Abstract. In recent years evidence has emerged that the amount of isoprene emitted from a leaf is affected by the CO 2 growth environment. Many -though not all -laboratory experiments indicate that emissions increase significantly at below-ambient CO 2 concentrations and decrease when concentrations are raised to above-ambient. A small number of process-based leaf isoprene emission models can reproduce this CO 2 stimulation and inhibition. These models are briefly reviewed, and their performance in standard conditions compared with each other and to an empirical algorithm. One of the models was judged particularly useful for incorporation into a dynamic vegetation model framework, LPJ-GUESS, yielding a tool that allows the interactive effects of climate and increasing CO 2 concentration on vegetation distribution, productivity, and leaf and ecosystem isoprene emissions to be explored. The coupled vegetation dynamics-isoprene model is described and used here in a mode particularly suited for the ecosystem scale, but it can be employed at the global level as well.Annual and/or daily isoprene emissions simulated by the model were evaluated against flux measurements (or model estimates that had previously been evaluated with flux data) from a wide range of environments, and agreement between modelled and simulated values was generally good. By usCorrespondence to: A. Arneth (almut.arneth@nateko.lu.se) ing a dynamic vegetation model, effects of canopy composition, disturbance history, or trends in CO 2 concentration can be assessed. We show here for five model test sites that the suggested CO 2 -inhibition of leaf-isoprene metabolism can be large enough to offset increases in emissions due to CO 2 -stimulation of vegetation productivity and leaf area growth. When effects of climate change are considered atop the effects of atmospheric composition the interactions between the relevant processes will become even more complex. The CO 2 -isoprene inhibition may have the potential to significantly dampen the expected steep increase of ecosystem isoprene emission in a future, warmer atmosphere with higher CO 2 levels; this effect raises important questions for projections of future atmospheric chemistry, and its connection to the terrestrial vegetation and carbon cycle.

The models differ by more than a factor of 5 for specific times and locations, which indicates that there are large uncertainties in emission estimates for at least some locations and seasons. The good agreement obtained for the EXPRESSO study field sites, however, suggests that the model can predict reasonable estimates if representative field measurements are used to parameterize the model.

International audienceWe have measured simultaneously the methane (CH4) and carbon dioxide (CO2) surface concentrations and water–air fluxes by floating chambers (FC) in the Petit-Saut Reservoir (French Guiana) and its tidal river (Sinnamary River) downstream of the dam, during the two field experiments in wet (May 2003) and dry season (December 2003). The eddy covariance (EC) technique was also used for CO2 fluxes on the lake. The comparison of fluxes obtained by FC and EC showed little discrepancies mainly due to differences in measurements durations which resulted in different average wind speeds. When comparing the gas transfer velocity (k600) for a given wind speed, both methods gave similar results. On the lake and excluding rainy events, we obtained an exponential relationship between k600 and U10, with a significant intercept at 1.7 cm h− 1, probably due to thermal effects. Gas transfer velocity was also positively related to rainfall rates reaching 26.5 cm h−1 for a rainfall rate of 36 mm h− 1. During a 24-h experiment in dry season, rainfall accounted for as much as 25% of the k600. In the river downstream of the dam, k600 values were 3 to 4 times higher than on the lake, and followed a linear relationship with U10