September 03, 2021
How Can Scientists Better Detect Signs of Forest Water Stress from Space?
Using microwave remote sensing to observe vegetation water content.
Hot droughts are becoming more common because of climate change, but scientists still do not know how forests respond to water stress conditions. Field measurements are too sparse, and most satellite measurements cannot detect early signs of water stress in forests. A team of researchers with expertise from field measurements, remote sensing, and numerical models identified opportunities and challenges for using water content from remote sensing to detect water stress. Microwave measurements from space can be used to estimate vegetation water content and detect water stress across all forests on Earth.
Vegetation water content is a useful measure of forest health and function because plants lose internal water when they are stressed or when they die. The research team identified the need for a satellite observational system to measure water content, which would provide data on day-to-day water fluxes and help identify the earliest signs of water stress in forests.
This research review describes how extensive and frequent estimates of vegetation water content from microwave remote sensing could improve scientists’ ability to detect signs of water stress and anticipate critical conditions for fire and mortality in forests across the world. Vegetation water content estimates could also allow for inference of belowground soil moisture and root water uptake conditions across large scales, which is challenging otherwise. Additionally, this review identified the need to establish relationships between vegetation water content and ecosystem-scale water potential to be able to detect signs of stress across different forest systems, and to be able to effectively link remote sensing measurements with terrestrial biosphere models. Improving methods will also be critical for distinguishing variations in water content due to changes in surface water (dew and rainfall interception) and changes in water stored inside plants. Moreover, this review points to the need for field campaigns that will help establish the volume-potential relationships at ecosystem scale, which are critical to define thresholds for wilting, mortality, and fire risks in different forests. Finally, the monitoring of forest water stress could greatly benefit from geostationary measurements of vegetation water content, as it would provide information at a sub-daily scale, which could be more directly related to field measurements and improve the quantification of water stress.
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
This work is a result from discussions initiated at the Sensing Forest Water Dynamics from Space: Towards Predicting the Earth System Response to Droughts workshop, which supported by the W.M. Keck Institute for Space Studies. This research was partially supported by the Next Generation Ecosystem Experiments-Tropics, funded by the Biological and Environmental Research (BER) Program within the U.S. Department of Energy’s (DOE) Office of Science. This research was also funded by NSF and NASA Terrestrial Ecology. The research carried out at the Jet Propulsion Laboratory, California Institute of Technology, was under a contract with the National Aeronautics and Space Administration. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. DOE.
Konings, A.G., et al. "Detecting Forest Response to Droughts with Global Observations of Vegetation Water Content." Global Change Biology 27 (23), 6005–6024 (2021). https://doi.org/10.1111/gcb.15872.