Abstract
Climate change may alter ecosystem functioning,
as assessed via the net carbon (C) exchange (NEE)
with the atmosphere, composed of the biological processes
photosynthesis (GPP) and respiration (Reco). In addition,
in semi-arid Mediterranean ecosystems, a significant fraction
of respired CO2 is stored in the vadose zone and emitted
afterwards by subsoil ventilation (VE), contributing
also to NEE. Such conditions complicate the prediction of
NEE for future change scenarios. To evaluate the possible
effects of climate change on annual NEE and its underlying
processes (GPP, Reco and VE) we present, over a climate/
altitude range, the annual and interannual variability of NEE, GPP, Reco and VE in three Mediterranean sites. We found that annual NEE varied from a net source of around
130 gC m−2 in hot and arid lowlands to a net sink of similar
magnitude for alpine meadows (above 2,000 m a.s.l) that are
less water stressed. Annual net C fixation increased because
of increased GPP during intermittent and several growth
periods occurring even during winter, as well as due to
decreased VE. In terms of interannual variability, the studied
subalpine site behaved as a neutral C sink (from emission of
49 to fixation of 30 gC m−2 year−1), with precipitation as
the main factor controlling annual GPP and Reco. Finally, the
importance of VE as 0–23 % of annual NEE is highlighted, indicating that this process could shift some Mediterranean ecosystems from annual C sinks to sources.
