Tolerance and resistance facilitate the invasion success of Alternanthera philoxeroides in disturbed habitats: A reconsideration of the disturbance hypothesis in the light of phenotypic variation

Disturbance has been considered to exert positive, neutral or negative effects on the invasion of exotic plants. Disturbance may enhance the invasion success of alien invasive species by both increasing their performance and alleviating the harmful effects caused by disturbance via tolerance and resistance, especially in the late phases of the invasion process. However, the tolerance and resistance of invasive plant clonal populations to disturbance has been mostly neglected. In the present study, we explored the phenotypic responses of clonal populations of the invasive plant Alternanthera philoxeroides and three co-occurring non-invasive clonal species, i.e., Alternanthera sessilis, Myriophyllum aquaticum and Jussiaea repens, to different levels of fluctuating flood disturbances using a mesocosm experiment. Different functional traits related to morphology, biomass allocation and physiology were used to evaluate plant performance. We found that A. philoxeroides coped with fluctuating flooding by performing phenotypic maintenance that affected ramet number, stolon length, specific leaf area, leaf mass ratio, chlorophyll content, nitrogen content and non-structural carbohydrate content; additionally, the plant experienced variations in stolon diameter, stolon mass ratio, root mass allocation and maximum fluorescence. In addition, Alternanthera philoxeroides demonstrated several similarities with the three non-invasive species in terms of the observed phenotypic responses to different disturbance statuses. Furthermore, Alternanthera philoxeroides showed respective advantages in tolerance compared to A. sessilis, photosynthetic capacity compared to M. aquaticum and especially J. repens, and resistance to submergence compared to all three non-invasive species. Overall, tolerance and resistance facilitated the successful invasion of A. philoxeroides in disturbed habitats. Furthermore, the native constructive species J. repens was severely harmed by fluctuating flooding; thus, fluctuating flooding could create an empty niche that promotes the expansion of the clonal populations of A. philoxeroides.