Study On Soil Moisture Variation And Profit And Deficit Of Typical Forest In Loess Alpine Hilly Region

Datong County of Qinghai Province is located in the northwest arid and semi-arid area,with fragile ecological environment.In order to promote economic development and restore its ecological environment,the construction of forest and grass vegetation has become an important soil and water conservation project in ecological management in the past 50 years.Due to the limitation of traditional afforestation density theory and afforestation technology,as well as the insufficient estimation of local water resources and environmental capacity,the long-term water deficit of the stand will eventually lead to ecological degradation such as slow growth of the stand.In order to guide the stand structure layout under the condition of limited water resources,optimize forest management measures,and improve the quality of forest management,this paper took the yinmentan small watershed in Datong County as the research area,combined with field investigation and monitoring,traditional statistical analysis,geostatistical analysis and other research methods,systematically analyzed the soil moisture distribution characteristics of typical forest land in the watershed from March 2019 to March 2020.It mainly includesPopulus cathayana,Betula platyphylla,Picea crassifolia,Larix principis rupprechtii,Sabina przewalskii and Pinus tabulaeformis.The main conclusions are as follows:(1)Analyze the distribution characteristics of soil moisture in both vertical and horizontal directions:As the depth of the soil layer increases in the vertical direction,SWC first increases and then decreases slowly and eventually stabilizes.From a horizontal perspective,SWC gradually decreases as the slope position rises,and On the same slope,there is a slope bottom agglomeration effect.Stand types,soil profile depth and slope position changes will all have a significant impact on SWC.The order of influence degree is:stand type>soil depth>slope position.(2)The variation of shallow soil water storage in the Anmentan small watershed is much greater than that of the deep soil;during the year,the degree of variation is the smallest in winter and the largest in summer.Soil water storage has moderate or strong spatial autocorrelation in different soil layers,and the spatial dependence tends to decrease with increasing depth.The soil water storage in autumn is significantly higher than that in other seasons.The soil water storage in 1m thick layer reaches 211.55mm,while the soil water storage in spring is the lowest in all seasons.The soil water storage in 1m thick layer is 58mm lower than that in autumn on average.The soil water storage in summer and winter is between the two seasons.(3)The stand type determines the profit and loss of soil moisture,and the excessive stand density is the main reason affecting the stand stability in the study area.Based on the study of water consumption characteristics of forest trees,we divided the water deficit of loess Alpine hilly region into:water deficit of high forest density-Larix principis rupprechtii in shady slope and semi shady slope;water deficit of high forest density-Larix principis rupprechtii in shady slope and semi shady slope;Water deficiency of high water consumption tree species-Populus cathayana and Betula platyphylla.Throughout the growing season,most Pinus tabulaeformis forests and some Larix principis-rupprechtii forests had no surplus and basically achieved the forest water balance;most Sabina przewalskii forests and Picea crassifolia forests had a soil moisture surplus greater than 20 mm;The soil moisture deficit of most Populus cathayana forests land and some Larix principis rupprechtii forests with relatively high planting density and Betula platyphylla forests was more than 20 mm,and soil moisture needs to be supplemented to supply forest consumption in the coming year.The most serious water deficit in the forest land studied are Populus cathayana forest(density of1667 plants/hm~2)on the sunny slope at the low position,Betula platyphylla forest(density of 952-1111plants/hm~2)located in the middle shady slope and semi-shady slope and Larix principis rupprechtii forest(density of 2500 plants/hm~2)located in the middleshady slope and semi-shady slope.The above forests all have different degrees of soil water deficit,and the density of the forests needs to be adjusted.The current densities of Pinus tabulaeformis forests,Picea crassifolia forests and Sabina przewalskii forests with ages of 10?20 years in the study area are all less than and close to the appropriate density,which are 1333-2500 plants/hm~2,1600-2300plants/hm~2,1667-2500plants/hm~2,the growth is suitable,but it is necessary to pay attention to the subsequent management of the forest density as the forest age increases.