Background
Carbon uptake by plants on land removes ~25% of anthropogenic CO2 emissions from the atmosphere, much of which is stored in long-lived forest woody biomass. The extent to which drought affects tree growth could therefore affect both the total ecosystem carbon uptake and the amount of time it spends sequestered in the ecosystem. Droughts reduce photosynthesis and growth and increase the risk of mortality, but the nature of drought itself is changing due to warming. Anthropogenic warming has increased the vapor pressure deficit (VPD) of the atmosphere, which increases evaporative demand and thus (all else being equal) increases the rate of water loss from plants and the soil. Increasing VPD negatively affects plant function on its own, but it also increases the risk of compound “soil drought” and “atmospheric drought,” wherein low soil moisture coincides with high temperatures and high VPD. During combined soil and atmospheric drought, plants are thus exposed to moisture stress on both ends of the soil-plant-atmosphere continuum.
Project Objectives and Methods
In our proposed project we ask: are the negative effects of “soil drought” (low soil moisture) and “atmospheric drought” (high VPD) on growth greater than the sum of their parts? Do the legacies of combined soil and atmospheric drought persist longer? And what plant traits and characteristics make tree growth more sensitive to combined soil and atmospheric drought? We will answer these questions in the conterminous U.S. using large tree-ring databases spanning tens of thousands of individual trees across hundreds of sites and dozens of species. We will specifically examine how growth reduction during and time-to-recovery following soil-only drought, atmosphere-only drought, and combined soil and atmospheric drought vary along aridity gradients, among species with different physiological and hydraulic traits, and among trees of different sizes.
Potential Impact of the Project
The proposed project fits within the “Synthesis Studies” Science Research Area, especially the Carbon Cycle and Disturbance sub-topic, as it will “generate new knowledge and mechanistic understanding” on how an important sink for anthropogenic CO2 emissions (forest growth) responds to the kind of global change-type drought that will become increasingly common in a warmer world. Current ecosystem models struggle both to simulate turnover time of carbon and to simulate the legacy effects of drought on plant production and growth. Understanding the extent to which combined soil and atmospheric drought affect forest growth and its post-drought legacies would improve theoretical understanding of forest carbon cycling and guide improvement of ecosystem models, including how both soil and atmospheric drought responses and legacies are shaped by specific plant traits. Our proposed research could also inform understanding of how drought in a warmer world could affect the composition of U.S. forests, which have already been reshaped by anthropogenic land use and fire management in much of the U.S. Our focus on the physiological, hydraulic, and morphological traits that make trees susceptible to combined soil and atmospheric drought will shed light on how forest structure and composition may change in a hotter world.
