Drought effects on the carbon balance and VOC emissions of a tropical rainforest ecosystem

Trees contribute substantially to the carbon cycling between the biosphere and atmosphere. Tropical ecosystems in particular are known to exchange not only CO₂ with the atmosphere, but also a wide variety of volatile organic compounds (VOCs). With their high reactivity and short life time, VOCs are known to play not only a crucial role in atmospheric chemistry but also in plant signaling and interactions. Due to climate change periods of sustained drought are thought to increase in future and have the potential to alter the carbon balance of tropical ecosystems drastically. However, combined VOC and CO₂ flux measurements are rare and thus a quantitative understanding of carbon exchange fluxes in rainforest species during and after drought periods has not yet been reached.

Thus, we used the unique opportunity to study changes of VOC and CO₂ flux patterns of the rainforest mesocosm of Biosphere 2 (University of Arizona) in response to an experimentally induced drought period and during the recovery (Biosphere 2 Water, Atmosphere, and Life Dynamics experiment; B2-WALD). This provides us novel information about stress responses of a rainforest ecosystem and its ability to recover, specifically to drought stress. Real-time fluxes of CO₂ and VOC exchange were measured by means of ¹³CO₂ laser spectroscopy and proton-transfer-reaction time-of-flight mass-spectrometry (PTR-TOF-MS) using leaf chambers on five different tree and understory species.

While photosynthesis decreased during the drought, changes in VOC flux patterns were more diverse. For example, isoprene emissions increased with dry conditions, whereas fluxes of acetone declined. Here we will present and discuss our first results on leaf gas exchange measurements of different VOCs and CO₂ and their response to drought and recovery.