Effects of resource availability on (a) relative yield totals (RYT), and (b) D_max.

<p>Shown are means (± 1 SE) across two- and four-species mixtures per resource treatment. Treatments manipulating resource availability are abbreviated with: F-S- = no fertilization, no shading, F-S+ = no fertilization, shading, F+S- = fertilization, no shading, and F+S+ = fertilization, shading. Results of tests for overall means of RYT ≠ 1 and D_max ≠ 0, respectively, for each resource treatment are indicated for different levels of significance with * p ≤ 0.05, ** p ≤ 0.01 and *** p ≤ 0.001. Levels of significance from linear mixed effects models (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158110#pone.0158110.t002" target="_blank">Table 2</a>) for effects of shade, fertilization and their interaction are given in the upper right corner.</p

Summary of linear mixed-effects model analyses for community biomass production, relative yield totals (RYT), transgressive overyielding (D_max), net diversity effects (NE), trait-independent complementarity effects (TICE), trait-dependent complementarity effects (TDCE) and dominance effects (DE).

<p>Plant communities of different species richness, functional group and growth stature composition were grown at different levels of resource availability manipulating light supply by shading and nutrient supply by fertilization.</p

Species pools of the biodiversity experiment.

<p>Listed are studied species and taxonomy, growth height [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158110#pone.0158110.ref064" target="_blank">64</a>], assignment to functional groups (grasses or forbs), different growth statures (small or tall), and species pools.</p

Effects of resource availability on (a) net diversity effects (NE), (b) trait-independent complementarity effects (TICE), (c) trait-dependent complementarity effects (TDCE) and (d) dominance effects (DE).

<p>Shown are means (± 1 SE) across two- and four-species mixtures per resource treatment. Treatments manipulating resource availability are abbreviated with: F-S- = no fertilization, no shading, F-S+ = no fertilization, shading, F+S- = fertilization, no shading, and F+S+ = fertilization, shading. Results of tests for overall means of diversity effects ≠ 0 for each resource treatment are indicated for different levels of significance with * p ≤ 0.05, ** p ≤ 0.01 and *** p ≤ 0.001. Levels of significance from linear mixed effects models (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0158110#pone.0158110.t002" target="_blank">Table 2</a>) for effects of shade, fertilization and their interaction are given in the upper right corner.</p

Results of linear mixed-effects model analyses predicting trait-independent complementarity effects (TICE), trait-dependent complementarity effects (TDCE) or dominance effects (DE) from trait-based predictors.

<p>Results of linear mixed-effects model analyses predicting trait-independent complementarity effects (TICE), trait-dependent complementarity effects (TDCE) or dominance effects (DE) from trait-based predictors.</p

Effects of resource availability on aboveground biomass production in communities of different species richness.

<p>Values are means (± 1 SE) per resource treatment and species-richness level. Values are staggered along the x-axis for enhanced clarity. Treatments manipulating resource availability are abbreviated with: F-S- = no fertilization, no shading, F-S+ = no fertilization, shading, F+S- = fertilization, no shading, and F+S+ = fertilization, shading.</p

Species—level relative yields (RYs).

<p>Shown are means (± 1 SE) across communities of different species richness and grown at varying resource availability. The threshold for greater biomass production of a species in the mixtures than expected from its monoculture (RY > 1) is indicated with a dotted line. Results of overall tests for RY ≠ 1 for each species are indicated with * p ≤ 0.05, ** p ≤ 0.01 and *** p ≤ 0.001. Different letters indicate significant differences among species in their RYs. Hatched bars = forbs, open bars = grasses; filled bars = small-statured species, unfilled bars = tall-statured species.</p

Realized species richness.

<p>Realized species richness as a function of time (A) as mean values (±1SE) per sown species-richness level across colonization periods, and (B) as mean values (±1SE) per colonization period across species-richness levels. For symbols see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0101928#pone-0101928-g003" target="_blank">Figs. 3</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0101928#pone-0101928-g004" target="_blank">4</a>.</p

Different Assembly Processes Drive Shifts in Species and Functional Composition in Experimental Grasslands Varying in Sown Diversity and Community History

<div><p>Background</p><p>The prevalence of different biotic processes (limiting similarity, weaker competitor exclusion) and historical contingency due to priority effects are in the focus of ongoing discussions about community assembly and non-random functional trait distributions.</p><p>Methodology/Principal Findings</p><p>We experimentally manipulated assembly history in a grassland biodiversity experiment (Jena Experiment) by applying two factorially crossed split-plot treatments to all communities: (i) duration of weeding (never weeded since sowing or cessation of weeding after 3 or 6 years); (ii) seed addition (control vs. seed addition 4 years after sowing). Spontaneous colonization of new species in the control treatment without seed addition increased realized species richness and functional richness (FRic), indicating continuously denser packing of niches. Seed addition resulted in forced colonization and increased realized species richness, FRic, functional evenness (FEve) and functional divergence (FDiv), i.e. higher abundances of species with extreme trait values. Furthermore, the colonization of new species led to a decline in FEve through time, suggesting that weaker competitors were reduced in abundance or excluded. Communities with higher initial species richness or with longer time since cessation of weeding were more restricted in the entry of new species and showed smaller increases in FRic after seed addition than other communities. The two assembly-history treatments caused a divergence of species compositions within communities originally established with the same species. Communities originally established with different species converged in species richness and functional trait composition over time, but remained more distinct in species composition.</p><p>Conclusions/Significance</p><p>Contrasting biotic processes (limiting similarity, weaker competitor exclusion) increase functional convergence between communities initially established with different species. Historical contingency with regard to realized species compositions could not be eradicated by cessation of weeding or forced colonization and was still detectable 5 years after application of these treatments, providing evidence for the role of priority effects in community assembly.</p></div

Summary of studied colonization period × seed addition treatments across initial species-richness levels.

<p>Vertical arrows indicate the time, when weeding was stopped in different treatments: (S0−)  =  never weeded, no seed addition, (S0+)  =  never weeded, seed addition (2005), (S3−)  =  cessation of weeding after three years (2004), no seed addition, (S3+)  =  cessation of weeding after three years (2004), seed addition (2005), (S6−)  =  cessation of weeding after six years (2007), no seed addition, and (S6+)  =  cessation of weeding after six years (2007), seed addition (2005).</p