Effects Of Different Stocking Rates On Water Use Efficiency Of Stipa Breviflora Desert Steppe

As an important indicator to characterize the water-carbon balance,water use efficiency reflects the carbon,water cycle and their coupling relationship in terrestrial ecosystems.The desert steppe located in the hinterland of the Eurasian continent has caused serious grassland degradation due to the disturbance of long-term overgrazing.Therefore,a systematic study of the response mode of water use efficiency to grazing is the key to a comprehensive understanding of the stability and sustainable use of terrestrial ecosystems.In this study,an experimental area was set up in the desert steppe of Stipa brevifloras in Siziwang Banner,Ulanqab City,Inner Mongolia Autonomous Region,and the sample plot was arranged by a completely randomized block experimental design.There were control treatment area(CK),light grazing treatment area(LG),moderate grazing treatment area(MG)and heavy grazing treatment area(HG),and each treatment was repeated three times.From 2017 to 2019,the basic quantitative characteristics and water use efficiency of Stipa brevifloras desert steppe constructs,life-form functional groups,plant communities,and ecosystem carbon,water flux,and topsoil physicochemical properties were monitored.One-way analysis of variance was used to the basic quantitative characteristics and water use efficiency of constructive species,functional groups,and plant communities,as well as ecosystem carbon and water fluxes,were tested for significance.Factor analysis,canonical correlation analysis,and correspondence analysis were used to analyze the basic quantitative characteristics of different vegetation tissue levels.The correlation analysis was carried out with water use efficiency,and the grey correlation analysis method was used to analyze the correlation of water use efficiency between different vegetation organization levels and the regulation effect of different vegetation organization levels and soil surface physicochemical properties on ecosystem water use efficiency.The main findings are as follows:(1)The intrinsic water use efficiency and instantaneous water use efficiency of Stipa brevis were decreased with the increase of stocking rate,while the long-term water use efficiency of the constructive species showed an increasing trend.Compared with the intrinsic water use efficiency and instantaneous water use efficiency of the constructive species,the long-term water use efficiency of the constructive species was more closely related to its basic quantitative characteristics.The long-term water use efficiency of the constructive species was negatively correlated with its height,and positively correlated with its coverage,density and the standing crop of biomass.(2)With the increase of stocking rate,the water use efficiency of perennial grasses,perennial forbs and annual-biennial plants showed an upward trend,and the water use efficiency of shrubs and semi-shrubs was the highest under moderate grazing treatment.The water use efficiency of perennial grasses has a negative correlation with its height,and is restricted by the basic quantitative characteristics of perennial forbs and shrubs and semi-shrubs.The water use efficiency of perennial forbs was negatively correlated with its coverage.(3)Compared with the control treatment area,the water use efficiency of plant community showed an increasing trend in the light grazing and moderate grazing treatment areas,and showed a decreasing trend in the heavy grazing treatment area.The relationship between plant community water use efficiency and its standing crop of biomass is different under different stocking rates.Under light grazing,the correlation between the two is not obvious;under moderate grazing,the two are negatively correlated;under heavy grazing,the two are positively correlated.(4)The water use efficiency of the ecosystem is affected by the stocking rate,and the change rule is not obvious,but the water use efficiency of the ecosystem in the moderate grazing treatment area is relatively high.Soil nutrient content and soil stoichiometric characteristics are the main index systems that affect the water use efficiency of the ecosystem,while the index system of soil physical properties has less effect on the water use efficiency of the ecosystem.(5)The long-term water use efficiency of the Stipa brevifloras is higher than that of the ecosystem;the long-term water use efficiency of Stipa brevifloras is most affected by its own quantitative characteristics,followed by the intrinsic water use efficiency and instantaneous water use caused by photosynthesis.The least influential factor is the water use efficiency of the ecosystem.The change of the basic quantitative characteristics of the functional groups themselves is the most basic material basis that affects its own long-term water use efficiency and ecosystem water use efficiency.The long-term water use efficiency of functional groups is affected by their own quantitative characteristics in the order of perennial grasses,annual-biennial plants,perennial forbs and shrubs and semi-shrubs.This order is also the effect of long-term water use efficiency on ecosystem water use efficiency.The long-term water use efficiency of the plant community is less affected by the water use efficiency of the ecosystem than its own basic quantitative characteristics.