Analyzing the major drivers of NEE in a Mediterranean alpine shrubland

Two years of continuous measurements of net ecosystem exchange (NEE) using the eddy covariance tech- nique were made over a Mediterranean alpine shrubland. This ecosystem was found to be a net source of CO2 (+ 52± 7 g C m 2 and + 48± 7 g C m 2 for 2007 and 2008) during the two-year study period. To understand the rea- sons underlying this net release of CO2 into the atmosphere, we analysed the drivers of seasonal variability in NEE over these two years. We observed that the soil water availability - driven by the precipitation pattern - and the photosynthetic photon flux density (PPFD) are the key factors for under- standing both the carbon sequestration potential and the du- ration of the photosynthetic period during the growing sea- son. Finally, the effects of the self-heating correction to CO 2 and H2O fluxes measured with the open-path infrared gas analyser were evaluated. Applying the correction turned the annual CO2 budget in 2007 from a sink ( 135± 7 g C m 2 ) to a source (+ 52± 7 g C m 2 ). The magnitude of this change is larger than reported previously and is shown to be due to the low air density and cold temperatures at this high eleva- tion study site.