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It is important to understand the dynamic responses of ecosystems to climate change in the context of global warming.The alpine treeline is regarded as highly sensitive and a potential indicator of climate change,and it has thus become one of the hotspots of global climate change research.Most research on treeline dynamics has been conducted at a population scale,and there have been comparatively few studies of treeline dynamics at a community scale.In this study,treeline dynamics were analyzed at a community scale in the Changbai Mountains by examining the change in relationship between the Betula ermanii community and the tundra community. The treeline of the northern slope of the Changbai Mountains was selected as the study area.The diameter-class structure of the Betula ermanii population and ecologically important values of shrubs and grasses were investigated in 145 quadrats.The age structure of the population was reflected by the diameter-class distribution,and the diameter-class distribution was used to understand the population dynamics of Betula Ermanii.Number of trees,basal area,and aggregation index I were selected to determine the diameter-class distribution characteristics.Because there were many species of shrubs and grasses at the study area,eight common plants(relative frequency0.05) were selected to evaluate treeline change on a community scale.Hierarchical clusters were successively carried out from two to six groups.Three classification methods were adopted in each hierarchical cluster.Finally,the eight common plants were divided into two groups that were subordinate to the forest and tundra community.Conventional statistics including means,standard deviations,skewness,kurtosis,and variation coefficients were used to reveal changes in the dynamics of shrubs and grasses.Canonical correlation analysis between each of the Betula ermanii population variables(shrub,single,and integration of shrub and single-trunk) and the common plants group-variable were carried out to discover the dynamic responses of the shrubs and grasses to population changes of the three kinds of Betula ermanii.The relationships between nine factors of the Betula ermanii population and the values of 8 ecologically important common plants were evaluated using univariate correlation analyses. The diameter-class structure of Betula ermanii was a reverse-J shape.The population had an expanding trend,leading to an advancing treeline.Forest plants such as Rhododendron chrysanthum and Deyeuxia angustifolia also showed evidence of advance.However,tundra plants,such as Vaccinium uliginosum Var.alpinum,Dryas octopetala Var.asiatica,Rhododendron confertissimum,Carex callitrichos V.Krecz and Carex siderosticha,simultaneously retreated at different speeds in the context of population expansion and treeline shift.The distribution of various forest plants changed at different speeds.The stronger competitive power a kind of plant had,the closer its advance speed to that of Betula ermanii.Likewise,the retrogression speeds of tundra plants were negatively related to competitive power.Also,for a given plant,the closer relationship it had with typical tundra plants,the faster the plant retreated.Therefore,there were obvious differences in the speed of shifts when various forest plants advanced and tundra plants retreated.The population expansion of Betula ermanii drove the shifts of shrubs and grasses,which belonged to the forest community.Treeline shift was not only manifested by tree change,but also by the forest community invading the tundra community.The change differences among various species compositions were very obvious.The difference was particularly obvious during the replacement process of various plants in accordance with their competitive status in the community. |
