Physiological drought tolerance and the structuring of tallgrass prairie assemblages

Drought is a defining characteristic of many grasslands worldwide. Yet we have little understanding of how drought structures grassland communities and the degree to which physiological drought tolerance advantages plants in grasslands. We characterized physiological drought tolerance (Ψcrit) for a large number of species in a mesic grassland community (Konza Prairie, KS, USA). We then examined the relationships between Ψcrit and a number of other key functional traits, and tested whether physiological tolerance of drought underlay success across a number of ecological contrasts—topographic position, burn frequency, and grazing—with 17 years of abundance data. Physiological drought tolerance of Konza species covered almost the full range known to plants globally. Consistently, physiologically drought-tolerant species had thin roots, while associations with other traits were inconsistent across functional groups. In this mesic grassland, physiological drought tolerance appears to increase the abundance of plants in xeric uplands, but does not in the mesic lowlands. Physiological drought tolerance did not alter species responses to changes in burning or grazing. In contrast to Ψcrit, species with high root tissue density were more abundant in uplands and lowlands than species with low root tissue density largely irrespective of grazing or burning regimes. In all, drought appears to have a limited role in structuring the Konza plant community. As such, more severe or frequent droughts in the region would likely restructure the Konza plant community in ways that are currently not observable.