Changes in assembly rules along a stress gradient from open dry grasslands to wetlands

Abstract

Running headline: Assembly rules along a long stress gradient Summary 1. A central issue of community ecology is finding rules that explain the composition and abundance of co-existing species. Nowadays two main processes, environmental filtering and limiting similarity are thought to play the main roles in structuring communities. Their relative importance under different environmental conditions, however, is still not properly clarified. 2. We studied the strength and the effect of environmental filtering (causing convergence) and limiting similarity (causing divergence) in 137 sample plots along an extremely long environmental gradient ranging from open sand grasslands to highly productive marshes, using a trait based approach. The main environmental gradient (i.e. productivity) was characterised by the Normalized Difference Vegetation Index, an indicator of aboveground live biomass. Cover of the plant species was estimated visually. Values of 11 plant traits were collected from field measurements and databases. Mean and dispersion of the trait values of the plots were quantified by community-weighted means and Rao's quadratic entropy. Trait convergence and divergence were tested by randomization tests, followed by the study of changes in effect size along the productivity gradient by fitting generalized additive mixed models (GAMM). 3. For vegetative traits we found mainly convergence, indicating the filtering effect of environmental constraints, while traits related to regeneration showed divergence. 4. The strength of convergence in vegetative traits generally decreased as productivity grew, indicating that while under harsh conditions environmental constraints strongly limit the possible trait values; under more benign conditions various water and nutrient-use strategies are adaptable. At high productivity, the strength of divergence in regenerative traits decreased. Since the larger diversity of vegetative traits found here reduces competition, the importance of diverse reproductive strategy is probably lower. 5. Synthesis: Our results partly support the stress-dominance hypothesis, but reveal that assembly rules are more complex. The relative importance of environmental filtering and limiting similarity depends on the trait and on the environmental conditions of the habitat. Traits related to resource use are generally limited by environmental filtering, and this restriction is weakening as conditions become more favourable, while traits related to regeneration are constrained by limiting similarity and are more diverse under harsh conditions.

Environmental filtering limits functional diversity during succession in a seasonally wet tropical secondary forest

“…To evaluate the magnitude and direction of the deviation of each plot from the null distribution, we calculate standardized effect sizes (SES) using probit‐transformed p ‐values following Lhotsky et al. () because FRic and FDis (for all and individual traits) of the null communities are not normally distributed and right‐skewed (Figures and ). More negative SES values indicates stronger environmental filtering, i.e., less variation in trait range, spacing or abundance of trait combinations than expected, while more positive SES values indicate greater niche complementarity, i.e., more variation in trait range or abundance of trait combinations than expected (Bernard‐Verdier et al., ; Lhotsky et al., ).…”

Seed mass diversity along resource gradients: the role of allometric growth rate and size‐asymmetric competition

“…, Lhotsky et al. ). Finally, we discuss our theoretical results with respect to previous theoretical and empirical studies of seed mass variation and the potential contribution of our modeling approach to the broader field of “trait‐based ecology” (McGill et al.…”

“…, Lhotsky et al. ). Although such studies provide clear evidence for variation in seed size along resource gradients, the interpretation of the observed patterns and the conclusions derived from them lack a solid basis due to the lack of theoretical models.…”

New plant trait records of the Hungarian flora

“…Using traits instead of species identities can lead to a better understanding of the mechanisms underlying the observed patterns and processes of communities. Trait-based approach may also help in finding general rules that are valid for communities building from different species pools (Fortunel et al 2014), and using this approach quantitative comparisons can be made between communities that have no species in common (Webb et al 2010).In a recent research project (Lhotsky et al 2016) we collected plant trait data to explore the changes in the importance of community assembly rules along environmental gradients. Although large international efforts have been made to build more and more complete trait databases, and several databases and datasets are accessible 1994)), several species of the Hungarian flora -especially endemic species and species with continental range -were missing from them.…”

“…In a recent research project (Lhotsky et al 2016) we collected plant trait data to explore the changes in the importance of community assembly rules along environmental gradients. Although large international efforts have been made to build more and more complete trait databases, and several databases and datasets are accessible 1994)), several species of the Hungarian flora -especially endemic species and species with continental range -were missing from them.…”