Analyzing the major drivers of NEE in an alpine Mediterranean shrubland

Abstract Two years of continuous measurements of net ecosystem exchange (NEE) using theeddy covariance technique were made over a Mediterranean alpine shrubland. Theseecosystems are little studied, since they have little CO 2 exchange potential. Neverthe- 5 less, their high susceptibility to environmental changes is far from being understood,introducing some uncertainty in terrestrial CO 2 and water vapour assessments. Highaltitude sites might be undergoing a transition from sink to source of CO 2 , due to theirhigh vulnerability to climate change. Indeed, this ecosystem was found to be a netsource of CO 2 (+52gCm −2 and +48gCm −2 for 2007 and 2008) during the two-year 10 study period. To understand the reasons underlying this net release of CO 2 into theatmosphere, we analysed the drivers of seasonal variability in NEE across these twoyears. We observed that the soil water availability – driven by the precipitation pat-tern – and the photosynthetic photon flux density (PPFD) are the key factors for under-standing both the carbon sequestration potential and the duration of the photosynthetic