Biodiversity and belowground interactions mediate community invasion resistance against a tall herb invader

Aims Species-rich plant communities are hypothesized to be more resistant against plant invasions because they use resources in a more efficient way.However,the relative contributions of aboveground competition and belowground interactions for invasion resistance are still poorly understood.Methods We compared the performance of Knautia arvensis transplants growing in plots differing in plant diversity both under full competition and with shoots of neighbors tied back to determine the relative strength of aboveground competition in suppressing this test invader without the confounding effect of shading.In addition,we assessed the effects of belowground competition and soil-borne pathogens on transplant performance.Important Findings Both aboveground competition and plant species richness strongly and independently affected invader performance.Aboveground biomass,height,leaf mass per area and flowering of transplanted individuals of K.arvensis decreased with increasing species richness of the host community.Species-rich and species-poor communities both imposed equally strong aboveground competition on K.arvensis.However,belowground interactions(especially belowground root competition)had strong negative effects on transplant performance.In addition,the presence of grasses in a plant community further reduced the performance of K.arvensis.Our results suggest that belowground competition can render species-rich host communities more suppressive to newly arriving species,thus enhancing community invasion resistance.

Time course of plant diversity effects on Centaurea jacea establishment and the role of competition and herbivory

Aims Invasion resistance in experimental plant communities is known to increase with increasing diversity and further to depend on the presence of particular functional groups.To test whether these effects also hold true for the invader establishment phase beyond the seedling stage,we studied survival and performance of Centaurea jacea L.(brown knapweed)planted into experimental grassland communities of varying plant biodiversity over three consecutive years.Moreover,we analysed the role of insect herbivory and biomass of the recipient community for mediating diversity effects.Methods In 2005,seedlings of Centaurea were transplanted into experimental grassland communities(the Jena Experiment)covering a species richness(1–60)and functional group richness(1–4)gradient.Half of these transplants and the community surrounding them in each plot were sprayed with insecticide while the other half served as control.In 2006 and 2007(during the second and third year after transplantation),we recorded survival,growth-related(e.g.transplant biomass,height)and reproduction-related traits(e.g.number of flower heads).Annual data on community aboveground biomass served as covariate to investigate mediating effects of aboveground competition with the recipient community.Important Findings Species richness was the most important factor responsible for Centaurea limitation.Higher levels of diversity decreased survival and all performance traits in both years.These diversity effects were partly driven by community biomass,but not fully explained by that covariate,suggesting the importance also of further processes.The influence of functional group richnesswas strong in the second year after transplantation and weaker in the third year.Among the particular functional groups,only the presence of legumes showed strong negative effects on Centaurea survival and weak negative effects on growth and reproduction,the latter two being mediated by biomass.Insect herbivore reduction considerably benefited Centaurea in sprayed monocultures,where it grew significantly larger than in all other diversity levels and than in the control subplots.We conclude that effects of plant community properties on invading individuals change in the course of establishment,that plant species richness effects are also important during later stages of establishment,and that biomass(especially at high diversity)and herbivory(especially at low diversity)of the recipient community are important in mediating community effects on invaders.