Coupling Relationships Between Artificial Stand Structure And Soil And Water Conservation Function In The Loess Region Of Western Shanxi

Artificial afforestation and restoration of vegetation are usually used to prevent and control soil erosion and improve the ecological environment in the Loess Plateau.This study focused on the three kinds of typical Robinia pseudoacacia,Pinus tabulaeformis pure and mixed plantations in Caijiachuan Watershed of the Loess Plateau,Western Shanxi Province.After studying the impact factors of the horizontal,vertical structure,and soil and water conservation function,the coupling relationships between different stand structures and soil and water conservation functions were quantified,using the structural equation modeling methods.And then,suitable forest structure optimization measures were proposed,to solve the key issues about stand density regulation and forestry management in the region.The results could provide some references for the reconstruction of the Loess Plateau forest structure and the coordinated management of water and soil conservation.The specific findings include those as follows:(1)In terms of stand structure,the distributions of DBH and tree height of the two species of Robinia pseudoacacia forests,Pinus tabulaeformis forests,Robinia pseudoacacia and Pinus tabulaeformis mixed forests,and Populus davidiana and Pinus communis secondary forests,were all single-peaked.The distribution of their crown amplitude gradually decreased with the increase of crown width.The stand density and tree competition index were highly correlated;the canopy density of mixed forests was generally higher than that of the other three stands;the stand growth is generally more uniform.Among them,the distribution of secondary forests was more uniform than that of artificial forests,which indicated to a certain extent that,secondary forests were more suitable for site conditions in this area than artificial forests.The uniform angles were mostly cluster-like,with a small amount of random and uniform distribution.The difference in neighborhood comparison of mixed forests was smaller than that of pure forests,and the extent of difference and spatial distribution between mixed forests were closer to that of secondary forests.The larger the leaf area index and forest index,the more complex the vertical structure of forests.(2)In terms of soil and water conservation function of the stand,the function of conserving water in mixed forests was better than that of other forests.The conservation of soil and sand-retaining function of secondary forests and mixed forests was superior to Robinia pseudoacacia and Pinus tabulaeformis forests,while Robinia pseudoacacia has advantages in maintaining soil nutrients.The biodiversity protection function of secondary forests and pure forests was s better than mixed forests.(3)Using forest structure and its function coupling model based on structural equations,the functions of water and soil conservation of Robinia pseudoacacia,Pinus tabulaeformis,and mixed forests were mainly for sand interception and sediment conservation and water conservation.The water and soil conservation function factors that were more sensitive to structural factors were mainly the production flow,sand production,and canopy interception.The appropriate regulation structural factors affecting soil and water conservation functions were stand density,canopy closure,forest tree competition index(middle and low competition intensity),angle scale(uniform distribution),size ratio(uniformity of forest growth,small difference),forest layer index and leaf area index.In addition,topographic factors(slope,aspect,elevation)had a significant effect on the structural and functional coupling processes of forest stands.(4)Based on the characteristics of different forest stands,the reference path coefficient ratio provided a quantitative optimization of forest structure optimization measures,including thinning,selective cutting,replanting of Robinia pseudoacaci,or Pinus tabulaeformis,to adjust existing forest stands.The optimal regulation and control objectives were as below:the stand density was about 1600 plants/hm2;the canopy density was about 0.7,the tending period was not less than 0.6;and the angle scale was within[0.375,0.475].To achieve a uniform distribution;size ratio should be stable at about 0.5;the forest competition index was less than 1.41.Meanwhile,in accordance with the near-natural management measures,adopted to optimize the control of forest structural factors,the existing forest stands were gradually converted to forest layers,which are mostly distributed in two layers(forest layer index>0.37)and leaf area index to[2.29,5],Multi-layered Pinus tabulaeformis and Robinia pseudoacacia mixed forest with 6:4 to 7:3 coniferous and broadleaf ratio to significantly enhance various soil and water conservation functions,thereby solving the technical difficulties in the structural transformation of the Loess Plateau and providing references to coordinated management of soil and water conservation.