Land-atmosphere drivers of landscape-scale plant water content loss using satellite observations

Drought events limit photosynthesis across the globe often with joint water-limitation and high temperatures. While effects of prolonged droughts on vegetation have received much attention, environmental drivers of short-term, landscape-scale plant drying, especially at the initial stages of drought, are less understood. We evaluate three-day scale plant water content drying using vegetation optical depth observations from NASA Soil Moisture Active Passive (SMAP) measurements. Following rainfall, landscape-scale satellite observations show that during soil drying, plants lose water content while solar radiation, vapor pressure deficit, and sensible heat flux increase. Using a Granger Causality framework, we quantify the degree to which these environmental factors drive observed plant drying across the diverse biomes of Africa. We find that soil drying drives widespread plant water content loss across drier biomes of Africa, with regional effects of surface heating/atmosphere drying. We detect interactions between these factors that tend to reinforce plant water content loss. For example, soil drying increases surface temperatures which we find further dries plants at the landscape scale. Our findings highlight the role of both surface and atmospheric dryness as well as soil-plant-atmosphere interactions in driving plant drying at the initial stages of drought. Our study also indicates that novel satellite vegetation optical depth observations show notable responses to short term surface and atmospheric variability. Such large-scale plant behavior provides benchmark data for representation of plant function in Earth system models.