitrogen as predictors of Vcmax in the tropics is likely to be more accurate than using leaf nitrogen alone but more data are needed to generate more robust trait relationships and test the alternative hypotheses. Using both leaf nitrogen and leaf phosphorus as predictors of photosynthetic traits substantially reduced present day tropical forest gross primary production (GPP) compared with simulations in which nitrogen alone was used to predict Vcmax and Jmax. However, there was little effect on the change in sink strength over the past 50 years. Preliminary results indicate that the range in modelled, present day tropical GPP caused by alternative methods used to predict traits is huge (>10Pg C yr-1), and the variability in the change in the sink strength over the past 50 years is also substantial. Accurate parameterisation of these photosynthetic traits and their variability with nutrient availability and environmental conditions such as light, temperature and aridity, is thus essential.