In this study, a team of researchers determined how the productivity in U.S. grasslands varies with precipitation and vapor pressure deficit. The vapor pressure deficit is the amount of moisture in the air relative to its potential maximum. In fact, grassland productivity is more sensitive to changes in vapor pressure deficit than to changes in precipitation.
This study suggests that new approaches to representing detailed physical characteristics of plants, such as stomatal closure and hydraulic traits, would improve the ability of Earth system models to predict how ecosystems respond to droughts.
Plants open and close stomata (tiny openings on plant leaves) to balance carbon dioxide uptake and water losses, while avoiding desiccation of xylem (plant tissue that conducts water through the plant). Water limitations can reduce photosynthesis rates and therefore growth. However, species differ in their degree of stomatal closure at a given level of aridity. Recent evidence has shown coordination between stomatal closure strategies and the loss of xylem conductance under drought. The two coordinated behaviors can be summarized by the concept of anisohydricity—defined as the slope of leaf water potential variations relative to soil water potential. Isohydric plants exert tight regulation of stomata and the water content of the plant, whereas anisohydric plants do not. Understanding the effects of large-scale variability in ecosystem-scale anisohydricity is crucial, as it may affect the severity of droughts and heatwaves through land-atmosphere interactions, the carbon uptake of terrestrial ecosystems, and plant mortality patterns.
However, anisohydricity and related plant traits have traditionally been determined from in situ measurements, and scaling such measurements to entire ecosystems is challenged by the coexistence of multiple species with a wide variety of drought response traits. Novel remote-sensing–based techniques can now detect the integrated drought response of ecosystems and produce estimates of anisohydricity. In this study, scientists use remote-sensing–based estimates of ecosystem-scale anisohydricity to show that the sensitivity of U.S. grassland vegetation growth, as assessed by average summertime normalized difference vegetation index, is controlled by the degree of anisohydricity. Anisohydric grasslands are three times more sensitive to vapor pressure deficit than isohydric grasslands. Anisohydric grasslands are also more sensitive to precipitation than isohydric ones, but these sensitivities are generally weaker.
The researchers conclude that increases in vapor pressure deficit rather than changes in precipitation will be a dominant influence on future grassland productivity. Current Earth system models do not explicitly represent many plant hydraulic processes, including the variations in water conductivity and resistance in plant tissue that affect isohydricity. Nevertheless, models with an explicit representation of plant water transport are becoming more common and could be incorporated into the next generation of large-scale Earth system models. Arguably a greater challenge is the incorporation of variation in anisohydricity by plant type in such models.
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A.P.W. was funded by Columbia University's Center for Climate and Life. P.G. was supported by a U.S. Department of Energy Early Career Award and a National Science Foundation Faculty Early Career Development Program (CAREER) grant.
A.G. Konings, A.P. Williams, and P. Gentine, "Sensitivity of grassland productivity to aridity controlled by stomatal and xylem regulation." Nature Geoscience 10(4), 284-288 (2017). [DOI: 10.1038/ngeo2903]