Convergent patterns in multitrophic biodiversity effects on yield across ecosystems

In ecosystems managed for food or fiber production,there is often no space for biodiversity.For example,intensive cereal monocultures are usually managed towards optimized yields using herbicides,fungicides and insecticides.In such systems,biodiversity is essentially unwanted,except maybe when it improves soil health or carbon fixation.Consequently,it has been difficult to convincingly show if and where biodiversity is economically important in intensive production systems.

How forest edge-center transitions in the herb layer interact with beech dominance versus tree diversity

Aims Forest fragmentation and the associated augmentation of forest edge zones are increasing worldwide.Forest edges are characterized by altered plant species richness and community composition.As the tree layer and its species composition has been shown to influence herb layer composition,changes in tree species composition or richness may weaken or strengthen edge effects in forest ecosystems.We studied effects of the edge-center transition,tree species composition and their potential interaction on the understory vegetation in the Hainich National Park,Germany’s largest connected deciduous forest,allowing to cover large edge-center transects.Methods We established 12 transects in an area of 75 km^(2) of continuous forest,6 beech-dominated and 6 in multispecies forest stands.Each transect reached from the forest edge up to 500 m into the forest interior.Vegetation relevés were conducted in regular,logarithmic distances along each transect.Important Findings Herb species richness was influenced by an interaction of edge effects and tree diversity level.With increasing distance from the forest edge,herb species richness remained constant in multispecies forest stands but rapidly decreased in beech-dominated forest stands.Further,herb richness was higher in the interior of multispecies forest stands.Percent forest specialists increased and percent generalists decreased with distance from the edge and this contrasting pattern was much more pronounced in beech-dominated transects.By using structural equation modeling,we identified litter depth mediated by tree species composition as the most important driver of herb layer plant species richness.

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.