Effects Of Allelopathy Of Plant Communities On Their Invasibility

Successful plant invasions depend,at least partly,on interactions between introduced plants and native plant communities.The invasibility of native plant communities and invasiveness of alien species are equally important.Introduced species that successfully invade native communities are frequently regarded as having distinguished characteristics such as strong competitive ability,high nutrient absorption and utilization,ruderal strategy and capable of clonal growth.There is increasing evidence that several below-ground processes such as root foraging' and interactions with soil microbes contribute to the outcome of competition.Allelopathy is also regarded as one part of the below-ground processes.While allelopathic effects of introduced invaders on native resident species have received much attention,the reverse,i.e.allelopathic effects of native residents on introduced plants,have been largely neglected.Therefore,three greenhouse experiments were conducted to test whether allelopathy of native plant communities decreases their invasibility and whether the characteristics of native communities affect such allelopathic effects.Activated carbon was utilized as an allelopathy neutralizer in all experiments,and severd as controls.Nine pairs of related(congeneric or confamilial)invasive and non-invasive introduced plant species(i.e.18 species)were grown in the presence or absence of a native grassland community,which consisted of three forbs and three grasses,with or without activated carbon in the soil.Activated carbon reduced the survival rate and growth of introduced plants in the absence of the native plant community.However,its net effect on the introduced plants was neutral or even slightly positive in the presence of the native community.This might suggest that the native plant community imposed allelopathic effects on the introduced plants,and that these effects were neutralized orreduced by activated carbon.The invasive and non-invasive introduced plants,however,did not differ in their tolerance to such allelopathic effects of the native plant community.Thus,although allelopathy of native plant communities might increase their resistance against introduced plants,there was no evidence that tolerance to allelopathy of native plant communities contributes to the degree of invasiveness of introduced plants.The invasive species Solidago canadensis was grown as target species in the absence of native communities or presence of native grassland communities with high or low species diversity,which consisted of four species or only one species,respectviely,with or without activated carbon in the soil.Activated carbon significantly reduced the growth of the invasive species in the absence of the native plant community.However,such a negative effect of activated carbon on the invasive species was more neutral when species diversity of the native communiy was higher.This suggests that the native plant community imposed allelopathic effects on the introduced plants,and that such an effect increased with increasing species diveristy of the native community.The invasive species S.canadensis was grown as target species in the absence of native communities or presence of native grassland communities of high or low density,which consisted of two legumes,two forbs and four grasses,with or without activated carbon in the soil.Activated carbon reduced the growth of the invasive species in the absence of the native plant community.However,the effect activated carbon on the invasive species was less negative in the presence of the native community of low density and became positive in the presence of the native community of high density.Thus,the native plant communities imposed allelopathic effects on the invasive species,and that such an impact increased with increasing density.These results indicate that allelopathy of native communities exists commonly and is potentially a mechansim for native communities to resist invasion by alien species.Furthermore,allelopathy of native communities increses with increasing species diveristy and density of the communities.However,there was no evidence that tolerance to allelopathy of native plant communities contributes to the degree of invasiveness of introduced plants.