The influence of nutrient gradients on the photosynthesis-leaf nitrogen relationship in Peruvian Andes tropical forests
As an integral part in carbon cycle, photosynthetic capacity has been increasingly incorporated in vegetation-climate models for predicting ecosystem productivity. To predict variations in photosynthetic capacity over large spatial and temporal scales, assumptions are made on the relationship between photosynthetic capacity (A) and leaf nitrogen (N) content. However, A-N relationships are unlikely to be fixed, particularly in tropical ecosystems where phosphorus (P) availability limits photosynthesis, or in high altitude tropical ecosystems where metabolism may be temperature limited. Given the lack of detailed data on A-N relationships in tropical ecosystems, we undertook a multi-institutional field study in mid-2011 to quantify photosynthetic capacity (determined from A-Ci plots) and associated leaf traits (leaf mass per unit leaf area (LMA), foliar N and P concentrations) along nutrient gradients in several Peruvian lowland forests, and along an altitudinal gradient in the Peruvian Andes (100m-3500m above sea level).
Preliminary analysis focussing on Peruvian lowland forests show that rates of A at any given N were significantly lower in forest species growing on P-limited sites. Decreases in A per unit N appear to be associated with lower allocation of leaf N to the CO2 fixing enzyme, Rubisco. However, no altitudinal trend for photosynthetic capacity was found despite foliar N:P ratios showing a decreasing trend with altitude (which indicates P-limitation at many lowland sites). Contrary to general hypothesis, changes in growth temperature and nutrient availability across altitudinal gradient did not translate to significant changes in leaf carbon metabolism. Collectively, our results demonstrate the influence of nutrient gradients on A-N relationships in tropical forests, which account for one-third of global terrestrial primary productivity.