| The Qinghai-Tibet Plateau is known as the "regulator" and "activator" of climate change in the northern hemisphere.The unique climate,geography and habitat conditions have bred rich biological species and formed a complex plateau ecosystem.However,the energy conversion and material circulation processes in this area are slow and the anti-interference ability is extremely poor.Once the ecological balance is destroyed,it is difficult to restore.How to improve the biodiversity and ecological service functions of forest communities in this area,accelerate the succession process of forest communities,and promote the speed of vegetation restoration is the focus of ecologist research.In this paper,the forest community of the Sejila Mountain in southeastern Tibet,the core area of the Qinghai-Tibet Plateau,was taken as the research object.The classic community survey method was used in the altitude range of 3000m-4700 m.Through large-scale field survey sampling experiments,the diversity characteristics and diversity maintenance mechanisms of the forest community in the Segila Mountains were studied from three dimensions of species diversity,phylogenetic diversity and functional diversity along the elevation gradient.At the same time,the spatial pattern and quantitative characteristics of the regenerating species of Abies georgei Orr var.smithii in the forest community of Sejila Mountain were analyzed,and the spatial distribution of the renewal layer of dominant tree species was studied.In order to further understand the community succession law of the Segila Mountains,three typical plots were selected in the experimental study.Combined with field control experiments at the level of 3 factors and 2 years of follow-up monitoring,the relationship between the renewal characteristics of the main built species in the forest area at different succession stages and the heterogeneity of typical habitats was explored.The theory of triple mechanism of community succession was verified,which provided a certain reference for the protection and restoration of vegetation on the Qinghai-Tibet Plateau.The main research conclusions are as follows:1.Species diversity of forest communities in Sejila Mountain,Tibet A total of 50 families,137 genera,and 225 species of seed plants were found in the Sejila Mountain in Tibet.The total species richness of the forest community in Sejila Mountain shows a decreasing distribution pattern with the increase of altitude gradient.Arbor diversity shows a unimodal distribution pattern along the altitude gradient.The diversity of shrubs and herbs shows a bimodal distribution pattern along the elevation gradient.The species richness of shrubs and herbs on the shady slope is significantly lower than that of the sun slope.Herbaceous plants generally have more growth advantages than shrubs in communities.The abundance of species in different life forms is affected by different types of environmental factors.The main environmental factors affecting the distribution of tree species richness along the elevation gradient are the degree of disturbance,the aspect,and the content of organic carbon and total nitrogen in the soil;what affects shrub species is altitude and slope;what affects herbaceous species are altitude,slope,and humidity.Arbor species richness is not significantly affected by spatial variables.Both spatial and environmental variables significantly affect the species richness of shrubs and herbs.2.Developmental diversity of forest communities in Sejila Mountain,Tibet There is a typical positive correlation between the phylogenetic diversity index of Sejila Mountain community and the change of species richness.From the comparison of the observations with the zero model,the arbors and shrubs don’t not show significant differences.While the difference in herbs is more and more significant with the increase of species richness.The Phylo Sor index of trees,shrubs,and herbs in the Sejila Forest community and altitude distance show a typical negative correlation.When the phylogenetic average pairwise distance(S.E.S.D’pw)and nearest neighbor distance(S.E.S.D’nn)of trees,shrubs,and herbs are at the same altitude or a small altitude difference,the phylogenetic β diversity ratio between plots changes more slowly than that at zero model.Phylogenetic β diversity shows aggregation.When the altitude span is 400 m or 800 m,the standardized phylogenetic β diversity presents a random pattern.The phylogenetic structure of arbor trees in the Sejila Mountains at low and high altitudes is clustered,and the mid-altitude areas are discrete.Environmental and spatial variables also significantly affected the phylogenetic structure of arbors,shrubs,and herbs.3.Functional diversity of forest communities in Sejila Mountain,Tibet The three major types of functional diversity indices of the Sejila Forest Community—functional richness index,functional uniformity index,and functional dispersion index—are internally related and independent of other indices.Functional diversity has a positive correlation with species richness and species diversity.Probability of species functional differences increases as species diversity and abundance increase.At different altitudes of forest communities,the functional structure index of the communities is significantly different.The average paired trait distance(S.E.S PW)and nearest trait neighbor distance(S.E.S NN)standardized at the same altitude also differ.The FD index of functional diversity of arbors,shrubs and herbs shows a significant positive correlation with species richness.The change of community functional traits shows a significant negative correlation with the change of spatial distance,indicating that the similarity of community functional traits gradually decrease with the increase of spatial distance.Environmental and spatial variables have significant effects on the functional traits of arbors,shrubs,and herbs.Phylogenetic information has different effects on three different community functional structure indexes.The S.E.S.PW index is significantly affected by phylogenetic information,and the S.E.S.NN index has almost no effect.4.Spatial pattern of renewed forest community species in Sejila Mountain,Tibet Among the mono-species forest communities in the Sejila Mountains,Abies georgei Orr var.Smithii,Picea likiangensis var.Linzhiensis and Sabina saltuaria exhibit different distribution characteristics with aggregate distribution,random distribution,and uniform distribution with changes in spatial scale.The monotypic community shows spatial aggregation on a small scale and random distribution on a large scale.This indicates that in the future community succession process,the monotypic community in Sejila Mountain may all tend to be randomly distributed.Different mixed forests show different spatial distribution patterns with changes in spatial scale.In the mixed forests of Abies georgei Orr var.smithii and Picea likiangensis var.linzhiensis,three different association modes of negative association,positive association,and no association alternate with spatial scale,indicating that the interspecies competition between the two groups is fierce;the spatial distribution of the mixed forests of Abies georgei Orr var.smithii and Sabina saltuaria,and the mixed forests of Picea likiangensis var.linzhiensis and Sabina saltuaria are all negatively correlated on a small scale,but not correlated on other scales.It shows that with the growth and development of individual species in forest communities,the competition between species gradually tends to coexist.Through the renewal survey of the research plots,it was found that the dominant tree species in the arbor layer are consistent with the newer dominant tree species,which fully indicates that the natural dark coniferous mixed forest community in Sejila Mountain is in a stable stage of renewal.5.The relationship between the renewal characteristics of the founding species of forest communities and the heterogeneity of typical habitats in Sejila Mountain,Tibet The theory of triple-mechanism community succession is further confirmed in the study of the secondary succession of the forest community in Sejila Mountain,Tibet.The arbor regeneration process showed three effects: inhibition,promotion and tolerance.Shrub micro-habitat promotes the settlement of arbor seedlings in early succession grasslands.It inhibits the settlement of arbor seedlings in the forest community in the late stage of succession.Shrub vegetation can inhibit the biomass of seedlings.However,due to different habitat conditions and different tree species,species have different ways of distributing biomass above and below ground,resulting in three effects of shrub-grass micro-habitat on the rhizome ratio during seedling growth,including inhibition,tolerance,and tolerance.The litter has a positive effect on the settlement of arbor seedlings in the grassland in the early succession stage and a negative effect on the habitat in the late succession stage.The litter has no significant effect on the growth of the seedlings,but only interacts with the formation of the habitat,showing a patience effect.The need for water during seed germination makes seed burying have a positive effect on seedling settlement.Seed burying affects the biomass allocation of the seedling roots in the early stages of succession,shows negative effects on seedling root growth,but does not show other effects on seedling growth during succession.In summary,the low-altitude system development and functional structure of the arbor layer are aggregated,inferring that environmental filtration plays a leading role,which may be closely related to the interference of low altitude.The species richness at mid-altitude is high,and the phylogeny and functions are in a discrete structure.It is inferred that competitive exclusion plays a dominant role at mid-altitude.At high altitudes,the environmental pressure increases,and the phylogeny and functional traits are in an aggregated structure,and environmental filtration also plays a leading role.Undergrowth shrubs and herbs present discrete structures at low and mid-altitudes,and aggregate structures at high altitudes.Competition between low-and mid-altitude species plays an important role in coexistence of species under the forest.High altitudes are dominated by environmental filtration.The experimental results also revealed the importance of the niche process in the construction of forest communities on the Qinghai-Tibet Plateau.Environmental and spatial variables have a significant impact on the β diversity of species,phylogeny,and function.The spatial variables explain the diversity more than environmental variables.We infer that the seed dispersion in the forests of the Sejila Mountains elevation gradient may play an important role.Therefore,niche differentiation and seed distribution restrictions may jointly drive the distribution pattern of the forest community along the elevation gradient in Sejila Mountains.The research on the renewal mechanism of the species in the Sejila Mountains further confirmed the "triple mechanism community succession theory".The renewal process of secondary succession of arbor in the forest community of Sejila Mountain in Tibet showed three effects: inhibition,promotion and tolerance.The growth of arbor seedlings depends on the environmental factors of water and light in different environmental growth processes.Their regeneration is affected by biological factors and abiotic factors.It is an extremely complex ecological process.According to the different responses of abies georgei var.smithii and picea likiangensis var.linzhiensis Cheng et L.K.Fu to habitats in different succession stages,the seeds were sown in the process of vegetation restoration in Sejila Mountains.In the grassland,the shrub vegetation and litter cover were increased at the emergence stage and the cover after emergence was reduced.In forests,targeted tending measures such as appropriate removal of shrubs and litter promote the renewal of forest communities to promote and accelerate the community succession and vegetation restoration of Sejila Mountain. |
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