| Water is an important ecological limiting factor in vegetation growth in arid and semi-arid regions of northern China.In the soil-plant-atmosphere continuum system,atmospheric precipitation is intercepted,distributed,and permeated,and completes the transport process from the atmosphere to the soil through the tree canopy,litter layer,and soil layer,respectively.The last step of the hydrological cycle is the transpiration of plants,mediating the return of water from the soil to atmosphere.Understanding the forest hydrology processes and the hydrological effect of forest land is necessary to devise strategies for optimizing plant water utilization,regulation countermeasures,forest land water distribution,and water transport.It is also essential for finding the suitable plant water environment capacity and density for vegetation growth and structure optimization and regulation in sandy regions.It provides a scientific basis for ecological water use in sandy regions and for studying vegetation growth and ecological protection.The Horqin Sandy Land in northern China is a transition zone between the northeast plain and the Inner Mongolia Plateau,as well as between the grassland and the desert,making this an ecologically fragile area in northern China.This study was conducted in Zhangwu County,Liaoning Province,on the southern edge of the Horqin Sandy Land,based on previous studies of the existing shelter forest construction and natural growth conditions of Populus alba var.pyramidalis,Pinus sylvestris var.mongolica,Salix cheilophila,and Atraphaxis bracteata.Previous studies have been carried on transpiration,water consumption,rainfall interception,litter and soil properties and their hydrological effects.In order to provide a theoretical basis for the restoration and maintenance of vegetation in this region,the water carrying capacity of vegetation and the ecological protection function of forest trees were comprehensively analyzed.（1）The variation trend of meteorological and soil factors in the study area showed the typical environmental change characteristics of semi-arid areas.Combined with rainfall frequency year analysis,therefore,2019 was a wet year,while 2020 was a dry year.All tree species showed the same seasonal trend of sap flow.In general,the sap flow of various tree species in wet years was higher than that in dry years,indicating that wet years have sufficient rainfall,higher soil moisture content,and more intense water consumption through plant transpiration.The daily sap flow of each tree species changed as an inverted U-shaped curve.At noon,the sap flow decreased,the plants took a short"nap"and their stomata tended to close.While,nocturnal sap flow of all tree species still occurred,indicating that water replenishment at night is relatively strong.Through correlation and stepwise regression analysis of sap flow and environmental factors on monthly,daily,and hourly scales,the results showed that transpiration of each tree species is mainly affected by water vapor pressure deficit and total solar radiation.（2）The time-delay effect of sap flow was determined using the dislocation contrast method.Then,the Penman Monnteith equation was used to calculate the stomatal conductance of each tree species.The stomatal conductance tended to be lower before and after the growing season and higher in the middle,but the seasonal variation was not remarkable.The dimensionless decoupling coefficient was affected by the negative feedback of aerodynamic conductance that was lower in wet years and slightly higher in dry years.However,the difference was not significant.The process of plant sap flow is more dominated by water vapor pressure deficit,and stomatal conductance has a strong control over transpiration.The established Jarivs type model comprehensively considered the effects of water vapor pressure deficit and total solar radiation on stomatal conductance of plant leaves and determined the simulation accuracy of each model.The simulation effect of the model in wet years was better than that in dry years,and the simulation effect of the tree P.pyramidalis and the shrub A.bracteata was better.（3）In this study,the formula gs=A-b VPD-1/2 was used to calculate and analyze the sensitivity of b(mmol.m-2s-1 Kpa-1),representing the effect of stomatal conductance on water vapor pressure deficit.The water regulation strategies of trees and shrubs were obtained,which reflect the drought tolerance of vegetation in arid areas;and the stomatal conductance of various tree species was more sensitive to environmental changes in dry years than in wet years.（4）During the study period,the rainfall type was mainly light rain and moderate rain.he highest proportion of the total runoff was from the penetration of rainfall through each tree species,followed by canopy interception and stemflow;rainfall penetration through the arbor was most affected by rainfall magnitude,diameter at breast height,and under-branch height.Shrub stemflow was most affected by rainfall magnitude,diameter,and canopy closure.The revised Gash model effectively simulated the canopy interception of P.mongolica plantation in sandy regions.（5）Based on the input and output data of forest land water,the minimum water nutrient area and water environment capacity of each tree species were calculated,indicating that the water niche of forest trees was significantly compressed in dry years compared with that in wet years.Similarly,factor analysis was carried out in combination with various indicators of vegetation structure and eco-hydrological function to obtain tree height,minimum water nutrient area,penetration rate,accumulation of semi-decomposed layers of litter,capillary water holding capacity,natural water content,fractal dimension（which are the factor variables for the degree of soil wind erosion control）,maximum soil water holding capacity,and water environment capacity of the forest land（which are the factor variables for the degree of soil water erosion control）.（6）According to the field wind erosion measurement,the windbreak effects of A.bracteata,S.cheilophila,P.pyramidalis,and P.mongolica inside and outside of the forest were4 H and 0.5 H,2 H and 1 H,0.5 H and 4 H,and 1 H and 2 H,respectively.Due to the effective windbreak distance outside the forest,the sediment transport in the shrub forest was greater than that in the arbor forest.The proportion of fine sand collected was relatively high,the composition of the particles of different sizes carried by wind erosion is also closely related to the amount of sediment transported.Based on the above research conclusions and data,this study used the Summation-Curve Method（SCM）to simulate and analyze the long-term wind and sand control effects of single and different species of trees and shrubs.Our results indicated that the sand blocking effect was significant inside shrub forests and outside arbor forests.Under different tree species configurations,the effective protection range of shrubs and arbor species was still limited on the leeward side of the forest belt,but could effectively prevent wind erosion on the windward side.A study of wind and water combined with erosion showed that from the perspective of external force,single hydraulic erosion might weaken the effect of wind erosion.However,considering the location and occurrence of sediment transport on the leeward side of the forest belt and the erosion ditch,and combined with the long-term composite erosion of the SCM simulation analysis,the compound erosion has potential hazards.（7）In summary,for the management of sandy land ecological environments,arbor forests such as P.pyramidalis and P.sylvestris are still the core of shelterbelt construction,which display good hydrological effects of woodland and effective prevention of wind and water erosion.However,while the transpiration water consumption and the canopy interception are relatively large in the growing season,the minimum water nutrient area and water environment capacity are greatly limited in the dry years.Therefore,the stand structure is transformed,the density is reduced,and shrub species are introduced.For example,A.bracteata and S.cheilophila,can reduce environmental stress in the forest,such as high temperature and water vapor pressure deficit,improve the sensitivity of plant stomatal conductance,reduce transpiration,and effectively solve the problem of insufficient soil moisture and vegetation carrying capacity.It should be noted that the construction of shelter forests should also consider the direction,location,and long-term effects of compound erosion,and combine the above studies to maximize the ecological protection function of forest trees. |
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