BIOGENIC EMISSION FROM THE MEDITERRANEAN PSEUDOSTEPPE ECOSYSTEM PRESENT IN CASTELPORZIANO

Emission rates and fluxes of biogenic components emitted by a Mediterr anean Pseudosteppe were measured in the BEMA test site of Castelporzia no during the 1993 and 1994 field campaigns. Enclosure and micrometeor ological techniques were used. Although the emission Was comprised of isoprene, semi-volatile aldehydes, acetic acid and monoterpenes at tra ce levels, the most relevant compound in air was isoprene. Basal emiss ion rates for isoprene (normalized at 30 degrees C and 1000 mu E PAR) as defined by Guenther et al. (1991, J. geophys. Res. 96, 10,799-10,80 8) were obtained for this ecosystem by combining experimental observat ions and predictions based on the Guenther algorithm. It is shown that the Mediterranean Pseudosteppe is a strong isoprene emitter with a ba sal emission rate of 0.45 mu g m(-2) s(-1) during the flowering season . At the end of the maximum physiologically active season basal emissi on rate ranged only in 0.1-0.15 mu g m(-2) s(-1). A close dependence f rom light and temperature for the isoprene emission is observed. The d ecline in emission rates seems to be associated with a reduction in ph otosynthetic activity linked to senescence of the vegetation present i n this ecosystem. The results obtained indicate that the Mediterranean Pseudosteppe is an ecosystem characterized by a rapid and strong vari ability in isoprene emission. It represents a source of isoprene compa rable to deciduous forest areas only during the flowering season (from the middle of March to the middle of May) whereas it becomes a minor source during the end of the maximum physiologically active season. (C ) 1997 Elsevier Science Ltd.