Carbon and energy fluxes over Mediterranean maquis

Mediterranean maquis is a shrubland ecosystem that is widely spread throughout the Mediterranean Basin, and comparable vegetation is found in similar climates worldwide. Little is known about the energy and carbon exchange over maquis, and learning more about CO2 fluxes and the relationship between NEP and environmental variables is important to understand the interaction between this ecosystem and climate. Micrometeorological data were collected near the northwest coast of Sardinia, Italy, over maquis for several periods during 1997-2007. The climate has cool, wet winters and hot, humid summers. There is a significant water deficit from May through September each year. An eddy covariance system (using a 3-D sonic and an open-path IRGA) were used to measure fluxes of momentum, sensible heat, latent heat, and CO2. All flux and meteorological data were quality-controlled. Gaps in the data were filled following the procedures reported by Reichstein et al. (2005). The vegetation is mainly sclerophyll species with some shrubs reaching a maximum height of 2.0 m. An LI-2000 leaf area instrument was used to estimate the LAI. Soil chambers in an open-system configuration were used to measure soil CO2 exchange approximately every four hours. The decoupling coefficient (Ω) was calculated according to Jarvis and McNaughton (1986) and the ecosystem surface conductance was estimated using the Penman-Monteith equation (Monteith, 1965). The ratio (H+LE)/(Rn-G) was approximately equal to 0.92; indicating good energy balance closure. The Bowen ratio values were high in the winter and clearly decreased during the summer drought season. CO2 uptake was highest in the spring and fall and lowest during the summer drought period. The NEP varied from year-to-year depending on drought and temperature conditions. In general, NEP was relatively low compared to other forest ecosystems. Surface conductance was clearly depressed during the summer drought period. A good relationship was found between GPP and LE.