Study On Management Model Of Larix Gmelinii –birch Mixed Forest In The North Of The Great Khing’an Mountains Based On Structural Adjustment

This research focuses on the management and utilization of the second-generation forest(post-harvested forest)of the mixed forest of Dahurian larch(Larix gmelinii)and birch(Betula platyphylla)in the Great Khing’an Mountains,which is an important ecological security barrier and national timber strategic reserve base in northern China.Through extensive field measurements,guided by the theory of sustainable management,near-natural forest management and based on optimizing the forest structure,and using forest growth and succession as the entry point,this study proposes a management model for the mixed forest.The main conclusions are as follows:1.The structural characteristics of the post-harvested forest and the original forest of larch and birch were studied in terms of species composition,diameter structure,stocking structure,mixed degree,size-class distribution,angular scale,and vertical structure.The species composition of the post-harvested forest is reasonable,but there are more small-diameter trees and fewer medium and large-diameter trees,and the stocking structure is not reasonable due to the lack of diameter structure.The mixed degree is weak to moderate,the size-class distribution is relatively uniform,and the angular scale is characterized by random distribution,tending to be clustered.There is no obvious vertical structure.The original forest has a more complete diameter structure,and the stocking and spatial structures are more reasonable than those of the post-harvested forest.2.Based on the structural characteristics of the post-harvested forest of larch and birch and the needs of national and regional ecological and socio-economic development,a target-structured management system consisting of a total goal and three levels of management objectives was proposed.The total goal is to maintain forest ecological benefits and sustainable development,while taking into account timber reserve resources.According to regional socio-economic needs,the first-level management objective(species target),the second-level management objective(forest type target),and the third-level management objective(functional target)are determined in a progressive manner based on species,use,and benefits.3.Twelve classic non-linear models of height-diameter relationship were fitted,and it was finally determined that the Zeide model can best explain the relationship between height and diameter of larch,while the Ratkowsky model can best explain the relationship between height and diameter of birch.4.An entry-limit model was established with the total cross-sectional area and the number of trees as variables,a growth probability model was established with the average diameter at breast height(DBH),the number of trees in the diameter class,and the residual cross-sectional area as variables,and a retention probability model was established with the average DBH and residual cross-sectional area as variables for small,medium,and large diameter classes.The growth dynamic transition probability matrix was constructed based on the calculated results of the growth probability and retention probability of each diameter class.5.Using the stand structure of the top-level original forest as a reference and referring to reasonable structural indicators of mixed forests with different ages and needle-leaf and broad-leaf species,the optimal stand structure that satisfies the structural indicators of species composition,diameter structure,stocking structure,and tree density was calculated.The optimal stand structure has a species composition of 8 larch to 2birch,an average q value of 1.21,a stocking ratio of small,medium,and large diameter classes of 1:4:5,and a tree density of 1100 trees/hm~2.6.The adjustment of selective logging is divided into two stages.The first stage is the optimization adjustment stage,aiming to transition the structure of the current forest to the optimal forest structure.Thinning is carried out based on the principle of prioritizing the harvesting of small diameter trees while retaining medium diameter trees,with a harvesting interval of 8a and a harvesting intensity of 5.92%～15.19%,making current forest structure approaches the target structure.The second stage is the sustainable utilization stage,aiming to continuously protect and maintain the optimal structure achieved in the first stage,and to realize the ecological functions of the forest while balancing the reserve of timber resources.Single-tree selection is carried out based on the principle of first harvesting large diameter trees,poorly growing small and medium diameter trees,and retaining a small number of healthy mother trees.Harvesting is conducted with an interval of 20a and a harvesting intensity of 36.40%,achieving sustainable and cyclical harvesting.