Response Of Soil Water On Poplar Shelter Forest Degradation In Bashang Area Of Hebei Province

The Three-North Shelter Forest Program in semi-arid region of China has a history of 40 years,which has achieved remarkable results.The shelter forest plays an important role in ecological services.Bashang area of Hebei Province is the key area of the construction of ecological shelter forest.In recent years,poplar shelter forest has experieneed a series of degradation phenomena,such as overripening and aging,withered shoots and branches,which has aroused the common concern of all sectors of society.Soil water is a key limiting factor for the growth of shelterbelt in Bashang area,and an important factor for uncovering the degradation mechanism of shelterbelt.However,there is still a lack of large sample data based on field observations.In order to explore the change of stand structure and the response of soil water to the degradation of Poplar shelter forest,56 tree forest plots with different degrees of decline,including Poplar and Pinus sylvestris var.mongolica,and 34 grassland,farmland,natural regeneration Poplar and artificial regeneration of Pinus sylvestris var.mongolica,were selected as the research objects in this study.In 90 typical plots,the vertical distribution of soil water at depth of 2 m was studied by artificial soil drilling combined with drying method,and the differences of soil moisture among different vegetation types were analyzed.At the same time,the growth status and regeneration of trees and shrubs in the plots were investigated to fully understand the changes of stand structure and composition of degraded ecosystem during the degradation of poplar shelter forest.Based on field investigation and experimental analysis,the results showed that:(1)In the degradation process of Poplar,the critical threshold was 20%of the proportion of dead shoots to trunk.Less than 20%is gradual natural degradation,while more than 20%is mutant severe degradation,which lacks obvious transition and shows serious decline directly.The median value of the mutant severe degradation sample plot is 55%,and the degradation characteristics are obvious,which is in marked contrast with gradual natural degradation.(2)From plot scale to individual tree scale,the relationship between DBH and tree height of poplar shelter forest follows logarithmic function model,that is,with the increase of DBH,tree height follows logarithmic function,which increases rapidly at first and then slowly.With the degradation of Poplar shelter forest,the canopy density of trees decreases,while the coverage of understory herbaceous plants will follow the logarithmic function relationship and show an upward trend.(3)At the depth of 60-90 cm underground,soil water concentration occurs under different vegetation types,that is,soil moisture tends to be similar,and then gradually decreases in the downward Poplar and Pinus sylvestris plots,while soil water of other vegetation types inereases gradually.The soil water of 0-60 cm surface of natural regeneration Poplar and artificial regeneration of Pinus sylvestris var.mongolica was significantly lower than that of other vegetation types,indicating that they consumed more soil moisture on the surface.(4)The soil water content of mutant severe degradation Poplar plot at depth of 0-60 cm was 8.27±2.59%,which was significantly higher than that of gradual natural degradation Poplar shelter forest plot(6.199±2.02%)(p=0.0026).Under 60 cm depth,there was no significant difference in soil water degraded Poplar land and healthy poplar land(P>0.05),which indicated that the significant response of soil water to the degradation of Poplar forest was only reflected in the surface 60 cm.(5)The depth of 60 cm was an important turning point of vertical distribution and changing reasons of soil water content among different vegetation types and decline Poplar.The depth of 0?60 cm is mainly affected by plant roots and precipitation,while the depth below 60 cm is mainly influenced by soil texture.In this study,a large sample plot method was used to systematically study the eco-hydrological process of soil water replenishment and increase after poplar shelter forest degradation.The results of this study have certain reference value for the restoration of overripe shelter forest in Bashang area,in order to provide a reasonable explanation for the degradation mechanism of shelter forest in Bashang area from the perspective of soil water,and provide a theory for the current ecological engineering construction in Bashang area.