Determinants of Terrestrial Ecosystem Carbon Balance Inferred from European Eddy Covariance Flux Sites

Abstract

Pioneering work in the last century has resulted in a widely accepted paradigm that primary production is strongly positively related to temperature and water availability.This has lead to the hypothesis that with global warming the northern hemispheric forest ecosystems may increase their net carbon sink. However, terrestrial carbon sink at ecosystem level (NEP) is depending on the net balance between assimilation (GPP) and ecosystem respiration of carbon (TER). Both parts of the carbon balance have been shown to be very sensitive to climate recently, in the context of heatwaves or gradual temperature increases. Through an analysis of nearly 100 annual pan-European eddy covariance flux data sets we find that the common climate relationships for primary production do not hold for NEP. We can explain this behavior by the parallel decrease of respiration when climatic factors become limiting gross primary production. In particular, water availability has an effect in addition to GPP on net carbon balance while the multivariate effect of mean annual temperature is small and not significant. These results indicate that temperature-based projections of net carbon balance may be misleading and delicate interactions between water-carbon cycle interactions should be scrutinized before predictions of net effects of climate change on northern hemisphere can be reliably performed.