Correlations between net primary productivity and foliar carbon isotope ratio across a Tibetan ecosystem transect

Abstract

Warming climate could affect leaf‐level carbon isotope composition (δ13C) through variations in photosynthetic gas exchange. However, it is still unclear to what extent variations in foliar δ13C can be used to detect changes in net primary productivity (NPP) because leaf physiology is only one of many determinants of stand productivity. We aim to examine how well site‐mean foliar δ13C and stand NPP co‐vary across large resource gradients using data obtained from the Tibetan Alpine Vegetation Transects (1900–4900 m, TAVT). The TAVT data indicated a robust negative correlation between foliar δ13C and NPP across ecosystems (NPP=−2.7224δ13C‐67.738, r2=0.60, p<0.001). The mean foliar δ13C decreased with increasing annual precipitation and its covariation with mean temperature and soil organic carbon and nitrogen contents. The results were further confirmed by global literature data. Pooled δ13C data from global literature and this study explained 60% of variations in annual NPP both from TAVT‐measures and MODIS‐estimates across 67 sites. Our results appear to support a conceptual model relating foliar δ13C and nitrogen concentration (Nmass) to NPP, suggesting that: 1) there is a general (negative) relationship between δ13C and NPP across different water availability conditions; 2) in water‐limited conditions, water availability has greater effects on NPP than Nmass; 3) when water is not limiting, NPP increases with increasing Nmass.