Forest community assembly is driven by different strata‐dependent mechanisms along an elevational gradient

Abstract

Aim The mechanisms driving forest community assembly along elevational gradients remain elusive. The distinct strata in heterogeneous forest are subject to differing assembly mechanisms. This study aims to evaluate how different ecological mechanisms driving forest community assembly across strata and spatial scales, and to identify how these assemblages respond to different abiotic or biotic variables along an elevational gradient. Location Yulong Mountain within Hengduan Mountains, Southwest China. Taxon Seed plants. Methods We sampled plant species along a 1,200 m elevational gradient across forest strata (trees, shrubs and herbs) at different spatial scales (neighbourhood and community). We integrated phylogenetic and functional diversity to disentangle the roles of ecological mechanisms in structuring community assembly. We also determined the relative effects of climatic, soil and biotic variables and their interactions on these community assemblages. Results The phylogenetic and functional diversity of trees were lower at the extremes of the elevational gradient, where they were constrained by water availability or temperature. While communities with higher species diversity exhibited an overdispersed structure at mid‐elevations. For shrubs, overdispersion was exhibited at high elevations despite low species richness, which could be consistent with facilitative interactions or competition exclusion. However, most communities of herbs appear to have a markedly random structure, despite some incongruence between phylogenetic divergence and functional convergence, as well as influences from overstorey trees and local light conditions. The relative effects of abiotic variables on community structure were larger at the community scale than at the neighbourhood scale. Main conclusions Our study emphasises the importance of considering phylogenies and traits in understanding the strata‐ and scale‐dependent mechanisms of forest community assembly. Collectively, results indicate that contrasting assembly mechanisms interact across different strata along elevational gradients to shape diverse forest communities.

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

“…Therefore, greater microsite availability at the larger spatial scale (100 m 2 ) led to the presence of plant species with different trait values and consequently higher trait divergence. With respect to the dependence of community assembly on spatial scale, our results are not in agreement with previous studies which showed that contrasting assembly mechanisms are responsible for different trait diversity patterns along a gradient of spatial scale [27,30,32,33,43].…”

“…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]. In contrast to the described niche-based assembly processes, the assemblages might also result from stochastic processes (neutral theory) [28,31].…”

“…Regarding forest ecosystems, relevant studies have so far been either conducted in (sub)tropical forests [27,33] or have focused solely on tree species in the overstory layer [4,34], woody plant species [35] or tree sapling communities [17]. In recent years, there has been increasing research focus on understory (herbaceous) forest communities [18,21,24,30,36]. However, less consideration has been given to temperate forest understory communities in Europe, particularly in regard to the comparison of different forest vegetation types.…”

“…Compared to other widely spread vegetation types (e.g., grasslands), forest understories are ecologically specific and most often resource-limited habitats, where the overstory tree layer induces both abiotic constraints (resource filtering, e.g., shading) and complex biotic interactions, as mature trees are strong competitors and major consumers of space, light, water and nutrients [30,37,38]. From an ecological perspective, the understory in mature, closed canopy forest stands can be seen as a highly selective environment, where shade tolerance affects the ability of plants to cope with other stressors [39,40].…”

“…By contrast, acidic and nutrient-poor soils usually harbor considerably lower species richness, as not many plant species are able to tolerate low pH values in the temperate forest understories [41,42]. One of the focal questions in forest functional ecology still pending is related to how the role of abiotic vs. biotic constraints is expected to change along ecological gradients such as altitude, tree species composition and forest stand structural features [30,40].…”