| One of the main objectives of forest management is to promote tree growth and improve stand quality on a sustainable basis.Close-to-nature forestry management is a promising way of forestry management that meets sustainable standards.The target tree management system with a single tree as the operating object is the biggest difference between close-to-nature forestry management and other forestry management systems,and it is also the most important means to realize the close-to-nature transformation of artificial forest.The key and core technology lies in the selection of target trees and the determination of target tree density in each growth stage.The adjustment of stand density and target tree density runs through the whole life cycle of forest.Each stage of forest forward succession corresponds to an optimal density.Dynamic programming is an effective method to solve multi-stage decision-making problems and stand density optimization problems.In this study,Phoebe bournei plantation in Jindong Forest Farm of Hunan Province was selected as the research object.A dynamic programming model was constructed to determine the optimal stand density and target tree density at each growth stage of the stand under three different site conditions.Based on the comprehensive analysis of the current characteristics of forest stand,site conditions and the future development direction of forest,the operation methods of forest types under three kinds of site conditions were designed,and the full-cycle management measures for close-to-nature transformation of Phoebe bournei plantation were put forward.The main research contents and conclusions are as follows:(1)The construction of dynamic programming model of optimal density of Phoebe bournei plantation.Combined with the close-to-nature theory and the law of forest succession,the stand growth process was divided into five stages,and the stand number density and breast height basal area were selected as two-dimensional state variables to reflect the change of stand growth and stand density in each stage.The number of trees per hectare and basal area per hectare to be cut were selected as decision variables,and the decision-making variables of each stage were the optimal cutting amount,and the obtained state variables were the optimal retention density of each stage.According to the change law of stand density and basal area growth in each stage,the state transition equation was written.The maximum total harvest of each stage was taken as the objective function.The Richards growth function was used to fit the stand height growth model and breast height basal area growth model.The tree height growth models under three site conditions were as follows:H=25.089*(1-0.757*EXP(0.056*t)^(1/(1-0.528)),R2=0.802;H=22.764*(1-0.673*EXP(-0.061*t))^(1/(1-0.668)),R2=0.817;H=20.756＊(1-0.606*EXP(-0.065*t))^(1/(1-0.733)),R2=0.901;Thebasal rea growth model was:B=4.942*SI^0.846(1-EXP(-0.039*(N/1000)^0.292*t))^1.296,R2=869;The fitting accuracy of each growth model was high.Based on the stand growth model,a complete dynamic programming model was constructed to determine the optimal stand density and target tree density in each growth stage of Phoebe bournei plantation.(2)The determination of optimal density and target tree density of Phoebe bournei plantation.By introducing the dynamic programming model into Lingo 11.0,the optimal retention density of the stand at each growth stage under three different site conditions was obtained,and the felled trees in each stage were regarded as interference trees.According to the ratio of target tree to interference tree under each site condition,the optimal density of target tree at each stage was obtained.In the stand establishment stage and competitive growth stage,Phoebe bournei did not reach the standard and age of target tree selection;In the quality selection stage,the optimal retention target tree density of the stand under three site conditions was 364 trees/hm2,702 trees/hm2,1026 trees/hm2,respectively;In the target tree growth stage,and the optimal retention target trees density of the stand under three site conditions was 153 trees/hm2,327 trees/hm2 and 702 trees/hm2,respectively.In the stand volume growth stage,the optimal retention target tree density of the stand under three site conditions was:100 trees/hm2,136 trees/hm2,248 trees/hm2,respectively.The maximum total harvest of the stand under three site conditions was 764.93 m3/hm2,554.21 m3/hm2 and 410.23 m3/hm2,respectively.(3)The design of forest type operation method of Phoebe bournei plantation.The site conditions,tree growth and stand current situation of Phoebe bournei artificial forest were analyzed.In terms of the selection of tree species,the native tree species,constructive species and dominant tree species should be selected as far as possible to cultivate Phoebe bournei artificial pure forest into multi-tree species mixed forest.In terms of diameter structure,the single peak mountain diameter distribution of the evenaged forest should be gradually transferred to the inverted "J" diameter distribution of the uneven-aged forest.In terms of tree height structure,the growth of dwarf trees and undergrowth vegetation should be promoted,and the stand should be gradually guided from single-layer structure to multi-layer complex community structure.Based on the dynamic programming model,the objective forest shape under three kinds of site conditions in 60 years are obtained,from the composition and proportion of tree species,the final goal of close-to-nature transformation of Phoebe bournei plantation was described from the aspects of stand hierarchical structure,forest development goal,target tree density,target tree diameter and stand volume.The forest development type of the stand with site index 24 was Phoebe bournei-Schima superba heterostratified mixed forest,the forest development type of stand with site index 22 was Phoebe bournei-Taxus heterostratified mixed forest,and the forest development type of stand with site index 20 was Phoebe bournei-Cunninghamia lanceolata heterostratified mixed forest.(4)The suggestions on full-cycle close-to-nature transformation and management of Phoebe bournei plantation.In the stand establishment stage,tending measures were taken,such as applying special fertilizer,weeding and loosening the soil comprehensively,so that most of the planted seedlings survived until they were established,and ensured that the seedlings have sufficient nutrients and light.In the competitive growth stage,added organic fertilizer to improve the stand soil to promote the growth of Phoebe bournei manure,thinned the slow-growing and poor-quality Phoebe bournei in places with excessive density.The density should be controlled within a reasonable range.After the quality selection stage,the first generation target tree was selected and cultivated,and the stand density,target tree density and selective cutting intensity were determined according to different site conditions.In this stage,the cutting intensity was high,and the interference tree can be cut many times.In the target tree growth stage,continued to cultivate the target tree,selectively cut down the interference tree,harvested the target tree that reaches the target diameter.The forest gap formed under the interference tree and the harvested target tree,and replanted the seedlings of native tree species adapted to the site conditions to promote and protect the regeneration under the forest;In the stand volume growth stage,began to cultivate the second generation target trees,maintained the natural regeneration of the forest gap,and formed a multi-layer,mixed,even-aged and close-to-nature forest. |
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