Experimental drought and plant invasion additively suppress primary pine species of southeastern US forests

Climate change and non-native invasive species are two predominant drivers of global environmental change, yet little is known about how they might interact to affect native communities and ecosystems. Drought and plant invasions are intensifying in ecosystems worldwide, including ecologically and economically important pine forests of the southeastern United States. These stressors can alter resource availability and plant competition outcomes, and may together exert additive, synergistic, or offsetting effects on native species, but such outcomes are difficult to predict. We used a factorial common garden experiment to determine how simulated drought, invasion by Imperata cylindrica (cogongrass), and their interaction affected seedling survival and performance (relative growth rates of height and diameter, and biomass) of two native pine species, Pinus elliottii var. densa (South Florida slash pine) and Pinus taeda (loblolly pine). In general, loblolly pine outperformed slash pine over the course of the experiment, but the directions and magnitudes of each species' responses to the treatments were similar, with the two stressors often exhibiting additive negative effects on pine seedling performance. For both species, invasion significantly suppressed seedling survival, drought reduced relative growth rates in height, and drought and invasion had an additive negative effect on diameter compared to ambient conditions with resident plant communities. The suppressive effects of drought on these primary pine species suggests that increasing drought in the region could scale up to affect forest stand dynamics. Furthermore, the experimental demonstration of cogongrass impacts on pine seedling survival and performance should further motivate land owners and property managers to remove this noxious invasive species. To predict the long-term outcome of drought and invasion on forest stands, and more broadly on vegetation dynamics in ecosystems affected by these global change agents, additional evaluations of their separate and interactive effects are needed. Nonetheless, these results experimentally demonstrate that stress from experimental drought combined with competition from an aggressive grass invader can significantly suppress seedlings of primary pine species of southeastern US forests.