Scambi di carbonio e vapor d'acqua di un ecosistema a macchia mediterranea durante il periodo estivo, misurati con la tecnica della correlazione turbolenta

CARBON DIOXIDE AND WATER VAPOR EXCHANGE OF A MEDITERRANEAN MACCHIA ECOSYSTEM DURING THE SUMMERTIME, MEASURED WITH THE EDDY COVARIANCE TECHNIQUE This section reports trends in the net ecosystem exchange (NEE) of a Mediterranean macchia ecosystem at the 'Arca di Noe' natural reserve in Alghero (Sardinia, Italy). NEE was monitored throughout the summer 1997 by means of the eddy covariance technique (EC). The EC system was made of a Gill Solent 3d sonic anemometer and a Licor 6262 gas analyzer, in a closed path configuration; fluxes were monitored on a continuous basis, mid May through early July, then from the end of July to the first decade of September, with a percent time cover of 95% for sensible heat, 73 and 75 % for water vapour and carbon dioxide fluxes, respectively. Continuous monitoring showed that stand growth was going on early in the season only. At these stage, the values of the canopy drought stress index were slightly above 0.25, and daytime trends of NEE were regular, paralleling those of irradiance. Later on, with the onset of severe water stress (canopy stress index down to 0.14), NEE became unimodal, peaking increasingly earlier in the morning and remaining negative for shorter time spans. Daily carbon dioxide budgets became negative, starting early in July, so that, on a seasonal basis, the stand appeared to loose carbon. Continuous monitoring of fluxes, started in 1998, will provide yearly carbon budgets, allowing for better assessments of the health status of the ecosystem.