Abstract: As surface temperatures rise around the globe, so does the vapour pressure deficit (VPD). Given the impact both factors have on plant growth and water use, there will likely be significant implications for the productivity and functioning of forests worldwide. Species found in moist tropical forest may be at particular risk due to their high sensitivity to VPD, and indeed there is already evidence of hot-dry periods limiting growth and impacting mortality of tropical trees. However, it is often challenging to disentangle the independent impacts of temperature, VPD, and soil moisture on plant physiological responses.
In this study, we isolated the effects of temperature (6 °C difference) and VPD (~1 kPa daytime difference) under well-watered conditions on the growth and physiology of nine tropical tree species native to the Australian Wet Tropics. Saplings were kept in climate-controlled glasshouses under three treatment regimes: 1) cool temperatures and low VPD, 2) warm temperatures and low VPD, and 3) warm temperatures and high VPD. After three months of treatment, we measured the impacts on leaf-level gas exchange, thermal tolerance, and integrated growth.
Our results show that sapling growth rates for all studied species were higher in the warm treatments vs the cool treatments, but that elevated VPD reduced growth, to differing extents depending on species. We present data demonstrating acclimation of photosynthesis and stomatal conductance, dark respiration, minimum conductance, and photosynthetic heat tolerance to both temperature and VPD. Our study shows that even moderate increases in VPD can decrease productivity and growth rates in tropical species independently from temperature or soil moisture deficit. These results have important implications for predicting the impacts of climate change on tropical forests and for developing strategies to mitigate its effects.
