ISOPRENE AND MONOTERPENE EMISSIONS FROM A EUCALYPTUS PLANTATION IN PORTUGAL

Measurements of isoprene and monoterpene emission rates were made from a mature and an immature Eucalyptus globulus tree in a commercial mon oculture plantation in central Portugal in 1994 using a branch enclosu re sampling system with analysis by gas chromatography/flame ionizatio n and gas chromatography/mass selective methods. Isoprene was the domi nant compound emitted and represented over 90% of the total assigned v olatile organic compound plant emissions during the day. Other identif ied species were alpha-pinene, sabinene, beta-pinene, myrcene, limonen e, cineole, linalool, alpha-terpineol, and, tentatatively, cis- and tr ans-ocimine and an unidentified octatriene. When they were adjusted to standard conditions of temperature (30 degrees C) and light (1000 mu mol(-2) ms(-1)) VOC emissions from a 1 year old sapling were 5 times h igher (49 and 5.2 mu g Cg(-1) dry wt h(-1) for isoprene and total mono terpenes, respectively) than those from a 7-year-old tree (15 and 0.7 mu g C g(-1) dry wt h(-1) respectively). On a projected leaf area basi s these differences were not so apparent (isoprene; 5 and 4 mg m(-2) h (-1), young and old trees, respectively; monoterpenes, 0.6 and 0.2 mg m(-2) m(-1), respectively). Emission rates from both trees were closel y correlated with incident light flux and temperature, with daytime ma xima, and with nighttime minima. Existing models describing emissions in terms of light, temperature, and empirical coefficients were found to adequately predict emissions from the young tree but to grossly ove restimate emission Hates from the mature tree. This finding has implic ations for the extrapolation of emission data obtained in the laborato ry with immature trees to the canopy, regional, or global scales, alth ough additional measurements are required to determine whether the res ults presented here can be generalized.