Sensitivity of grassland productivity to aridity controlled by stomatal and xylem regulation

The terrestrial water and carbon cycles are coupled through plant regulation of stomatal closure. Both soil moisture and vapour pressure deficit-the amount of moisture in the air relative to its potential maximum-can govern stomatal closure, which reduces plant carbon uptake. However, plants vary in the degree to which they regulate their stomata-and in association, xylem conductance-in response to increasing aridity: isohydric plants exert tight regulation of stomata and the water content of the plant, whereas anisohydric plants do not. Here we use remote-sensing data sets of anisohydricity and vegetation greenness to show that productivity in United States grasslands-especially anisohydric ones-is far more sensitive to variations in vapour pressure deficit than to variations in precipitation. Anisohydric ecosystem productivity is over three times more sensitive to vapour pressure deficit than isohydric ecosystem productivity. The precipitation sensitivity of summer productivity increases with anisohydricity only for the most anisohydric ecosystems. We conclude that increases in vapour pressure deficit rather than changes in precipitation-both of which are expected impacts of climate change-will be a dominant influence on future grassland productivity.