When can we expect negative effects of plant diversity on community biomass?

Although experiments overwhelmingly show biodiversity increases ecosystem functioning, relationships in natural communities are more variable. This raises the question of whether and when theory would predict negative diversity effects. We argue that plant communities containing more stably coexisting core species should have higher biomass production. However, variation in numbers of transient species, whose abundances fluctuate strongly and which cannot stably coexist, may not consistently affect biomass. Distinguishing changes in core and transient species richness is critical. For instance, recent attempts to use novel causal modelling approaches have implied negative effects of biodiversity on biomass. However, we find these approaches also result in negative relationships when applied to experiments, where we know there is a causal, positive effect of diversity. We suggest that transient species contribute disproportionately to the variation in diversity isolated in thes..., The PaNDiv experiment was established in Autumn 2015. It contains factorial manipulations of plant diversity, functional composition, nitrogen, and foliar pathogen exclusion. The experiment contains 216 2m x 2m plots. Experimental plant communities vary in species richness (1, 4, 8, 20 species), functional composition and functional diversity, nitrogen enrichment (in the form of urea; 0, 100 kg.ha 1.y 1) and foliar fungicide treatment (Score Profi by Syngenta Agro AG, 24.8 % difenoconazole and Ortiva by Syngenta Agro GmbH, 22.8 % azoxystrobin). Plots are separated by 1m grass path,s and the whole experiment is mown twice a year, reflecting typical extensive grassland management in this area.
A pool of 20 common, perennial grassland species was used. To manipulate functional composition, in terms of resource use strategy, these were divided into 10 slow and 10 fast-growing species based on specific leaf area (SLA) and leaf Nitrogen. Species combinations were randomly selected from the re..., # When can we expect negative effects of plant diversity on community biomass?

[https://doi.org/10.5061/dryad.m0cfxppd6](https://doi.org/10.5061/dryad.m0cfxppd6)

## Description of the data and file structure

Data were collected in the PaNDiv experiment that manipulates plant species richness, plant functional composition, N enrichment and foliar fungal removal.The whole experiment consisted of 216 plots of 2 m 2 m separated from one another by a 1 m path and arranged in four blocks. The species pool consisted of 20 perennial herbs and grasses typical of mesic, Central European grasslands. We classified the species, as 10 fast growing species or as 10 slow growing species, using literature values of leaf N content and specific leaf area (SLA). All 20 monocultures were established, together with 15 four species, 15 eight species and 4 twenty species plots. Plots with four and eight species could contain either only fast or only slow species, or a mix of the two strategies. This allowe...,