Modeling and measurements show plant water loss responds more to dry soil than dry air at the Barro Colorado Island during the 2015-2016 El Niño drought event.
Water stress from dry soil and vapor pressure deficit (VPD) can both limit plant transpiration and hence plant response to drought. However, separating the response of plant functioning to these two interactive stresses is challenging. Using statistical models fitted to two types of field observation data and results from a land surface model with an added capability to simulate water movement in the soil and water transport within the plant at a tropical forest site in Panama, this study found that dry soil is more important than VPD in limiting plant water loss at the site during the El Niño drought of 2015-2016.
Carbon sequestered by tropical forests during normal and wet years can be released during drought years due to tree mortality and reduced ecosystem productivity. Recent drought-related plant mortality has been attributed to increasing VPD associated with climate change. This research disentangled the relative impact of VPD and soil water stress on canopy conductance that controls plant transpiration at a tropical forest site in Panama. The results highlighted the need for new data collection as well as new model development to improve understanding of tropical forest responses to drought.
In this research, field data and numerical modeling were used to isolate the impact of dry soil and vapor pressure deficit (VPD) on evapotranspiration (ET) and gross primary productivity (GPP) at a tropical forest site in Barro Colorado Island (BCI), Panama, focusing on their response to the drought induced by the El Niño event of 2015-2016. Numerical simulations were performed using a plant hydrodynamic scheme (HYDRO) and a heuristic approach that ignores stomatal sensitivity to leaf water potential in DOE’s Energy Exascale Earth System Model (E3SM) Land Model (ELM). The sensitivity of canopy conductance to (VPD) obtained from eddy-covariance fluxes and measured sap flux shows that, at both ecosystem and plant scale, soil water stress is more important in limiting canopy conductance than VPD at BCI during the El Niño event. The model simulations confirmed the importance of water stress limitation on canopy conductance, but overestimated the VPD impact compared to that estimated from the observations. During the dry season at BCI, seasonal ET, especially soil evaporation at VPD > 0.42 kPa, simulated using HYDRO and ELM, was too strong and will require alternative parameterizations.
Contacts (BER PM): Daniel Stover, SC-23.1, Daniel.Stover@science.doe.gov (301-903-0289)
PI Contact: Ruby Leung, Pacific Northwest National Laboratory, firstname.lastname@example.org
This research was supported by the U.S. Department of Energy Office of Biological and Environmental Research as part of the Terrestrial Ecosystem Science program through the Next Generation Ecosystem Experiment (NGEE) Tropics project.
Fang, Y., Leung, L. R., Wolfe, B. T, Detto, M., Knox, R. G., McDowell, N. G., et al. (2021). Disentangling the effects of vapor pressure deficit and soil water availability on canopy conductance in a seasonal tropical forest during the 2015 El Niño drought. Journal of Geophysical Research: Atmospheres, 126, e2021JD035004. https://doi.org/10.1029/2021JD035004