Effects of vegetative propagule pressure on the establishment of an introduced clonal plant, Hydrocotyle vulgaris.

Some introduced clonal plants spread mainly by vegetative (clonal) propagules due to the absence of sexual reproduction in the introduced range. Propagule pressure (i.e. total number of propagules) may affect the establishment and thus invasion success of introduced clonal plants, and such effects may also depend on habitat conditions. A greenhouse experiment with an introduced plant, Hydrocotyle vulgaris was conducted to investigate the role of propagule pressure on its invasion process. High (five ramets) or low (one ramet) propagule pressure was established either in bare soil or in an experimental plant community consisting of four grassland species. H. vulgaris produced more total biomass under high than under low propagule pressure in both habitat conditions. Interestingly, the size of the H. vulgaris individuals was smaller under high than under low propagule pressure in bare soil, whereas it did not differ between the two propagule pressure treatments in the grassland community. The results indicated that high propagule pressure can ensure the successful invasion in either the grass community or bare soil, and the shift in the intraspecific interaction of H. vulgaris from competition in the bare soil to facilitation in the grassland community may be a potential mechanism.