Biogenic hydrocarbon emissions and landcover/climate change in a subtropical savanna

Biogenic non-methane hydrocarbon (NMHC) emissions strongly influence the ch emical composition of the troposphere. Thus, variations in emissions of the se compounds are expected to cause changes in concentrations of important a tmospheric trace gases. Here, we assess the relative magnitude of potential changes in NMHC (e.g., isoprene and monoterpene) emissions using field flu x measurements from a subtropical savanna parkland/thorn woodland site in c onjunction with model predictions of climate and landcover change. NMHC emi ssions of about 40 plant species were characterized. Grasses, as a group, h ad low emission rates. Several common woody species had high emission rates . However, there was little evidence of emissions being consistently relate d to woody plant taxonomy, growthform or functional groups. Above-canopy me asurements were used to validate modeled isoprene flux predictions of about 2 mg C m(-2) h(-1) for savanna parkland/thorn woodland and ca. 0.7 mg C m( -2) h(-1) for the regional landscape, which is a mixture of savanna parklan d/thorn woodland and cropland. Linkage of the biogenic emissions model with a plant succession model indicated that landcover change since the early 1 800s has elicited a 3-fold increase in total NMHC emissions. This increase reflected changes in vegetation species composition (from domination by gra sses which were typically 'low emitters', to shrubs and trees, many of whic h were 'high emitters') and increases in foliar density. Field measurements on two common shrub species indicated that isoprene emission increased exp onentially with increases in leaf temperature from 20 to 40 degrees C and w ere not suppressed by drought stress. Accordingly, our model predicted that projected increases in ambient temperature (3 to 6 degrees C) emissions co uld produce a 2-fold increase in biogenic NMHC emissions. Cloud cover, prec ipitation, relative humidity, and winds also exerted some control over NMHC emissions, but their influence was highly variable and difficult to estima te. Although our results are specific to southern Texas USA, they indicate the magnitude of change in NMHC emissions that could occur at other locatio ns when climate and vegetation composition are altered. (C) 1999 Elsevier S cience Ltd. All rights reserved.