Abstract ID: 341

What is the relationship between leaf nitrogen, leaf phosphorus and photosynthetic capacity in tropical forests and savannas? New data from tropical Africa.

This work examines the relationships between foliar nutrient concentration (nitrogen and phosphorus) and the parameters that determine the photosynthetic capacity of leaves, V_cmax and J_max. This study is based on a unique (large and new) dataset focused on tropical forests and savannas from West Africa: the data were obtained from 12 field sites in 3 countries, Ghana, Burkina-Faso, and Mali. Similar datasets are being produced for Brazil, Bolivia, Cameroon, and Australia. This work enables us to test our understanding of nutrient constraints on leaf physiology and to improve the modelling of leaf function across scales (e.g. from leaf to region). Pre-existing field data from tropical rain forests strongly suggest that the nitrogen-V_cmax relationship may differ strongly between temperate and tropical terrestrial forests. If this proves to be true more widely, it has important implications for global and regional modelling efforts. A test of this assumption is urgently needed and we present the largest data set hitherto available from tropical forests. Early results from our African dataset show that, on a mass basis, leaf [N] alone explains 45% of the variation in V_cmax, while [P] alone explains 28% of the variation of that parameter. The single best predictor of carboxylation capacity was specific leaf area (SLA), which explained 59% of the variation. Multiple correlation analyses showed that the interaction between SLA + [N] was the best model to predict V_cmax, with 61% of de variation explained. Furthermore, leaves with low [P] showed inferior photosynthetic nitrogen use efficiency (PNUE = A_max/[N]), suggesting that phosphorus limitation was very important for a subset of trees and shrubs examined in this study. We also examine similar relationships with J_max and consider local and regional scale patterns in both parameters.

Session:  Carbon - Scaling carbon fluxes to the region from measurements in plots, towers, and aircraft.

Presentation Type:  Oral