Isoprene emission and photosynthesis in a tropical forest canopy: Implications for model development

Isoprene emission by plants is the principal source of photochemically active reduced compounds in the troposphere, and tropical forest ecosystems are the largest single source of isoprene. The oxidation of isoprene plays a major role in controlling the redox potential of the troposphere and the dynamics of carbon monoxide, ozone, and methane. We used a combination of infrared gas analysis and gas chromatography/photoionization detection in the first study of isoprene emission and photosynthesis from canopy leaves in a tropical wet forest under controlled light and temperature conditions. Twelve of the 33 tree species surveyed produced isoprene. This is a similar proportion of emitting species to that found in other species-rich ecosystems surveyed to date. Canopy leaves did not show temperature saturation in their isoprene-emission responses, although isoprene emission saturated at high light intensities for some species. Photosynthesis rates saturated at high light intensities and declined at high temperatures. Both isoprene-emission capacity and photosynthesis capacity declined through the canopy, with the largest fluxes at the top of the canopy. In addition, the percentage of photosynthate lost as isoprene was lower in sub-canopy leaves than in leaves from the top of the canopy. Across all taxa, isoprene-emission capacity scaled positively with photosynthesis capacity. These results suggest that canopy models of isoprene emission from tropical forests can use estimates of photosynthesis capacity to predict isoprene emission, as long as these models consider the different responses of isoprene emission and photosynthesis to light and temperature.