Different responses to the 2011 drought between native- and exotic-dominated experimental communities in Texas

Abstract:

Global change includes invasion by non-native species, and invasion may affect ecosystem recovery from drought, especially when they cause species diversity declines. Here we test predictions in central Texas in a six-year experiment that compares mixtures of all exotic or all native species under two summer irrigation treatments (128 or 0 mm) that varies the amount of summer drought stress. A major drought (precipitation of 41% of normal) occurred in year 4 (2011), allowing a “natural experiment” test of how treatments affect drought resistance and recovery. Peak biomass was only 8% of pre-drought values in the drought year of 2011. Biomass resistance (drought/pre-drought biomass) and resilience (post-drought/pre-drought biomass) were similar among treatments. Native communities lost fewer species during the drought, and recovered more species after the drought than exotic communities. Over time, the responses in peak biomass to summer irrigation changed between native and exotic communities, with initially large responses in exotic communities in year 1 diminishing to non-significant responses by years 5 and 6. Native communities on the other hand, showed a small response to summer irrigation in year 1, but this response grew to become a large increase with irrigation by years 4-6. Reduced response to summer irrigation in exotic communities were associated with reduced species diversity and shifts in species composition towards C₄ grasses. The differential effects of the drought indicate that non-native species will alter grassland community responses to drought events.