Fluctuating flooding motivates the facilitation of a wetland invader: a probe into the stress gradient hypothesis

Stress gradient hypothesis proposes that the plant interaction tends to convert from competition to facilitation when facing accelerating stress. The present study aims to explore the interaction between two co-existing wetland plant invaders - Alternanthera philoxeroides and Myriophyllum aquaticum with exposure to flooding stress. Different plant assemblages including monospecific (single individual per species), homospecific (two individuals of same species) and heterospecific (two individuals of different species) treatments were subjected to different flooding regimes including mesic habitat, constant flooding and fluctuating flooding. The plant interaction was evaluated by biomass accumulation, biomass allocation and relative competition intensity index (RCI). We found that the biomass accumulation and allocation of A. philoxeroides were consistent across different treatments of community assemblage and flooding regime, while the root mass, biomass and root mass allocation of M. aquaticum significantly increased at the presence of A. philoxeroides. Moreover, the RCI of M. aquaticum decreased at the presence of A. philoxeroides. Especially, the RCI of M. aquaticum was 0.06 in the homospecific community, while that of M. aquaticum was -0.82 in the heterospecific treatment under fluctuating flooding condition. Overall, Alternanthera philoxeroides facilitated M. aquaticum, especially under fluctuating flooding condition, which complies with the stress gradient hypothesis. The water level drawdown can benefit the impedance of the growth of M. aquaticum and the deceleration of the co-invasion of A. philoxeroides and M. aquaticum.