Influence of precipitation dynamics on plant invasions: response of alligator weed (Alternanthera philoxeroides) and co-occurring native species to varying water availability across plant communities

Over the last few decades, rainfall has become more variable, with a worldwide increase in the frequency of extreme precipitation events. While increases in rainfall variability are expected to significantly affect plant species, the effects of precipitation dynamics on plant invasions remains understudied. We examined the growth response of Alternanthera philoxeroides to varying water availability at the community level when grown in a monoculture and mixed community with four commonly co-occurring native species. The native species were also grown together as a control to compare to the invasive species communities. Seven water treatments were applied to each community, including a ‘normal’ treatment (based on the average rainfall for the area), drought (water was one third of the normal treatment), flood (water was double the normal treatment), and other simulated extreme events (where water availability was more variable than the normal treatment). All plants were measured for growth (e.g., biomass and stolon length) and competitive traits (i.e., relative competitive dominance and relative interaction indices). Most growth traits in A. philoxeroides were enhanced with increased and/or more variable water availability in both the monoculture and mixed community. In contrast, differences in growth traits in the native plants were mostly non-significant across treatments in each plant community. Compared to the normal treatment, A. philoxeroides had higher relative competitive dominance (RDI) and interaction (RII) indices in response to flooding. Conversely, RII in the native species was not significantly different among treatments. Overall, our results suggest that A. philoxeroides is more responsive to flooding compared to the natives. Our study provides useful insights into growth response and competitiveness of A. philoxeroides to precipitation variability across different plant communities.