| Understanding the distribution patterns of vegetation, species composition and diversityand the relationships between them and environmental variables has important guidingsignificance for developing optimal strategies for conservation of species diversity,sustainable managing and utilizing plant resources. Based on field investigation andlaboratory experimental data, the present study analysed the minimum sampling areas ofseveral typical plant communities in Taibai Mountain using the method of fitting species-areacurves; the vegetation on the south slope and the north slope of Taibai Mountain wereseparately classified using cluster analysis, detrended correspondence analysis (DCA) andindicator species analysis (ISA), and species composition of each plant community was givena quantitative description; the relationships between the overall species composition of eachslope of Taibai Mountain and species composition of each single plant community andenvironmental variables were analysed with detrended canonical correspondence analysis(DCCA) and redundancy analysis (RDA), respectively, and partial detrended canonicalcorrespondence analysis (pDCCA) and partial redundancy analysis (pRDA) were carried outto evaluated the relative importance of each environmental variables on species composition;generalized linear model (GLM) and generalized additive model (GAM) were conducted to fitresponse of species diversity to various environmental variables. The major findings are asfollows:(1) The minimum sampling areas for tree, shrub and herb layers of several types of plantcommunities in Taibai Mountain all increased with the increase of the study accuracy level.When the sampling areas for tree, shrub and herb layers of these communities were separatelyset to be125-228m~2,18-170m~2and7-91m~2, it could meet below80%study accuracy leveland when they were set to be203-508m~2,62-296m~2and29-150m~2, it could meet below90%study accuracy level. All the plant communities related in this paper met below90%study accuracy level.(2) Eight plant communities were clearly identifiable on each slope and these communities presented zonational distributions along elevational gradients, however, theelevational ranges occupied by different communities had more overlaps at middle elevations(c.2250-3350m a.s.l.) than at two extremes. The species composition of differentcommunities existed great difference, and this kind of difference still existed even in the samecommunities occurred on between the south slope and the north slope.(3) Three communities which were not reported in previous literature were found andthey were Quences spinosa community, Acer caesium subsp. giraldii community and Carexcapilliformis var. major community.(4) There were respectively13and11environmental variables significantly explainingall species composition on the south slope and the north slope. On the south slope, elevationhad the strongest explanatory power, followed by annual mean temperature, and theexplanatory powers of crown density, soil thickness, stone cover, soil organic matter content,total nitrogen content, carbon: nitrogen ratio, total potassium content, aspect, soil pH value,litter thickness and alkali-hydrolyzable nitrogen content decreased successively. On the northslope, the order of the explanatory powers of environmental variables on species compositionsuccessively were annual mean temperature, elevation, crown density, aspect, soil totalpotassium content, available potassium content, soil thickness, stone cover, soil pH value,total phosphorus content and litter thickness. The relationships between species compositionof tree, shrub and herb and environmental variables on both slopes all showed the similarcharacteristics with all species composition.(5) The environmental variables that could significantly explain total species richnessand the response curves of species richness to environmental variables between the southslope and the north slope existed some differences. On the south slope, there weresignificantly unimodal relationships between total species richness and elevation, annualmean temperature, soil organic matter content, total nitrogen content, alkali-hydrolyzablenitrogen content and crown density, and there were significant linear relationships betweentotal species richness and litter thickness, soil carbon: nitrogen ratio and available potassiumcontent. On the north slope, total species richness showed significantly unidomal relationshipswith elevation, annual mean temperature and soil thickness, significant reverse hump-shapedrelationships with soil total nitrogen content, alkali-hydrolyzable nitrogen content and totalpotassium content, significant linear relationships with aspect and soil pH value, while hadmore complex relationships with soil available phosphorus content and crown density whichall showed reverse S-shaped relationships. The relationships between species richness ofdifferent life-form and environmental variables presented similar features with total speciesrichness, that is the environmental variables that could significantly explain species richness of the same life-form between on the south slope and the north slope were different, and sowere the response curves. However, generally, they could summarize as four kinds ofrelationships: increase linearly, decrease linearly, unimodal, reverse hump-shaped, reverseS-shaped and no obvious relationship.(6) The relationships between the species distribution and richness of each of five plantcommunities and environmental variables were analyzed. Species composition of Quercusaliena var. Acuteserrata community was mainly effected by elevation, slope, annual meantemperature and crown density and impact intensity being annual mean temperature>elevation> crown density> slope. Species richness was mainly influenced by elevation,aspect, annual mean temperature, soil thickness, soil pH value and total potassium content,which exhibited a significant reverse hump-shaped relationship with elevation, increasedlinearly from cold-moist aspect to warm-dry aspect, decreased linearly with increase of annualmean temperature and soil thickness, while had more complex changes with soil pH value andtotal potassium content gradients.The major environmental variables impacting species composition of Betulaalbo-sinensis community were annual mean temperature, soil thickness, carbon: nitrogenratio, total potassium content and crown density and effect intensity being soil carbon:nitrogen ratio> soil thickness> crown density> total potassium content> annual meantemperature. No environmental variables significantly impacting species richness of Betulaalbo-sinensis community were detected.Species composition of Betula utilis community mainly related to soil pH value andcrown density and impact intensity being crown density> soil pH value. Species richness wasmainly associated with soil carbon: nitrogen ratio and total phosphorus, and which displayeda significant reverse hump-shaped relationship with the former and significant linear increasewith increase of the later.The most environmental variable influencing species composition of Abies fargesiicommunity was aspect, followed by stone cover, and soil available phosphorus content,elevation, available potassium content, litter thickness, and crown density decreasedsuccessively. Species richness of Abies fargesii community showed a significant reversehump-shaped relationship with elevation, significantly unidomal relationships with litterthickness and soil total nitrogen content, significant linear relationships with soil organicmatter content and carbon: nitrogen ratio, which decreased with decrease of the twoenvironmental gradients. Species richness increased with increasing crown density, but whencrown density reached to60%, basically, it did not change any more.Species composition of Rhododendron capitatum-Salix cupularis-Carex capilliformis var. major community mainly related to aspect, soil organic matter content and crown densityand impact intensity being crown density> aspect> soil organic matter content. Speciesrichness of this community mainly related to elevation, annual mean temperature, litterthickness, soil organic matter content, total nitrogen content, soil carbon: nitrogen ratio,alkali-hydrolyzable nitrogen content, total phosphorus content and crown density. Speciesrichness linearly declined with increasing elevation, soil total nitrogen content,alkali-hydrolyzable nitrogen content and total phosphorus, and linearly increased withincreasing annual mean temperature and crown density, while demonstrated significantreverse hump-shaped relationships with litter thickness, soil organic matter content andcarbon: nitrogen ratio.Vegetation, species composition and diversity in Taibai Mountain were effected by avariety of environmental variables. However, topographic and climate variables played themost important role when considering from the entire south slope and the entire north slope ofTaibai Mountain, respectively. For single community, soil variables were more important.While vegetation variable (crown density) had full impact on not only overall vegetation,species composition and diversity on the south slope or on the north slope but also singlecommunity.
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