Deciphering the effects of temperature and vapor pressure deficit on plant carbon assimilation: Implications for plant productivity under global warming

Background/Question/Methods It is well documented that global warming will influence patterns of plant productivity due to the direct effects of temperature on photosynthesis and respiration. Indirect effects, such as those imposed by associated changes in vapor pressure deficit (VPD), are less well understood, largely because climate predictions are uncertain as to whether VPD will increase or stay constant with expected temperature rises. This uncertainty is troublesome owing to the strong effects of VPD on stomatal conductance and consequently on carbon assimilation. Therefore, in experimental warming studies, it is essential to explicitly consider the direct and indirect effects mediated by VPD. This research targets the degree to which uncertainty in VPD predictions propagates further uncertainty in future plant response to climate change. Using gas exchange and chlorophyll fluorescence, we measured photosynthetic rates as well as electron transport rate and chlorophyll fluorescence on naturally established tree seedlings of four species while varying VPD and temperature. Experiments were conducted in open-top warming chambers in Duke Forest to account for physiological acclimation to temperature.

Results/Conclusions We estimate the degree to which VPD levels influence photosynthesis with warming. Our results show a substantial VPD effect on photosynthesis with warming for all the species studied, indicating VPD as an important factor in climate vegetation feedbacks. Increasing VPD caused a decrease in net carbon assimilation that could account for up to half of the decrease in assimilation rates we saw with a ten degree increase from ambient. Measurements of net photosynthesis during the summer showed an optimum at around 30 degrees C. When plants were warmed to temperatures higher than this optimum, net photosynthesis rates decreased, mostly due to VPD effects and due to photorespiration, which may have been exacerbated by the stomatal closure resulting from VPD effects. Plants cooled to temperatures below their optimum for photosynthesis showed both lower gross photosynthesis levels as well as decreased respiration and photorespiration.