Community assembly processes along a sub-Mediterranean elevation gradient: analyzing the interdependence of trait community weighted mean and functional diversity

Abstract

Community-weighted-mean (CWM) and functional diversity (FD) describe two aspects of plant communities' functional structure. While they have been often used separately to infer assembly processes, their covariation can actually provide useful insights into the prevalence of a particular 2 assembly process over the other. We propose a framework where positive or negative covariation of these indices can be related to different assembly processes along an environmental gradient. We tested this framework in grassland communities along elevation gradient in Central Apennines by collecting species cover and traits of the most abundant species and calculating the effect size CWM and FD. We performed major axis regression for each effect size CWM-FD relationship for different belts along the elevation gradient. The observation that Plant Height showed a positive CWM-FD relationship only under more stressful conditions indicates that there may be a tendency towards habitat filtering. Seed Mass showed positive covariation in each belt may indicate the presence of both habitat filtering and limiting similarity acting with different intensity depending on the environmental stress level. Negative covariation between CWM-Plant Height and Seed Mass-FD under less stress suggest biotic filter, while positive covariation between CWM-Plant Height and both Seed Mass and SLA FD under stressful conditions suggest the presence of habitat filtering.Ultimately, the relationship of CWM and FD may provide information on how different communities assemble along an environmental gradient. Moreover, combining the information of CWM with the FD and environmental stress level might help to identify the processes behind the same functional pattern.

Patterns of Understory Community Assembly and Plant Trait-Environment Relationships in Temperate SE European Forests

“…In general, our results corroborate previous studies showing that trait convergence primarily occurs in resource-poor environments and mainly results from abiotic filtering, whereas the strength of trait convergence tends to diminish in less stressful conditions [10]. Increasing trait convergence along stress gradients has been demonstrated by many empirical studies [4,19,23,25,33]. The most stressful abiotic conditions in our study sites occur in IM site 6-KL, a high-altitude, single-species dominated overstory (spruce) forest on silicate parent material, resulting in acidic and nutrient-poor soils.…”

“…The range of trait values within a community is expected to decrease with increasing environmental adversity [22,23]. Abiotic filtering, and the resulting trait convergence, play a major role in harsher environments [24,25]. As abiotic filtering causes increased similarity of traits among species that passed through all abiotic filters and were established in a given location, species must then overcome the regulative constraints induced by interspecific competition [10,26].…”

“…Distinguishing patterns of trait convergence and divergence has been a notable challenge in community ecology [29], since different assembly processes can be responsible for trait convergence/divergence [5]. For example, trait divergence in more stressful abiotic conditions can be attributed to facilitation [25]. Another notation is that trait convergence in more favorable environments has been found to be related to the exclusion of weak competitors [24,27,30].…”

Disentangling the Effects of Disturbance from Those of Dominant Tall Grass Features in Driving the Functional Variation of Restored Grassland in a Sub-Mediterranean Context

“…Indeed, dominant species may remove from the community all weaker competitors (i.e., weaker competitor exclusion), acting as a biotic filter, which leads to the coexistence of more functionally similar species. Otherwise, it is possible that in the absence of dominant species, biotic interactions may lead to coexistence of functionally dissimilar species [32,38].…”

The Legacy of the Past Logging: How Forest Structure Affects Different Facets of Understory Plant Diversity in Abandoned Coppice Forests

“…Values for clonal traits were available for all those species whose relative cumulative cover reached 80% of the total cover of all species. This trait sampling approach is suggested for areas where there is a low turnover of species (i.e., beta diversity) [57,58] as is the case in our study (β diversity values of 7.6 expressed as β = γ/mean α, considering Jost correction as recommended by de Bello et al [59]. Table 1.…”