Disentangling Structural and Physiological Responses to Soil Versus Atmospheric Drought - a Growth Chamber Experiment

The degree to which forests will continue to sequester carbon dioxide from the atmosphere and subsequently mitigate climate change hinges on whether trees can make physiological and structural adjustments to cope with changing environmental stressors. Many temperate forests are expected to encounter greater drought stress in the coming decades, making information on their responses to water stress particularly critical. We still don't fully understand the interactive effects of drought stressors (i.e., declining soil moisture and rising vapor pressure deficit; VPD) on tree structure and function. Structural changes, particularly leaf angle adjustments, can increase water use efficiency by alleviating the need to close stomates, impacting the water and energy balance of forests.

In this study, we used two walk-in growth chambers to investigate the impact of soil moisture and VPD on both leaf physiology and leaf angle. A total of 84 tree saplings (42 Quercus alba and 42 Acer saccharum) were distributed between the two chambers. From July - August 2022, 25% of saplings experienced an atmospheric drought (gradually elevated VPD); 25% experienced a soil moisture drought; 25% experienced both atmospheric and soil moisture drought; and 25% experienced no change in environmental conditions. We measured structural changes in leaf angle using a Terrestrial Lidar Scanning system, backed by manual protractor data, as well as gas-exchange data via an LI-6800 portable photosynthesis system. The data were used to test the prediction that tree species that differ in their photosynthetic capacity and water use strategies will also differ in their capacity to adjust leaf angles. Given the increasing impact of droughts on plant productivity, better understanding of the structural and physiological adjustments by trees under water stress is imperative to accurately project the consequences of drought for global climate, both now and in the future.