Effects Of Changes In Groundwater Depth And Soil Physicochemical Properties On Plant Communities In The Lower Shule River

Groundwater,as the main limiting resource for vegetation growth in northwest China's arid regions,can change the local soil environment and microclimate through its fluctuating changes?up or down?,which will have a significant impact on species growth,distribution and functional structure of plant communities.Most domestic and international research on groundwater and natural vegetation in arid areas has focused on exploring the effects of groundwater on soil water content,soil salinity,and plant distribution.Less reveals the impact of changes in groundwater depth on plant functional traits in such communities,which is crucial for predicting how the arid region ecosystem responds to environmental changes.There is also a lack of theoretical support for arid region related research in explaining the groundwater-soil-vegetation triad.Therefore,this paper takes the Dunhuang Xihu National Nature Reserve of the Lower Shule River,a typical arid area in northwest China,as the study area.180quadrats?60 trees/shrubs and 120 herbs?were set up in five locations with significant changes in vegetation type and different depths of groundwater from north to south in the core areas of the reserve?Ma juan wan,Hou keng and Huo shao jing?.Groundwater depth,major community characteristics parameters?height,abundance and biomass,etc.?,10 key leaf traits and soil physicochemical property parameters?soil organic carbon,total soil nitrogen and total soil phosphorus,etc.?were obtained in the set-up quadrats mainly through field measurements and indoor experiments.Furthermore,a variety of statistical methods are used to analyze the effects of groundwater depth and soil physicochemical properties on plant communities in the lower reaches of Shule River.The main conclusions reached are as follows:1.The change of groundwater depth in the study area affects the distribution of water,nutrients and salt ions in the soil,and has a greater impact on the chemical properties of the soil.Among them,with the increase of groundwater depth,soil organic carbon?SOC?,soil total nitrogen?STN?,and soil total phosphorus?STP?and total dissolved salt content?TDS?showed an increase and then a decrease,while soil p H and soil bulk density?SBD?showed the opposite trend;with the increase of groundwater depth,soil water content?SMC?decreased significantly,while soil C:N showed a significant increase trend.The distribution of salt ion content in the soil mainly shows that the surface layer is mostly,and it decreases gradually,but also due to the difference in the depth of groundwater and the migration ability and law of the ion itself,the distribution of the middle layer is high and the distribution of the surface and bottom layers is small.2.Changes in groundwater depth in the study area are the main driving force for changes in the main characteristics and functional traits of plant communities.Among them,the analysis of the effect of groundwater depth changes on the major characteristics of the plant community showed that with the increase of groundwater depth,the type,structure and species composition of plant communities tend to be simple,and the whole study area constituted a distribution of the main types of Populus euphratica-Tamarix ramosissima-Phragmites australis community.Plant community cover,species abundance and species richness decreased progressively,while the opposite was true for the Pielou index;plant community height,community biomass,the Simpson index and the Shannon-Wiener index showed an increasing and then decreasing trend with increasing groundwater depth.The analysis of the effect of groundwater depth changes on the functional traits of plant communities showed that the community-level weighted mean?CWM?of leaf functional traits in tree/shrub and herb differed significantly in different groundwater depths?p<0.05?;with the increase of groundwater depth,the changes of CWM values in tree/shrub and herb were not the same.Both specific leaf area?SLA?and maximum leaf photosynthetic rate(A_max)increased with increasing groundwater burial depth,while leaf dry matter content?LDMC?and leaf thickness varied inversely.Leaf phosphorus content?LPC?at the tree/shrub and herb community levels remained relatively stable with groundwater burial depth.The indicators SLA_CWM,LDMC_CWM,A_{max CWM} and Height _CWM were more sensitive to changes in groundwater depth in both tree/shrub and herb.3.Changes in the physicochemical properties of the soil in the study area have a greater impact on the community-level plant functional traits than on the plant community main characteristics.Among them,analysis of the effects of changes in soil physicochemical properties on the major characteristics of plant communities showed that each index value of soil physicochemical properties has a certain suitable range,and only within this range the values of major community characteristics?cover,biomass,etc.?can be maintained at a high level.Analysis of the effects of changes in soil physicochemical properties on community-level plant functional traits showed that soil p H,soil total dissolved salt content,soil water content?0-40 cm?and soil total phosphorus content had the greatest influence on CWM values in herbaceous communities;soil p H,soil moisture content and soil organic carbon content had the greatest influence on CWM values in tree/shrub communities.4.The groundwater depth and key soil chemical properties in the study area are the main driving factors for species distribution and vegetation growth changes.The results of generalized canonical discriminant analysis and cluster analysis show that,the dominant species Agropyron cristatum,Apocynum venetum,Tamarix ramosissima and Phragmites australis are more suitable for growth in sites with shallow groundwater depth and high soil nutrient content,while Glycyrrhiza uralensis,Populus euphratica,Alhagi sparsifolia and Lycium ruthenicum are more distributed in sites with deeper groundwater depth and poorer soil.The results of co-collinearity screening and redundancy analysis indicated that 33.66%of the variation in the distribution of tree/shrub species could be explained by nine environmental factor variables and 32.15%of the variation in the distribution of herbaceous species could be explained by ten environmental factor variables.Through further analysis of the variance of environmental factors,it is found that changes in groundwater depth are the main driving force for species distribution in arid areas,which can explain the 21.16%changes in species distribution,followed by soil K⁺?9.94%?and soil total nitrogen?4.9%?,which together with the interaction of the three explained 41.7%of the variation in vegetation distribution.Finally,a structural equation model of groundwater depth-soil physicochemical properties-vegetation in the study area was constructed,and the main influence relationship among the three was found:the change of groundwater depth affects the growth of vegetation by changing soil structure and soil nutrient content.5.Based on the drought resistance,ecological niche and main characteristic parameters of plants,the optimal groundwater depth range of the dominant species was comprehensively judged and given.Among them,drought tolerance of dominant species showed that 8 dominant species were ranked in order of drought tolerance:Phragmites australis<Tamarix ramosissima<Agropyron cristatum<Apocynum venetum<Glycyrrhiza uralensis<Alhagi sparsifolia<Populus euphratica<Lycium ruthenicum;The results of the niche widths and niche overlaps showed that the average niche width of the eight dominant species in the three resource dimensions?groundwater depth,soil total nitrogen and soil K⁺?were in the following order:Agropyron cristatum<Populus euphratica<Phragmites australis<Apocynum venetum<Tamarix ramosissima<Glycyrrhiza uralensis<Lycium ruthenicum<Alhagi sparsifolia;in general,the overlaps of the species in the three resource dimensions were large,with greater niche overlaps in the soil K⁺and soil total nitrogen resource dimensions than in the groundwater depth resource dimension.Based on the above research results,the optimal range of groundwater depth for each dominant species was determined as follows:Agropyron cristatum?0.67-2.34 m?,Glycyrrhiza uralensis?0.67-5.86 m?,Phragmites australis?0.67-2.34 m?,Apocynum venetum?0.67-4.33 m?,Alhagi sparsifolia?2.34-5.86 m?,Tamarix ramosissima?0.67-4.33 m?,Lycium ruthenicum?2.02-5.86 m?,and Populus euphratica?2.34-3.06 m?.m),black-fruited wolfberry?2.02-5.86 m?,and poplar?2.34-3.06 m?;overall,soil nutrients and vegetation growth were maintained at their best within 2-4 m of the groundwater depth range in this study area.Through field surveys,laboratory experiments,and mathematical statistics,in-depth discussion of the relationship between groundwater depth-soil physicochemical properties-vegetation in inland river basins in typical arid regions.It has enriched the research results of the ecological research direction in arid areas,has important scientific value,and has a certain role in promoting the interdisciplinary research of ecology,geography and soil science.The research conclusions can provide a scientific basis for vegetation restoration,ecosystem management and the reasonable allocation of ecological water in arid areas.