Plant invasions and climate change, such as enhanced frequency and intensity of drought, are primary environmental threats, yet few studies have evaluated how they interact to affect ecosystems. Drought stress may have synergistic effects with invasion, further suppressing native plant species. Alternatively, invaders may offset drought effects by maintaining soil moisture. We coupled a factorial common garden experiment with a field experiment to assess the independent and interactive effects of cogongrass (Imperata cylindrica) invasion and drought on loblolly (Pinus taeda) and slash (Pinus elliottii) pine. Common garden treatments were a factorial combination of ambient or experimentally reduced (by 89%) precipitation (hereafter “drought”) and native understory plants (uninvaded) or native plants plus cogongrass (invaded). We also tracked survival of pine seedlings at nine field sites invaded by cogongrass and spanning a soil moisture gradient (from ~2-10%) across central Florida. Pine trees were grown in cogongrass-invaded or adjacent uninvaded plots and in plots where resident cogongrass or native understory species were removed with a broad-spectrum herbicide.
In the garden, severe drought in uninvaded plots reduced pine survival by more than 40% whereas invasion reduced loblolly survival by 20% and slash survival by 25% under ambient precipitation. Therefore in isolation, drought and invasion suppressed both pine species. However, in plots where the stressors interacted, loblolly survival in invaded plots differed under ambient [80%] and drought [58%] conditions. Conversely, invasion offset drought effects on slash pine such that survival was similar in invaded plots under ambient [57%] and drought [47%] conditions. Greater slash pine survival under invasion and drought was attributed to higher soil moisture and humidity in invaded plots. In the field experiment, slash but not loblolly pine survival was negatively related to soil moisture. Both loblolly and slash pine were promoted by invader and native plant removal, but these competitive effects were not mediated by soil moisture conditions. Thus, invasion partially ameliorated drought effects on slash pine by buffering environmental stress in the garden experiment, but not across the more benign conditions in the field experiment. These results suggest that invasive plants may interact in surprising ways with climate change factors but that outcomes may be dependent on the severity of environmental conditions.