Species Abundances Influence the Net Biodiversity Effect in Mixtures of Two Plant Species

Species abundances (evenness or identity of the dominant species in mixtures) usually are not rigorously controlled when testing relationships between plant production and species richness and may be highly dynamic in disturbed or early successional communities. Changes in species abundances may affect the yield of mixtures relative to yields expected from species monocultures [the net biodiversity effect (NBE)] by changing how species that differ in function are distributed in the plant community. To test the prediction that variation in species abundances affects the NBE via changes in the expression of functional differences among species (the complementarity effect), we grew perennial grasses and forbs in field plots in central Texas, USA, as equal-density monocultures and two-species mixtures in which relative abundances of species were varied. Function should differ more consistently between species of different growth forms than of the same growth form. We predicted, therefore, that the complementarity effect and influence of species abundances on the NBE would be more pronounced in grass/forb mixtures than in mixtures with species of the same growth form (grass/grass and forb/forb mixtures). The NBE varied with species evenness in two of the six species pairs studied and with identity of the dominant species in a third species combination. The NBE was sensitive to species proportions in both grass/grass and grass/forb assemblages. In all combinations in which the NBE differed with either evenness or identity of the dominant species, the variation resulted largely from change in the complementarity effect. Our results suggest that the NBE of mixtures is sensitive to effects of species ratios on complementarity

Do species evenness and plant density influence the magnitude of selection and complementarity effects in annual plant species mixtures?

Plant species richness influences primary productivity via mechanisms that (1) favour species with particular traits (selection effect) and (2) promote niche differentiation between species (complementarity). Influences of species evenness, plant density and other properties of plant communities on productivity are poorly defined, but may depend on whether selection or complementarity prevails in species mixtures. We predicted that selection effects are insensitive to species evenness but increase with plant density, and that the converse is true for complementarity. To test predictions, we grew three species of annuals in monocultures and in three-species mixtures in which evenness of established plants was varied at each of three plant densities in a cultivated field in Texas, USA. Above-ground biomass was smaller in mixtures than expected from monocultures because of negative \u27complementarity\u27 and a negative selection effect. Neither selection nor complementarity varied with species evenness, but selection effects increased at the greatest plant density as predicted

Variability in community productivity: mediation by vegetation traits

Plant productivity varies through time in response to environmental fluctuations. Learning to reduce temporal variability in primary productivity is a frequent goal of management with clear relevance to sustaining ecosystem services for an expanding human population. Reducing variability in productivity requires an improved understanding of how plant community traits interact with environmental fluctuations to influence plant growth dynamics. We evaluated links between two community traits, species diversity and species abundance-weighted values of specific leaf area (SLA; leaf area per unit of leaf dry weight), and temporal variability in grassland productivity at patch (local) and aggregate (multi-patch) spatial scales. Aggregate communities were created by combining patches of spatially-distinct communities of perennial plant species from grassland biodiversity experiments in Texas, USA. Inter-annual variability in aboveground net primary productivity (ANPP) of aggregate communities was analyzed as a function of two multiplicative components, mean temporal variability in the ANPP of patches and temporal synchrony in ANPP dynamics among patches. We used regression analyses to determine whether temporal variability in aggregate ANPP and its components were correlated with either species diversity or community-weighted SLA over 5 years. We found that temporal variability in ANPP of aggregate communities, calculated as the square of the temporal CV [(δ/μ)2] of ANPP, was strongly correlated with temporal variability in patch ANPP. Increasing mean SLA reduced ANPP variability of aggregate communities by increasing mean productivity (μ). Increased temporal changes in patch-scale SLA further reduced temporal variability in aggregate ANPP by reducing effects of precipitation fluctuations on productivity. Conversely, increasing species diversity over the narrow range measured increased temporal variability in aggregate ANPP. High diversity was associated with reduced dominance of temporally-stable C4 grasses. Our results implicate means and patch-scale temporal dynamics in community SLA as potential indicators of variability in grassland primary productivity through time