Plant traits have been recognized as valuable for providing insight into ecosystem responses to climate extremes, such as drought. Such approaches may be especially useful in grasslands where 1-3 dominant plant species are responsible for much of aboveground net primary productivity (ANPP). Grasslands cover >30% of Earth’s terrestrial surface and are particularly sensitive to drought as they are water-limited systems. This sensitivity to drought, defined as the relative reduction of ANPP during drought, can vary two-fold among grassland types (xeric to mesic), but the causes of this variability are still unclear. Here, we take advantage of a natural drought in 2012 to assess whether plant traits can explain the differential drought sensitivity seen in central US grasslands. Drought sensitivity was assessed in six central US grasslands spanning a precipitation gradient of >600mm from desert grasslands to tallgrass prairie. Following the 2012 drought, we measured leaf hydraulic traits of the dominant 5-7 species (representing >80% plant cover) within these same six grasslands to provide a mechanistic understanding of plant responses to drought. Community-weighted plant traits were calculated based on plant cover and regressed against site-level sensitivity to the 2012 drought and mean annual precipitation (MAP).
Drought sensitivity varied greatly along the precipitation gradient with arid sites being more sensitive than mesic sites. Leaf hydraulic conductivity was positively correlated with MAP where more mesic plant communities were dominated by species capable of moving water more efficiently. Leaf vulnerability to cavitation was positively correlated (r2=0.99) with drought sensitivity among the four northern sites where more drought sensitive sites were inhabited by species that were more sensitive to xylem cavitation. When the arid grasslands of New Mexico were included, however, the relationship was not significant, likely because the species inhabiting these monsoon-driven systems exhibit strong stomatal control (anisohydry) and never reach the critical water potential for xylem cavitation to occur. Community-weighed leaf TLP was not correlated with drought sensitivity or mean annual precipitation; however, the coefficient of variation (CV) of TLP (a measure of within-site trait variation) was significantly correlated with drought sensitivity (r2=0.75) with more sensitive sites having a lower CV of leaf TLP. This suggests that within-site variability in plant traits (i.e. trait diversity) may be more important than the trait values themselves for explaining drought sensitivity.