Study On The Spatial Pattern Dynamics And Construction Assembly Of Populus Euphratica Communities In The Upper Reaches Of Tarim River

With climate change and unreasonable exploitation of soil and water resources,natural Populus euphratica communities have been seriously degraded,which endangers the sustainable development of the desert oasis seriously.Therefore,investigating the spatial pattern dynamics and construction mechanism of riparian Populus euphratica forest communities in the upper reaches of Tarim River desert is important to curb ecological degradation,build vegetation protection and restoration system,and promote sustainable economic and ecological development.In this study,three fixed monitoring samples of 11 ha are established in the upper Tarim River,and the species composition and community structure of the Populus euphratica forest communities are analyzed based on the plant species survey data.Spatial point pattern analysis based on a completely stochastic model,eight species multi-degree distribution models,and species richness distribution pattern are used in combination with different habitats and sampling scales in order to explore the spatial pattern dynamics and construction mechanisms of Populus euphratica forest communities.The main findings of this study are as follows.(1)The vegetation structure of the desert riverine Populus euphratica community was simple and the species were relatively rare,including 2 species of arbor,3 species of shrubs and 10 species of herbs;The species hierarchy within the habitat was clear and the arbor layer and shrub layer were dominant in the vertical structure.The tree diameter structures in different habitats were bell-shaped and inverted pyramidal shaped,respectively,and lacking the complement of young trees.The community was poorly regenerated and belonged to the declining population(2)Species richness was the highest in riverine habitats,and decreased in terrace and sandy habitats with habitat deterioration,where diversity index and richness index were significantly and positively correlated with soil moisture content,and evenness index was significantly and positively correlated with soil salinity.The total diversity index of Populus euphratica forest communities was negatively correlated with the difference of soil water content,and the species diversity decreased with the increase of water content difference between the two sections.(3)The overall spatial pattern of the dominant species was aggregated.The species in the riverine habitat were mainly aggregated,and the intensity of aggregation decreased with the deterioration of the habitat.Seedlings were aggregated in both the floodplain and the sandy land habitats,and the aggregation intensity decreased with the increase of species age,and the old trees were randomly distributed.The spatial distribution was scale-dependent,and the aggregation intensity decreased with the increase of the scale,which tended to be randomly distributed or even uniformly distributed at larger scales.Species interactions dominated at small scales,while habitat heterogeneity dominated at large scales(4)The spatial correlations of species at different developmental stages showed strong scale dependence,and both intraspecie and interspecific correlations(positive and negative)all decreased with the increase of scale,and eventually they all tended to be unrelated.The negative spatial correlations among individuals at different developmental stages within species gradually increased with increasing diameter differences.The negative correlations among heterospecies were stronger,and the correlations decreases with the increase of scale;The positive correlations of conspecies were greater than that of heterospecifics,and heterospecies were more susceptible to the constraining effect of large diameter scale on small diameter scale,indicating that conspecifics were more likely to coexist than heterospecies at small scales.(5)Dead Populus euphratica showed strong negative interactions with seedlings of each species,indicating that the death of Populus euphratica did not have a positive effect on population regeneration due to the special environment of the arid area.(6)The results of the multi-degree distribution model fitting indicated that neutral processes played an important role in the formation of species abundance patterns in the floodplain,terraces and sandy land habitats.The ecological niche model fitted better in the riverine habitat and weakened with habitat deterioration.Drought stress triggered by a significant reduction in soil moisture resulted in lower fruiting rates of community species,reduced disperser abundance,and reduced seedling colonization,which may have contributed to the better fit of the neutral theory model.(7)Species richness was the highest in the riverine habitat and lower in the terrace and sandy habitats as the habitat deteriorated.Species richness increased with the increase of sampling scale.For habitats with low species richness,the richness reached a maximum at small scales and then did not change with the increase of scale.The variance and coefficient of variation of species richness were found to vary discontinuously with the increase of sampling scalem.The habitat variance of floodplain reached the maximum at 60m×60m,terrace and sandy land both reached the maximum at 100m×100m.In this study,we found that the structural characteristics,quantitative characteristics,species diversity,spatial pattern,multi-degree distribution pattern and richness pattern of desert riparian Populus euphratica communities all had habitat heterogeneity and strong scale dependence,which fully illustrated the important role of habitat heterogeneity and distance effect in the community construction process.In a conclusion,ecological niche and neutral processes played a role in the community construction process of desert riparian Populus euphratica forests in arid regions.