Identifying mechanisms of non-native plant invasions is critical for predicting invasions, understanding impacts on native communities, and developing management strategies. Enhanced acquisition of limiting resources is a frequently cited mechanism to explain the establishment of plant invaders. Climate models predict substantial alteration of rainfall patterns across much of the globe, thus enhanced water acquisition by invaders may provide a competitive advantage over co-occurring native species. However, understanding how climate change-associated changes in water availability will interact with invasion to dictate community structure is complex. Here we evaluated the competitive effects of a highly invasive C4 grass on native plant species under reduced water availability.
We established a greenhouse experiment to test for water competition between cogongrass and two native species (wiregrass and longleaf pine) under well-watered (900ml/day) and reduced (450ml every fourth day) treatments. Plant treatment combinations included each species grown alone, each native species in competition with the invader, and all three species planted together. A deuterium-labled water (D2O) tracer was applied to evaluate differential water uptake by each species under the treatment conditions. We also measured multiple performance indicators, including aboveground biomass, stomatal conductance (gs), and carbon assimilation (A) to examine species-specific responses to drought and competition.
Results/Conclusions
We found that D2O uptake, gs, A, and aboveground biomass of the invader was unaffected by competition with longleaf pine or wiregrass or by simulated drought conditions. Despite its superior performance, cogongrass δD values were significantly lower than either native species (p < .05 for both species), suggesting that it was not competing with native species for water. This result is likely because cogongrass displayed significantly higher instantaneous water use efficiency (A/gs) than either native species (p< .001 for both comparisons), and thus required less water to sustain growth and performance. It is also possible that the dense rhizome system of cogongrass sequestered water from previous watering events, thereby diluting the D2O tracer signature. In contrast, cogongrass competition reduced native species gs, A, and aboveground biomass by 50%, 65%, and 88%, respectively. These results suggest that cogongrass maintains consistent performance (A, biomass production) regardless of competition or water availability, while significantly suppressing native species. Furthermore, reduced water alone did not negatively affect native species performance, suggesting that the effects of plant invasions may outweigh climate change effects such as drought in structuring invaded plant communities.