Variation Characteristics Of Plant Community Diversity Along The Elevational Gradient In The Western Tianshan Mountains

Mountain areas are hot spots for biodiversity research,and elevation is an important spatial scale for studying the changes of plant diversity in mountain areas.The western Tianshan Mountains of China belongs to the world’s "Pan-Third Pole",which has rich biological resources and diverse vegetation types;it is characterized by a distinctive natural vertical vegetation zonation,and then it is a natural laboratory for investigating the elevational variation of plant diversity.In the present study,combining three diversity dimensions(species diversity,phylogenetic diversity and functional diversity)and two diversity indices(α and β),the vertical variations of plant diversity at two spatial scales(i.e.,regional scale [the entire region] and local scale [each vertical transect])for different life types(woody and herbaceous plants)using the data from eight vertical transects and 122 sampling plots on a vertical elevational gradient of1000–3290 m in the western Tianshan Mountains were systematically studied,the driving factors of the elevational patterns of plant diversity were explored,and the interrelationships among the three-dimensional plant diversities were discussed.The results showed that:(1)In terms of species diversity,the species richness indices of total species,herbaceous layer and woody layer in plant communities at the regional scale in the western Tianshan Mountains indicated a single-peaked distribution pattern with increasing elevation,and the species β-diversity showed a bimodal distribution pattern.At the local scale,the α-diversity indices of total species,herbaceous and woody layers varied along the elevational gradient in different transects,with the woody layer showing a single-peaked distribution pattern;while the species β-diversity presented a decreasing or irregular distribution pattern along the elevational gradient.The interpretation of species α-diversity by environmental factors at the regional scale was lower than that at the local scale;in which,climate was the main factor influencing species richness of total species and woody layer,while the vertical zonation differences in soils drove changes of species richness in herbaceous layer.The main influencing factors for local-scale herbaceous layer and total species were almost identical,but they differed between different transects.Dispersal limitation due to geographic distance was the main influencing factor of local-scale species β-diversity and regional scale total species β-diversity variation,the regional woody layer is mainly affected by environmental filtering,while the species composition of the herbaceous layer is mainly affected by environmental filtering and dispersal limitation.(2)In terms of phylogenetic diversity,the phylogenetic α-diversity of the total species,herbaceous layer and woody layer in plant communities at the regional scale in the western Tianshan Mountains showed a unimodal distribution pattern with increasing elevation,while the phylogenetic β-diversity all showed a decreasing trend.Climate was the main factor affecting the change of phylogenetic α-diversity of total species,soil had the greatest interpretation for herbaceous layer,and climate and soil jointly dominated the change of phylogenetic α-diversity of the woody layer.Dispersal limitation drove changes in the phylogenetic composition of total species along the elevational gradient,which together with environmental filtering affected the phylogenetic composition of the herbaceous layer and woody layer.At the local scale,the phylogenetic α-diversity of total species,herbaceous layer and woody layer in different transects showed diverse elevational patterns,the environment had a high interpretation for phylogenetic α-diversity(65%–99%).The community structure and soil and their interactions with other environmental factors dominated the phylogenetic diversity changes of total species and herbaceous layer in different transects,respectively,while the woody layer was more influenced by climate and soil.The phylogenetic structure of plant communities in most transects shifted from aggregation to dispersal with increasing elevation,which was influenced by habitat filtering and interspecific competition,respectively.Phylogenetic β-diversity varied irregularly along the elevational gradient in different transects and was mostly caused by phylogenetic turnover;the relationships between phylogenetic β-diversity and geographic and environmental distances exhibited difference,and both environmental filtering and dispersal limitation could be the main ecological processes driving changes in phylogenetic composition along the elevational gradient at the local scale.(3)In terms of functional diversity(herbaceous layer only),the functional richness index(FRic)of herbaceous layer in the western Tianshan Mountains at the regional scale presented a decreasing distribution pattern with increasing elevation,the functional evenness index(FEve)and functional divergence index(FDiv)did not show a significant change trend,the functional dispersion index(FDis)and Rao’s quadratic entropy index(Rao Q)showed a “U-shaped”distribution pattern,while the functional β-diversity represented a decreasing trend.Environmental factors explained less variation in functional α-diversity(< 20%),and both geographical and environmental distances significantly influenced functional β-diversity,and environmental filtering could be a significant predictor of changes in functional composition of the herbaceous layer at the regional scale.At the local scale,the change patterns of functionalα-diversity indices along the elevational gradient were diverse,with different influencing factors(including climate,soils,community structure and their interactions),but the environmental interpretation was generally high(40%–99%).Functional β-diversity varied unimodally and irregularly along the elevation gradient in different transects,respectively;and changes in functional β-diversity in some transects were caused by functional turnover,while that in some transects were co-dominated by functional turnover and functional nestedness.The relationships between functional β-diversity and geographic and environmental distances also varied in the same transects,with both environmental filtering and dispersal limitation likely to play a major role.(4)Interrelationships among the three-dimensional diversities showed that the intensities of phylogenetic signal of six functional traits of herbaceous layers differed in different transects at the local scale in the western Tianshan Mountains,and the divergence and conservation of phylogenetic signal coexisted in most transects,suggesting that the herbaceous layers formed random phylogenetic relationships along the elevational gradient.At the regional scale,the phylogenetic signal of functional traits in the herb layer were divergent.The species richness index of the herbaceous layer in most transects was significantly and positively correlated with the phylogenetic Faith’s PD index,which was consistent with the results at the local scale.The correlations between Rao Q index and species richness index and Faith’s PD index were mostly non-significant and even negative.There were significant positive correlations among the threedimensional β-diversity indices and the corresponding turnover components in the herbaceous layer at the regional scale,and most of the nestedness components also showed significant positive correlations,which were generally consistent at both spatial scales.Species and phylogenetic diversity at local and regional scales and α-and β-diversity had strong correlations and spatial consistency,while the relationship between functional diversity and species and phylogenetic diversity varied among various transects.In conclusion,the variation characteristics of the three-dimensional diversities of plant community along the elevational gradient in the western Tianshan Mountains differed to an extent at both spatial scales.The regional-scale diversity patterns conformed to the general law,while the local-scale diversity patterns were more diverse.The elevational patterns of plant diversity also significantly different among different life forms.The interrelationships among various plant diversity indices and their main influencing factors varied at both regional and local scales.The results in this study provide valuable theoretical basis for the conservation and scientific management of plant diversity in the western Tianshan Mountains,China.