Energy and CO2 exchanges of a Mediterranean shrubland ecosystem

The main objectives of this paper are to (i) describe the diurnal and seasonal variation in the energy fluxes of a Mediterranean shrubland ecosystem, (ii) assess the dynamics of CO2 fluxes, and (iii) investigate the relationship between NEP and environmental variables. Field observations were taken near west coast of Sardinia, Italy, over shrub vegetation for several periods during 1997-2005. Vegetation included sclerophyll species and some scattered shrubs of a maximum height of 2.0 m. The climate is semi-arid with a significant water deficit from May through September. Fluxes of momentum, sensible heat, latent heat, and CO2 were measured using an eddy covariance system consisted of a 3D sonic anemometer and an open-path IRGA mounted over the vegetation. All flux and meteorological data were quality-controlled. Gaps in the data were filled following the procedures reported by Reichstein et al., 2005. LAI values were estimated using LI-2000 under diffuse light conditions. Soil CO2 effluxes were measured using soil chambers in an open-system configuration along transects approximately every four hours. Ecosystem surface conductance was obtained from Penman-Monteith equation and the decoupling coefficient (Ω) was calculated according to Jarvis and McNaughton (1986). The energy budget closure from half-hour eddy covariance data was acceptable with an average (H+LE)/(Rn-G) ratio approximately equal to 0.92. The energy partitioning exhibited distinct seasonal pattern with Bowen ratio values clearly decreasing during the drought season. Peak CO2 uptake occurred during spring and autumn showing a clear decrease in summer. The estimate of NEP showed differences among years depending on drought and temperature conditions. The surface conductance was clearly depressed during long-term drought period. In general, NEP was relatively low compared to other forest ecosystems.