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

Warming climate could affect leaf-level carbon isotope composition (delta C-13) through variations in photosynthetic gas exchange. However, it is still unclear to what extent variations in foliar delta C-13 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 delta C-13 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 delta C-13 and NPP across ecosystems (NPP=-2.7224 delta C-13-67.738, r(2)=0.60, p < 0.001). The mean foliar delta C-13 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 delta C-13 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 delta C-13 and nitrogen concentration (N-mass) to NPP, suggesting that: 1) there is a general (negative) relationship between delta C-13 and NPP across different water availability conditions; 2) in water-limited conditions, water availability has greater effects on NPP than N-mass; 3) when water is not limiting, NPP increases with increasing N-mass.