Plant density affects measures of biodiversity effects

Aims:We tested for the effect of final sowing plant density(i.e.density of established seedlings)on the values of biodiversity effects[transgressive overyielding,net effect,complementarity effect(CE)and selection effect(SE),trait-dependent complementarity and dominance effect]in a glasshouse pot experiment.Methods:We conducted a single-season(4 months)glasshouse experiment.Species monocultures and mixtures containing up to four common meadow species from different functional groups were sown and subsequently thinned to five density levels(8–128 individuals per pot,i.e.200–3200 individuals m 2).Community functioning was characterized by yield(both living and dead biomass)of all constituent species.Important Findings:Our results show that plant density(final sowing density in our case,but this finding can be generalized)affects the yields of both monocultures and mixtures.As these and their relationships are the basis for calculation of biodiversity effects,these effects also varied along the density gradient.Net biodiversity effect,CE and SE all increased with density.The net biodiversity effect and the CE switched from negative to quite positive in the four-species mixture.Using Fox’s tripartite partitioning,trait-dependent complementarity was minor in comparison to the dominance effect.One of our experimental species did not follow the density–productivity relationship,called constant final yield(CFY),which was reflected in the biodiversity measures.The shape of the density–productivity relationship for experimental species affects also the values of biodiversity indices,particularly when species do not follow the CFY relationship.According to our data and recent simulation experiments,the values of commonly used biodiversity effects can be rather misleading if a species has,e.g.a unimodal dependence of yield for the density gradient and the density level used in the experiment is higher than the peak density.

Forest biodiversity, relationships to structural and functional attributes, and stability in New England forests

Background： Forest biodiversity is the foundation of many ecosystem services, and the effect of biodiversity on ecosystem functioning and processes （BEF） has been a central issue in biodiversity studies. Although many hypotheses have been developed to interpret global gradients of biodiversity, there has not been complete agreement on mechanisms controlling biodiversity patterns and distributions. Differences may be due to limited observation data and inconsistencies of spatial scales in analysis. Methods： In this study, we take advantage of USDA Forest Service forest inventory and analysis （FIA） data for exploring regional forest biodiversity and BEF in New England forests. The FIA data provide detailed information of sampled plots and trees for the region, including 6000 FIA plots and more than 33,000 individual trees. Biodiversity models were used to analyze the data. Results： Tree species diversity increases from the north to the south at a rate about 2-3 species per latitudinal degree. Tree species diversity is better predicted by tree height than forest age or biomass. Very different distribution patterns of two common maple species, sugar maple （Acer sdcchorum） and red maple （Acer rubrum）, highlight the vulnerability of sugar maple and its potential replacement by red maple on New England landscapes. Red maple generally already outperforms sugar maple, and will likely and continuously benefit from a changing climate in New England. Conclusions： We conclude that forest structure （height） and resources （biomass） are more likely foundational characteristics supporting biodiversity rather than biodiversity determining forest productivity and/or biomass. The potential replacement of red maple for sugar maple in the New England areas could affect biodiversity and stability of forest ecosystem functioning because sugar maple plays important ecological roles distinct from red maple that are beneficial to other tree species in northern hardwood forests. Such a change may not affect forest resilience in terms of forest productivity and biomass as these are similar in red maple and sugar maple, however, it would almost certainly alter forest structure across the landscape.

Can root trait diversity explain complementarity effects in a grassland biodiversity experiment?

Aims The positive relationship between plant biodiversity and com-munity productivity is well established.However,our knowledge about the mechanisms underlying these positive biodiversity effects is still limited.One of the main hypotheses is that com-plementarity in resource uptake is responsible for the positive biodiversity effects:plant species differ in resource uptake strat-egy,which results in a more complete exploitation of the avail-able resources in space and time when plant species are growing together.Recent studies suggest that functional diversity of the community,i.e.the diversity in functional characteristics(‘traits’)among species,rather than species richness per se,is important for positive biodiversity effects.However,experimental evidence for specific trait combinations underlying resource complemen-tarity is scarce.As the root system is responsible for the uptake of nutrients and water,we hypothesize that diversity in root traits may underlie complementary resource use and contribute to the biodiversity effects.Methods In a common garden experiment,16 grassland species were grown in monoculture,4-species mixtures differing in root trait diversity and 16-species mixtures.The 4-species mixtures were designed to cover a gradient in average rooting depth.Above-ground biomass was cut after one growing season and used as a proxy for plant productivity to calculate biodiversity effects.Important Findings Overall,plant mixtures showed a significant increase in biomass and complementarity effects,but this varied greatly between com-munities.However,diversity in root traits(measured in a separate greenhouse experiment and based on literature)could not explain this variation in complementarity effects.Instead,complementa-rity effects were strongly affected by the presence and competitive interactions of two particular species.The large variation in comple-mentarity effects and significant effect of two species emphasizes the importance of community composition for positive biodiversity effects.Future research should focus on identifying the traits asso-ciated with the key role of particular species for complementarity effects.This may increase our understanding of the links between functional trait composition and biodiversity effects as well as the relative importance of resource complementarity and other underly-ing mechanisms for the positive biodiversity effects.

Effects of plant species richness on stand structure and productivity

Aims Aboveground biomass production commonly increases with species richness in plant biodiversity experiments.Little is known about the direct mechanisms that cause this result.We tested if by occupying different heights and depths above and below ground,and by optimizing the vertical distribution of leaf nitrogen,species in mixtures can contribute to increased resource uptake and,thus,increased productivity of the community in comparison with monocultures.Methods We grew 24 grassland plant species,grouped into four nonoverlapping species pools,in monoculture and 3-and 6-species mixture in spatially heterogeneous and uniform soil nutrient conditions.Layered harvests of above-and belowground biomass,as well as leaf nitrogen and light measurements,were taken to assess vertical canopy and root space structure.Important Findings The distribution of leaf mass was shifted toward greater heights and light absorption was correspondingly enhanced in mixtures.How ever,only some mixtures had leaf nitrogen concentration profiles predicted to optimize whole-community carbon gain,whereas in other mixtures species seemed to behave more‘selfish’.Nevertheless,even in these communities,biomass production increased with species richness.The distribution of root biomass below ground did not change from monocultures to three-and six-species mixtures and there was also no indication that mixtures were better than monocultures at extracting heterogeneously as compared to homogeneously distributed soil resources.We conclude that positive biodiversity effect on aboveground biomass production cannot easily be explained by a single or few common mechanisms of differential space use.Rather,it seems that mechanisms vary with the particular set of species combined in a community.

Drought weakens the positive effect of plant diversity on community biomass

Drought can greatly impact the biodiversity of an ecosystem and play a crucial role in regulating its functioning.However,the specific mechanisms by which drought mediate the biodiversity effect(BE)on community biomass in above-and belowground through functional traits remain poorly understood.Here,we conducted a common garden experiment in a greenhouse,which included two plant species richness levels and two water addition levels,to analyze the effects of biodiversity on aboveground biomass(AGB),belowground biomass(BGB)and total biomass(TB),and to quantify the relationship between BEs and functional traits under drought conditions.Our analysis focused on partitioning BEs into above-and belowground complementarity effect(CE)and selection effect(SE)at the species level,which allowed us to better understand the impacts of biodiversity on community biomass and the underlying mechanisms.Our results showed that plant species richness stimulated AGB,BGB and TB through CEs.Drought decreased AGB,BGB and TB,simultaneously.In addition,the aboveground CE was positively associated with the variation in plant height.SEs in above-and belowground were negatively correlated with the community mean plant height and root length,respectively.Furthermore,drought weakened the aboveground CE by decreasing variation in plant height,resulting in a reduction in AGB and TB.Our findings demonstrate that the complementarity of species is an important regulator of community biomass in above-and belowground,the dynamics of biomass under environmental stress are associated with the response of sensitive compartments.

Complementarity among species in horizontal versus vertical rooting space

Aims Many experiments have shown a positive effect of species richness on productivity in grassland plant communities.However,it is poorly understood how environmental conditions affect this relationship.We aimed to test whether deep soil and limiting nutrient conditions increase the complementarity effect(CE)of species richness due to enhanced potential for resource partitioning.Methods We grew monocultures and mixtures of four common grassland species in pots on shallow and deep soil,factorially combined with two nutrient levels.Soil volume was kept constant to avoid confounding soil depth and volume.Using an additive partitioning method,we separated biodiversity effects on plant productivity into components due to species complementarity and dominance.Important findings Net biodiversity and complementarity effects were consistently higher in shallow pots,which was unexpected,and at the low nutrient level.These two results suggest that although belowground partitioning of resources was important,especially under low nutrient conditions,it was not due to differences in rooting depths.We conclude that in our experiment(i)horizontal root segregation might have been more important than the partitioning of rooting depths and(ii)that the positive effects of deep soil found in other studies were due to the combination of deeper soil with larger soil volume.

Genotypic diversity and genotype identity of resident species drive community composition

Aims Species-rich plant communities are more resistant to invasions.In the past decade it was demonstrated that genetic variation also has many ecological effects.In our study we aimed to test whether the patterns of response to the genetic diversity of a resident species differ between colonizing species of different growth forms and whether the response is affected by soil nutrients.Methods We established experimental stands of a common grass,Festuca rubra,harbouring three levels of genetic diversity(1,6 or 18 clonal genotypes,referred to as genotypic diversity)under two soil nutrient levels.In the fourth year after the stands were established,we sowed a mixture of four colonizers into the stands:a stoloniferous legume(Trifolium repens),a broad-leaf tussock grass(Anthoxanthum odoratum),a largerosette forb(Plantago lanceolata)and a small-rosette forb(Campanula rotundifolia).We observed species establishment and growth over 3 years.We tested whether colonization success depended on genotypic diversity,specific Festuca genotypes,soil nutrients and colonizer growth form.Important Findings The colonization success and biomass of the colonizers were significantly affected by the genotypic diversity and the genotype identity of the resident clonal grass.The response,however,differed between the colonizers.The strongest response to the genotypic diversity of the resident species was observed in the tussock grass with a growth form and architecture similar to the resident species.The large-rosette species responded in early stages of growth whereas the stoloniferous legume did not respond at all.The intraspecific genotypic diversity and genotype identity of the resident species play an important role in the assembly of plant communities.