Grass abundance shapes trait distributions of forbs in an experimental grassland

Abstract

Questions During community assembly, plant functional traits are hypothesized to be filtered both by prevailing abiotic conditions and by competition from the surrounding community. While trait responses to abiotic conditions are well documented, whether and how traits respond to the biotic community is less studied. We investigated trait responses in specific leaf area (SLA) and plant height for eight forb species in response to increasing grass abundance. We asked: (1) do species’ trait distributions respond predictably to increased grass presence; (2) do functionally similar species respond similarly in terms of trait means to increased grass presence; and (3) at the community level, does increasing grass presence drive change in functional diversity among forbs? Location Glacial Heritage Preserve, Thurston County, Washington, USA. Methods Eight forb species were grown in communities that differed in the proportion of grass seed sown into the plot (grass‐rich, mixed and forb‐rich) in an experimental grassland established under uniform abiotic conditions. SLA and height were measured for 29–33 individuals per species per treatment, and differences among species and treatments were tested using PERMANOVA and PERMDISP. To assess how Festuca abundance impacted trait distributions for the entire suite of forbs, functional richness (FRic), functional evenness (FEve) and functional dispersion (FDis) were calculated for each seeding treatment and compared using ANOVA. Results Trait means and/or dispersion of most species responded to grass abundance, but the direction and magnitude differed among species. FEve was lowest in the grass‐rich treatment for SLA, and FRic was significantly lower in the grass‐rich treatment for both SLA and height. FDis for the combined traits was lowest in the grass‐rich treatment. Conclusions This study provides clear evidence that the biotic community can shape plant trait expression within and across species, and supports the idea that at the species level, trait values are not fixed, but rather can vary in response to biotic factors. This variation can impact functional diversity as well as community‐level trait estimates and therefore also affect our understanding of ecosystem functions that are reliant on those estimates.

The functional trait space of tree species is influenced by the species richness of the canopy and the type of forest

“…However, results are contrasting and there is no clear evidence of similar species-specific responses (trait adjustments) to changes in species richness and composition. Insights from biodiversity experiments (carried out primarily with herbaceous species) have revealed a potential role of intraspecific variability for fostering species complementarity (Ashton et al, 2010;Mitchell and Bakker, 2016;Zuppinger-Dingley et al, 2014;Zhu et al, 2015). For instance, Zhu et al, (2015) assessed that 64% of the total net biodiversity effect measured on light capture compared to monocultures in wheat and maize intercrops was due to species plasticity.…”

Assessing the joint behaviour of species traits as filtered by environment

“…Our framework extends the commonly used metric of community level functional trait expression, the CWT, to include uncertainty and estimates of variation for trait expression at the community level. This extension has important implications for trait‐based analyses, as abiotic and biotic filters have been demonstrated to impact not only mean trait expression but also to alter expression of trait variability (Lipowsky et al., ; Mitchell & Bakker, ). Although collection of sufficient data to estimate variability is labour‐intensive, global meta‐analyses have indicated that this variability is important on a global scale (Siefert et al., ).…”

“…This extension has important implications for trait-based analyses, as abiotic and biotic filters have been demonstrated to impact not only mean trait expression but also to alter expression of trait variability (Lipowsky et al, 2015;Mitchell & Bakker, 2016). Although collection of sufficient data to estimate variability is labour-intensive, global meta-analyses have indicated that this variability is important on a global scale (Siefert et al, 2015).…”

Trait plasticity of intensive pasture species due to growth in mixture across seasons and nutrient addition levels

“…Knops & Reinhart 2000) or experimentally manipulated species composition (e.g. Mitchell & Bakker 2016;Bennett et al 2016). We are not aware of any ITV studies that have simultaneously manipulated both species composition and abiotic factors for communities with more than two species, although one study has applied incomplete manipulation of species composition (seed addition to existing communities) with nutrient addition (Siefert & Ritchie 2016).…”

“…Documentation of intraspecific trait variation (ITV) is increasingly recognised as crucial for understanding species and community level responses to temporal and spatial environmental variability (Albert et al 2010;Siefert et al 2015;Fajardo & Siefert 2016) and mechanisms of co-existence in plant communities (Bennett et al 2016;Mitchell & Bakker 2016). However, there is a lack of studies experimentally controlling both species composition and abiotic factors in studying the effects of trait plasticity on ITV and its community-level consequences.…”