Research And Application On Water Consumption Characteristics Of Typical Forests In Eastern Qinghai

The eastern part of Qinghai is located at the intersection of the Qinghai-Tibet Plateau and the Loess Plateau,which is a “sensitive area” for climate change in Asia and even the entire northern hemisphere and is of particular importance to national ecological security.To improve the ecological environment,since the 1980 s,with the large-scale "tree and grass planting activities in Northwest China",the forest coverage rate has been significantly improved;however,due to the constraints of traditional concepts,extensive use of natural resources and with the acceleration of social and economic activities,in the actual afforestation process,problems such as single vegetation composition,excessive density and unreasonable stand structure appeared,resulting in ecological and environmental problems such as vegetation degradation,water conservation capacity decline and forest water imbalance.The current mechanism of forest evapotranspiration water consumption in this area is not well understood,and the relationship between forest water consumption and site conditions is still unclear,and no reasonable stand density have been proposed.Therefore,this study analyzed the environmental capacity of precipitation resources in eastern Qinghai;clarified the water consumption characteristics of typical forests and formed the water consumption law in the growing season;based on water consumption and soil moisture variation characteristics of typical forests,the soil moisture vegetation carrying capacity system was constructed;the response mechanism of forest transpiration to density under long-time scale of thinning was explored,which supplemented the theory of typical forest vegetation allocation;Finally,the irrigation water demand of typical forest land is determined,and the local water and electricity production capacity is fully utilized,so as to improve the yield and water use efficiency,so as to provide theoretical basis and guidance for the rational utilization of precipitation resources,the alleviation of deep soil water deficit and the regulation and transformation of plantation in eastern Qinghai.In this study,the water consumption characteristics and soil moisture changes were investigated on typical forests in three small watersheds in Datong County,eastern Qinghai Province,and the sap flow,meteorological and hydrological data of typical forests with different site types,forest ages and densities were continuously observed in the field.Moreover,the variations of soil evaporation and forest canopy interception evaporation were also measured.The results are as follows:(1)This study investigated the relationship between canopy transpiration and meteorological factors under different water conditions.It was found that the impact of meteorological factors on canopy transpiration of various tree species would be limited by soil drought stress,indicating that the main environmental limiting factor of canopy transpiration of typical stands in the study area was soil water conditions rather than meteorological conditions.Meanwhile,the maximum temperature,wind speed and precipitation accounted for a smaller proportion of the canopy transpiration variation,while the minimum temperature and the canopy transpiration variation had little effect(r<0.5).The diameter at breast height of each typical tree species can explain more than 86% of the variation of canopy transpiration in this area,indicating that the difference of canopy transpiration of various tree species is mainly related to the characteristics of the forest itself.(2)There are differences in the transpiration of each stand under different site conditions(altitude,aspect,and slope),which is affected by soil moisture,meteorological factors,and the physiological structure of the forest.The difference of transpiration among stand ages was mainly explained by diameter at breast height,indicating that diameter at breast height can be considered as one of the parameters in large-scale stand transpiration simulation.(3)The density effect had a significant positive impact on the diurnal canopy transpiration and nighttime sap flow of each forest.After 20 years of thinning and tending,the stand water consumption revealed a gradual downward trend with the reduction of stand density,indicating that the water consumption of plantation can be reduced by thinning.Moreover,this study explored the composite mechanism of various environmental factors on nocturnal sap flow,indicating that the response mechanism of nocturnal sap flow to hydrological factors is mainly the comprehensive effect of soil moisture,wind speed and saturated vapor pressure difference,but the comprehensive control effect is limited(≤61%)to the mechanism of nocturnal sap flow.(4)Based on the estimation model of the overall canopy stomatal resistance and meteorological factors established by the Penman-Monteith equation model,the water consumption estimates of typical plots were obtained.Combined with the actual water consumption of forests calculated by the soil water location flux method,the relative error of the two is within 12%,and the calculation results are relatively reliable,indicating that this method is feasible to estimate the water consumption of typical forests.(5)According to the law of forest land water consumption and soil water storage,the water deficit in the eastern part of Qinghai is divided into two forms: water deficit caused by high stand density – P.crassifolia Kom.,B.platyphylla Suk,L.principis-rupprechtii Mayr on shady slopes;water deficit caused by tree species with high water consumption—P.cathayana Rehd on semi-shady and semi-sunny slopes.(6)Under the existing rainfall resources,the reasonable stand densities of P.crassifolia Kom.,P.cathayana Rehd,B.platyphylla Suk,and L.principis-rupprechtii Mayr are 1200,702,979 and 1212 plants/ ha respectively.The comparison found that the stand densities of all forests with soil moisture deficit were greater than their soil moisture vegetation carrying capacity,indicating that there are certain problems in the spatial configuration of the existing vegetation types,which need to be adjusted urgently.(7)The irrigation water demand of each typical forest was different,which showed that the water demand of P.cathayana Rehd was the highest,with a value of 112.52 ～ 218.08 mm,and the water demand of P.crassifolia Kom.is the lowest,with a value of 36.88 ～ 155.3 mm.Moreover,the irrigation water storage showed different trends with different site types and forest ages.