Climate change and species invasions are simultaneously affecting many ecosystems worldwide. However, few studies have investigated how climate-related physical stress and competition with invasive species may interact to regulate native plant community structure over time. Climate change has the potential to increase the success and exacerbate the effects of invasions. The southeastern US is expected to experience expanded invasion by Imperata cylindrica (cogongrass), a highly problematic invasive grass, and future climate scenarios predict increased drought conditions for the region. Our goal was to investigate the individual and interactive effects of I. cylindrica invasion and drought on the threatened herbaceous plant communities of longleaf pine forests. We hypothesized that drought would enhance the success of I. cylindrica and its ability to suppress longleaf pines and native plants. We planted longleaf pine seedlings and native herbaceous plant species in 40, 4 m × 4 m plots, to which we then applied a factorial combination of I. cylindrica invasion and rainout shelters to simulate drought. At six times over two years, we recorded plant community composition.
Contrary to our expectation, the drought and invasion treatments did not interact synergistically. Instead, invasion by I. cylindrica reduced the magnitude of the drought treatment such that soil moisture was higher in invaded drought than the uninvaded drought treatments. Additionally, the drought treatment did not significantly affect percent cover of I. cylindrica. By the third year of the experiment, I. cylindrica invasion alone reduced species diversity by 50% relative to controls. In plots experiencing both stressors, species diversity decreased by 35% relative to drought alone, suggesting drought offset the negative effect of invasion. Nonmetric multidimensional scaling ordination showed native plant community composition diverged greatly over time in response to invasion, but showed only minimal response to drought. This result corresponded to overall species losses in the invaded plots versus increased species numbers in the uninvaded plots. Imperata cylindrica plots maintained >60% live cover in February, while <30% live cover remained in native plots, suggesting the extended phenology of I. cylindrica may promote its dominance. Our research indicates that drought may ameliorate some of the effects of plant invasion under this future climate scenario; however significant losses of plant diversity are likely if invasions are left unchecked.