The propagule supply, litter layers and canopy shade in the littoral community influence the establishment and growth of Myriophyllum aquaticum

The exotic macrophyte species Myriophyllum aquaticum (Vell.) Verdc. is widely dispersed, mainly through stem fragments and rhizomes, in shallow littoral zones in which uniform canopy conditions are created by the pre-existing resident wetland plant species. Therefore, interactions between these two types of asexual propagules and the resident species may occur during the early establishment and growth phases of M. aquaticum. We tested the hypothesis that the establishment and growth of M. aquaticum are affected by three factors: the number of propagules supplied, the presence of a standing biomass of Carex japonica Thunb. and the presence of thick litter layers in the littoral zone. M. aquaticum rhizomes and stem fragments were introduced into a standing biomass of C. japonica in mesocosms using two types of sediments (littoral and sand-clay) and two propagule supply levels. After 4 months, the survival rates of both the rhizomes and stem fragments were high under all of the experimental treatments. The propagule supply positively affected the survival rates and growth of M. aquaticum. The survival of the rhizomes was unaffected by the presence of either C. japonica or littoral sediments, whereas the survival of the stem fragments was reduced by the presence of C. japonica. The presence of litter layers is a primary factor facilitating the growth of M. aquaticum propagules because of the high nutrient content of the litter. In addition, the presence of the standing C. japonica biomass and newly growing Eleocharis yokoscensis (Franch.et Sav.) Tang et Wang (a small ruderal species that, unexpectedly, grew rapidly following the removal of the standing biomass of C. japonica) reduced the growth of the M. aquaticum stem fragments and rhizomes, respectively. Our findings suggest that the loss of vegetative cover resulting from intense cattle herbivory and other factors in littoral zones may accelerate the invasion of M. aquaticum. An effective approach for preventing M. aquaticum invasion is to reduce the propagule supply and prevent propagules from dispersing.