Climate change is intensifying drought, thereby altering ecosystem structure and functions. Potentially compounding drought effects are non-native plant invasions, which also can suppress native species and modify ecosystems. However, while multiple global change stressors, including drought and invasions, are predicted to impose novel stress on ecosystems, most studies measure plant responses to only a single stressor. It thus remains unknown whether multiple stressors will have additive, synergistic, negative, or even offsetting effects on native plant performance. We used a factorial common garden design to examine the individual and interactive effects of simulated drought and invasion by non-native, invasive cogongrass (Imperata cylindrica) on the survival and growth (tree diameter and height) of longleaf pine (Pinus palustris), a species of critical ecological importance in the southeastern US. We implemented four treatments in a randomized block design (4 treatments x 10 blocks = 40 plots): 1) reduced precipitation (hereafter “drought”) with native understory species; 2) drought with native understory species and cogongrass; 3) ambient precipitation with native understory species; and 4) ambient precipitation with native understory species and cogongrass. To quantify drought and invasion effects on environmental conditions relevant to tree performance, we measured soil moisture, light levels, temperature, and humidity.
Longleaf pine survival was significantly reduced by drought (by 31% in 2014 and 23% in 2015), and cogongrass invasion also independently suppressed pine survival (20% reduction in 2014 and 23% in 2015) under ambient precipitation. However, cogongrass invasion strongly facilitated longleaf survival under drought conditions (by 27.5% in 2014 and 19.5% in 2015). There was a similar facilitative effect of invasion under drought conditions for tree diameter. Trees exposed to ambient precipitation had similar diameter (17 cm) regardless of invasion status, whereas tree size was significantly greater with invasion (17.5 cm) than with native species (15.5 cm) under drought conditions. Larger diameter trees may have resulted in more efficient water uptake and translocation for trees in invaded plots. Additionally, invaded plots had higher soil moisture and humidity and lower temperature than uninvaded plots, suggesting that the dense canopy created by cogongrass mitigates the environmental stress of drought, thereby promoting longleaf survival and performance. These results highlight the need for more experiments designed to uncover the interactive effects of multiple climate change stressors, the effects of which cannot be extrapolated from each stressor acting in isolation.