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title: Spatial distribution patterns of tree species and ecological processes across three forest types, southwest China
author: Mengesha Asefa
Issued Date: 2017-11
Abstract: One of the ultimate objectives of community ecology is to understand the mechanisms structuring species distribution pattern. A number of ecological processes including niche-based deterministic process, wherein species are sorted along environmental axes; neutral-based dispersal limitation, wherein species are distributed based on their capacity of movement; and/or random placement, are indicated to be the drivers of plant community assemblage. Although all these processes could most likely occur in plant community assemblage, however, the most important question yet in community ecology is whether their relative importance varies systematically across different ecological gradients. Furthermore, to gain deep insights into the coexistence mechanisms of plants (given species names are relatively information poor), it also worth to look into the evolutionary and functional dimensions of the species.
       In this dissertation, we tested in general four competing hypotheses (Random placement hypothesis, Dispersal limitation hypothesis, Environmental filtering hypothesis, Dispersal and environment hypothesis) to explore their relative importance in structuring community assembly using multiple summary statistics. We also considered the effect of life history stages of trees and spatial scales that potentially influence the relative role of these hypotheses on the structure of communities in the tropical (Xishuangbanna forest dynamics plot (Xishuangbanna FDP), sub-tropical (Ailaoshan FDP), and sub-alpine (Lijiang FDP) forests. Moreover, given the neglecting ability of species name-centric approach on the evolutionary and functional structure of species, we also applied the evolutionary history and functional traits of species to further explore the spatial signature of those hypotheses on the phylogenetic and functional assemblage of the community.
We used different biodiversity metrics at the community level (specie area relationship, species abundance distribution, and species beta diversity) and summary statistics at the species level (pair correlation function, K2 function, and wavelet variance) to test the relative importance of the aforementioned hypotheses using advanced spatial statistical methods. We found that species are structured non-randomly, being structured by both dispersal limitation and environmental filtering in all forest types. Unexpectedly, we did not find an evidence of shifts in the relative importance of underlying mechanisms with life history stages of trees in most of the summary statistics. In general, most of the summary statistics selected dispersal limitation as the dominant driving mechanism in all forest types across life history stages and spatial scales. However, our hypotheses were found to be scale dependent for species beta diversity. Dispersal limitation alone at large spatial scales, and in combination with environment at small spatial scale shaped species beta diversity mainly in the tropical and sub-alpine forests. We also found that the dissimilarity of species substantially decreased as we increased the spatial scale of analysis. Furthermore, the species level analysis also confirmed the community level result that dispersal limitation is found to be the primary mechanism for species conspecific aggregation in the tropical and sub-tropical forests, whereas environmental filtering accounted for the aggregation in the sub-alpine forest ecosystem. The results suggest that multiple summary statistics tend to be applied to extract the different facets of biodiversity so as to capture the multiple actions of ecological processes.
Furthermore, we tested the aforementioned ecological hypotheses on the phylogenetic and functional structure of the community. We found non-random patterns of phylogenetic and functional dissimilarity across life history stages and spatial scales. The result showed that the effect of ecological mechanisms was found to depend on the metrics used to compute phylogenetic and functional beta diversity. We found environmental filtering accounted for pairwise phylogenetic and functional turnover, whereas dispersal limitation, alone and in combination with environmental filtering, for the nearest neighbor lineage and trait turnover, suggesting that different metrics of measurement should be used to quantify distinct facets of the phylogenetic and functional structure of plant communities. Dispersal limitation alone, and jointly with environment were found to structure early and later life history stages of trees respectively for nearest neighbor (but not for pairwise) phylogenetic and functional beta diversity. However, the relative importance of dispersal and environment in general did not appear to change with spatial scale, as unexpected.
       We also used environmental variables and geographic space to explore how functional traits and lineages are structured by environmental factors across spatial scales. We found a clear environmental gradient that considerably structured trait and lineage assemblages, and consistent trait-environment association across spatial scales. Species with high resource acquisition-related traits were associated with the resource-rich part of the plot across the scales, whereas resource-conserving functional traits were distributed in limited-resource environments. Furthermore, we found phylogenetic and functional clustering. Similar functional strategies were also detected among distantly related species, suggesting that phylogenetic distance is not necessarily a proxy for functional distance.
Key words/phrases: Ecological processes, functional traits, life history stages, phylogenetics, spatial scale
Degree Level: 博士
Appears in Collections:西双版纳热带植物园毕业生学位论文_学位论文

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Recommended Citation:
Mengesha Asefa. Spatial Distribution Patterns Of Tree Species And Ecological Processes Across Three Forest Types, Southwest China[D]. 北京. 中国科学院研究生院. 2017.

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