DOI: 10.5061/dryad.1c59zw47k
title: Data from: When can we expect negative effects of plant diversity on community biomass?

description: 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
 these models. We highlight the need for improved approaches to
 analysing data from naturally assembled communities and call for increased
 attempts to compare results with experimental systems. Synthesis:
 understanding the functional consequences of biodiversity loss is critical
 but we need to be clear about what type of diversity change we are
 measuring and to focus on the loss of stably coexisting, core species.
description: 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 respective species pool (i.e., fast, slow, or all), with the
 constraint that all mixtures contained grasses and non-leguminous forbs.
 Legumes were excluded as they cannot be easily assigned to fast or slow
 pools. The plots were arranged in four blocks, and all species
 compositions occurred once per block. Each composition received the four
 combinations of fungicide x nitrogen, randomly allocated per block. To
 maintain species compositions, the experiment is weeded three times per
 year. We collected aboveground biomass twice per year,
 before the mowing from 2017 to 2023. The samples were taken by clipping
 plant material to 5cm above ground level, in two quadrats of 20 cm × 50 cm
 located in the centre of each plot. We weighed the biomass after 2 days of
 drying at 65°C. The plot target biomass production (i.e., of the sown
 species without weeds) was calculated by multiplying the percentage cover
 of weeds by the total biomass and subtracting this estimated weed biomass
 from the total weight. The percentage cover of all target species and
 weeds was measured on 1 m2 plot in the centre of each plot, at the same
 time as biomass was harvested. For the analysis of this
 dataset used in the Allan et al. 2025 (doi 10.1111/1365-2745.70071) see https://github.com/noemiepichon/negative_diversity_effects_opinion    
description: # 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 allowed us to create a large gradient in functional composition (mean SLA) and functional diversity (variance in SLA), independent of species richness. Species compositions for each plot were randomly selected from the respective species pool, and mixtures contained both grasses and herbs. Each species composition (54 in total) received the four combinations of N enrichment and fungicide application. Each block contained all 54 compositions but the treatments that a given composition received in a particular block were randomly selected. ### Files and variables #### File: PaNDiv\_biomass\_2017to2023.txt **Description:** ##### Variables * year: Year of data collection 2017 to 2023 * block: The experiment was arranged in 4 spatial blocks each containing all species compositions * plot: there were 216 plots * unique.composition.id: the particular set of species on the plot. Each of the 51 combinations received all four N x fungicide treatments * functional.composition: whether the plot contained Fast (F), Slow (S) or Mixed (M; both fast and slow) species * species.diversity: the number of species sown 1, 4, 8 or 20 * nitrogen: whether plots were fertilised: fertilised plots received 100 kg N ha-1y-1 as urea once in April and once late June * fungicide: whether plots had fungicide added, fungicide plots were sprayed four times during the growing season (beginning of April and June, late July and September) with "Score Profi" (24.8 % Difenoconazol 250 g.L-1) and "Ortiva" (32.8% Chlorothalonil 400 g.L-1 6.56% Azoxystrobin 80 g.L-1) * total.biomass: biomass samples were taken by clipping plant material to 5cm above ground level, in two quadrats of 20 cm 50 cm located in the centre of each plot. This is the sum of the two quadrats * realised.diversity: the number of species recorded in the central 1m2 of the plot, at the same time the biomass was harvested ## Access information Other publicly accessible locations of the data: * Biomass from 2017 and 2018 is available here [https://doi.org/10.5061/dryad.dbrv15f3j](https://doi.org/10.5061/dryad.dbrv15f3j)