Effects of species composition, land surface cover, CO2 concentration and climate on isoprene emissions from European forests

Emissions of isoprene from terrestrial vegetation are known to affect atmospheric chemical properties, like its oxidation capacity or the concentration of tropospheric ozone. The latter is of concern, since besides being a potent greenhouse gas, O-3 is toxic for humans, animals, and plants even at relatively low concentrations. Isoprene-emitting forests in the vicinity of NOx pollution sources (like cities) can contribute considerably to O-3 formation, and to the peak concentrations observed during hot summer weather. The biogenic contribution to O-3 concentrations is generally thought to increase in a future, warmer climate - pushing values beyond health thresholds possibly even more frequently and over larger areas - given that emissions of isoprene are highly temperature-dependent but also because of the CO2 fertilisation of forest productivity and leaf growth. Most projections of future emissions, however, do not include the possible CO2-inhibition of leaf isoprene metabolism. We explore the regional distribution of emissions from European woody vegetation, using a mechanistic isoprene-dynamic vegetation model framework. We investigate the interactive effects of climate and CO2 concentration on forest productivity, species composition, and isoprene emissions for the periods 1981-2000 and 2081-2100. Our projection of future emissions includes a direct CO2-isoprene inhibition. Across the model domain, we show that this direct effect has the potential to offset the stimulation of emissions that could be expected from warmer temperatures and from the increased productivity and leaf area of emitting vegetation. Changes in forest species composition that may result from climate change can play a substantial additional role in a region's future emissions. Changes in forest area or area planted in woody biofuels in general are not noticeable in the overall European forest isoprene budget, but - as was the case for changes in species composition - may substantially affect future projections in some regions of the continent.