Community Assembly And Turnover Mechanisms Of Subalpine Forests Along Elevational Gradient In Yulong Mountains In Northwest Yunnan, China

The mechanisms of community assembly and turnover are core questions in ecology. The approaches integrating the information of phylogeny and functional traits into community ecology have provided new insights into the investigations of mechanisms of community assembly. The framework combined the data of phylogenies, functional traits and environmental variables can provide a deeper understanding of community assembly along elevational gradient and to reveal how species within community response to climate change. Yulong Mountains is located in Northwest Yunnan, a central part of the global biodiversity hotspot of "Mountains of Southwest China", which was recognized as one of the extremely sensitive regions to climate change. In this study, we have established 19 monitoring plots (20 m x 50 m) across elevational gradient in subalpine forests of Yulong Mountain. Three dimensions of diversity, including species diversity, phylogenetic diversity and functional diversity were generated to investigate the diversity distribution patterns and community species composition along elevational gradient, acrossing conifer forest and sclerophyllous evergreen broadleaved forest. In this study, we aim to examing the mechanisms of community assembly and turnover, and how they response to environmental factors in the system of subalpine forest. The main results and conclusions were summarized as below:1. Species diversity along elevational gradient335 seed plant species representing 190 genera and 65 families were collected in the studied plots. Species richness decreased along elevational gradient. For tree species, the distribution pattern of species diversity was unimodal. While the diversity of shrub and herb species decreased linearly with elevation. The diversity of seedling showed no pattern along the elevational gradient. Environmental and spatial variables significantly affected the species diversity and species composition of community. Dissimilar species composition was observed at different elevations. Beta diversity significantly increased with the increase of the elevational distance. The species replacement component contributed more to beta diversity than the nestedness-resultant component. Beta diversity was significantly affected by variables of environment and space, and the percent of variance explained in beta diversity by spatial variables was higher than environmental variables.2. Phylogenetic diversity along elevational gradientThe community phylogenetic tree with high resolution and strongly supported values for internal clades was constructed by using DNA barcoding data. For tree species, the community phylogenetic structure is under-dispersion at low and high elevations and over-dispersion at middle elevations as measured by net relatedness index and nearest taxon index. Herb and shrub species showed phylogenetic over-dispersion at low and middle elevations while under-dispersion at high elevations. Over-dispersed phylogenetic structure was detected at high elevations for seedling species. Environmental and spatial variables had significantly effect on the community phylogenetic structure. Phylogenetic beta diversity increased with increasing elevational distance. The phylogenetic beta diversity was driven collectively by phylogenetic replacement and phylogenetic nestedness-resultant components. The percent of variance explained in phylogenetic beta diversity by spatial variables was higher than environmental variables.3. Functional diversity along elevational gradientThe variation of six functional traits (height, leaf thickness, specific leaf area (SLA), leaf carbon concentration (LCC), leaf nitrogen concentration (LNC) and leaf phosphorus concentration (LPC)) were decomposed into interspecific variation effect, intraspecific variation effect and covariation effect. The results revealed that species turnover explained the largest amount of variation in leaf morphological traits (leaf thickness and SLA) across the elevational gradient. However, intraspecific variability explained a large amount of variation in three other traits (height, LNC and LPC). The detection of limiting similarity in community assembly was improved when accounting for both intraspecific and interspecific variability. Therefore incorporating intraspecific variation to trait-based studies is essential to improve the understanding of species coexistence in communities.Traits of wood species had significant phylogenetic signal, while, herbaceous traits showed albeit weak phylogenetic signal. For tree species, functional structure (standard effect size of mean pair-wise functional distance and nearest neighbor functional distance) exhibited under-dispersion at low and high elevations, and over-dispersion at middle elevations. However, for understory species such as shrub, herb and seedling, over-dispersion was observed at low and middle elevations and under-dispersion was detected at high elevations. Environmental, spatial variables and phylogenetic structure significantly influenced the functional structure. Functional beta diversity increased along the rising of elevational distances, and was driven by both functional replacement and functional nestedness-resultant components. Furthmore, the percent of variance explained in functional beta diversity by spatial variables was higher than environmental variables as well.4. Community assembly and turnover mechanisms along elevational gradient based on a combined approach of species, phylogenies and functional traitsThe results based on species, phylogenetic and functional diversities indicated that environmental filtering could determine the under-dispersed structure at low and high elevations for tree species. Anthropological disturbance at low elevations and low temperature at high elevations acted as environmental filters in the process of species coexistence. The structure of community at middle elevations was over-dispersed, suggesting the effect of competition exclusion. For understory (shrub and herb and seedling) species, the structures were over-dispersed at low and middle elevations, indicating that competition exclusion was the main assembly mechanism in understory communities. Structure at high elevations was under-dispersed, which indicating environmental filtering accounted for the patterns of species, phylogenetic and functional structure for understory species. The results revealed that the community assembly mechanisms of different life-forms were different, and support the importance of niche-based deterministic process in subalpine forests. Furthermore, the spatial distance had significantly effect on the diversity distribution patterns, which suggested that dispersal limitation was also essential to species coexistence in community and community turnover. Niche and stochastic processes influenced the distribution patterns of species, phylogenetic and functional diversities along elevational gradient in subalpine forests of Yulong Mountain at local scale. The findings have important implications in the understanding of community assembly and turnover mechanisms and how they response to climate change.5. Community assembly and turnover mechanisms across temperate conifer-sclerophyllous evergreen broadleaved forest ectoneVegetation shift from conifer forest to evergreen broadleaf forest across 300 m elevation was found in a temperate conifer-sclerophyllous evergreen broadleaf forest ectone in Yulong Mountain. The relative abundance of Pinus yunnanensis decreased along with increasing elevation, while the relative abundance of Quecus guyayafolia increased. Quecus aquifolioides become the dominant species in high elevational community. Functional traits have significant phylogenetic signals, indicating the evolution of traits wereconserved. In P. yunnanensis communities, the phylogenetic and functional structure shifted from over-dispersion to under-dispersion along elevational gradient, indicating the mechanism shifted from competition exclusion to environmental filtering accordingly.In Q. aquifolioides communities, the competition exclusion shaped the over-dispersed structure of phylogeny and function. Phylogenetic and functional structures were over-dispersed in forest ectone, the results indicated competition exclusion acting on these ectone communities. Phylogenetic and functional beta diversities were positively correlated with species Bray-Curtis distance. Canopy cover possibly influenced the species richness and functional diversity, while soil water content in dry season may influence the phylogenetic and functional alpha diversity. However, we found the environmental and spatial distances could not explain the turnover of species, phylogenetic and functional diversity, thus biotic interaction such as competition could be a main factor in driving the multi-dimensional diversity turnover in conifer forest-evergreen broadleaf forest ectone.