The Effect Of Rapid Evolution And Allelopathy On The Invasiveness Of The Invasive Plant Ambrosia Trifida

Invasions of exotic species strongly influence species composition,structure and function of invaded ecosystems in China;damage biodiversity security and economic development;or even endanger human health.However,we do not know surely the mechanisms of exotic species invasions,which are complicated and various.It is inevitable that after introduction exotic species encounter novel biotic and abiotic environments that are different with those in their native ranges.As a result,exotic species may evolve to adapt to the new environments,promoting fitness and contributing to invasiveness.Ambrosia trifida is a harmful invasive exotic plant in North China.In native range in USA the invader experiences much stronger damage from above and belowground enemies than in invasive range in China.Thus,we hypothesized that A.trifida may evolve to increase competitive ability and nitrogen allocation to the photosynthetic apparatus in responses to the new environments in China.In order to test for the roles of the evolution of increased competitive ability hypothesis,the evolution of nitrogen allocation hypothesis,and the novel weapons hypothesis in invasion success of A.trifida,we first conducted an intraspecific competition experiment with A.trifida plants from 10 invasive and 10 native populations grown in pots in a common garden in the research base of Shenyang Agriculture University.Then,we determined the differences in allelopathy between A.trifida plants from the invasive and native range,and the differences in susceptibility to allelopathy of the invader between three native plants from the invasive range and four native plants from the native range.The main results are as follows:(1)Compared with A.trifida plants from the native range,A.trifida plants from the invasive range showed significantly higher growth vigor and competitive ability,which may be associated with their adaptive evolutionary responses to enemy release in China.(2)Compared with A.trifida plants from the native range,A.trifida plants from the invasive range showed significantly higher root to shoot ratio,which may be associated with their adaptive evolutionary responses to enemy release and/or high resource availability in China.Whatever the reasons,the decreased root to shoot ratio contributes to invasion success in resource-rich habitats in China(3)Compared with A.trifida plants from the native range,A.trifida plants from the invasive range allocated significantly higher fraction of leaf nitrogen to carboxylation and bioenergetics components of photosynthesis,showed higher photosynthetic nitrogen-use efficiency and maximum photosynthetic rate,but had similar leaf nitrogen content and evenlower specific leaf area.The increased nitrogen allocation to photosynthesis for the invader in China may be associated with its decreased nitrogen allocation to nitrogen-based defensive chamicals.(4)Compared with the four native plants from the native range,the three native plants from the invasive range on average were more susceptible to allelochemicals of the invader from noth ranges.Compared with A.trifida plants from the native range,A.trifida plants from the invasive range showed significantly stronger allelopathic effects on the seven native plants from the two countries.Our results indicate that the evolution of increased competitive ability hypothesis,the evolution of nitrogen allocation hypothesis,and the novel weapons hypothesis may act synergetically to contribute to invasion success of A.trifida in China.The increased competitive ability of the invader in China may be resulted from the increased nitrogen allocation to photosynthesis that may be the result of evolutionary responses to enemy release,and the increased allelopathy that may be the result of evolutionary responses to the na?ve native plants.To the best of our knowledge,this is the first time to confirm the roles of these hypotheses in contributing interactionally to invasion success of the same exotic plants.Little effort has been made to test the evolution of nitrogen allocation hypothesis.Our study provides new and direct evidence for this hypothesis.