| This study focused on four kinds of typical forest which Robinia pseudoacacia Linn.,the Pinus tabulaeformis Carr.,and its mixed forest,as well as secondary?Populus davidiana Dode and Quercus dentata Thunb.?in Caijiachuan watershed on the Loess Plateau in western Shanxi,China.The data of stand structures?stand density?SD?,tree height?TH?,diameter at breast height?DBH?,canopy density?CD?,tree crown area?CA?,the leaf area index?LAI?,angle scale?AS?,tree competition index?TCI?,tree size ratio?SR?,and the forest layer index?FLI??and soil and water conservation function?the function of conserving water,the function of maintaining soil nutrients,the function of sand-retaining?were obtained by the forest investigation and fixed observation,and these data were used to analysis the characteristics of stand structure and soil and water conservation.Also,a comprehensive analysis for soil and water conservation function was conducted based on these data,and the type of low-efficiency forest was determined according to the result of comprehensive analysis.The identification and grading of low-efficiency forest and the formation mechanism of low-efficiency were conducted in this research.Particularly,the structure factors which can be regulared were identificated and screen outted according to the coupling relationship of stand structure and soil and water conservation,and the influence pathes and influence intensities between structure and function,and the threshold and regulation ranges of these structure factors are quantified through the coupling relationship.This study aims to solve the key technology of stand structure optimization of low-efficiency forest,which mainly focuses on regulating stand density,so as to provide the scientific basis and theoretical supports for realizing precise control and spatial allocation optimization of stand structures of soil and water conservation forest on the Loess Plateau in western Shanxi,China.The main results of this paper are as follows:?1?In terms of stand structure,the stand density distribution of the R.pseudoacacia,the P.tabulaeformis and mixed plantations has obvious difference,while the stand density distribution of the secondary forest more uniform.There are some similarities and differences on the stand structure of these four typical forests.The horizontal stand structures of these four forests showes strong regularity,while those vertical stand structures showes weak regularity.The stand structures of mixed forest is more reasonable than that of pure forest,and such stand structures configurations more conducive to the forest to play the function of water and soil conservation.?2?Concerning the soil and water conservation function,the comprehensive capacity of water conservation of mixed forest is better than other three forest types.Soil fertility of secondary forest is higher than that of artificial forest,and the contents of TN and TP were higher than other indexes.The benefits of water storage and sediment reduction in secondary forest are better than artificial forest.The amount of soil and water loss in mixed forestland is less than that of R.pseudoacacia and P.tabulaeformis forestland.The comprehensive evaluation results of the soil and water conservation functions of the four kinds of stands indicated that it is urgent to carry out the research concerning the optimization of stand structure configuration of artificial pure forests.And the research on the optimization of stand structure configuration of low-efficiency R.pseudoacacia forest is preferred to carry out because the variation of forest ecosystem more than P.tabulaeformis forest.?3?The R.pseudoacacia forest is divided the normal forest and three low-efficiency forests which mild low-efficiency,moderate low-efficiency and severe low-efficiency guided by the soil and water conservation function according to the distribution characteristic curve of stand structure factor and the comprehensive index of soil and water conservation?SWBI,0?10?.The SWBI of normal forest ranged from 6 to 10,and the proportion of 63.59%.The SWBI of mild low-efficiency ranged from 4 to 6,and the proportion of 16.41%.The SWBI of moderate low-efficiency ranged from 2 to 4,and the proportion of 13.33%.The SWBI of severe low-efficiency ranged from 0 to 2,and the proportion of 6.67%.?4?Concerning the reasons for low-efficiency of soil and water conservation in R.pseudoacacia forest,the results of the coupling relationship of stand structure and soil and water conservation in three low-efficiency forest suggested that these stand structures that the SD,the TH,the CA and the LAI have significant impact on soil and water conservation function in the mild low-efficiency forest.The SD,the TH,the CD,the TCI and the AS have significant impact on soil and water conservation function in the moderate low-efficiency forest.In the severe low-efficiency forest,the SD,the TH,the TCI,the LAI and the CD have significant impact on soil and water conservation function.The results of the coupling relationship of structure and function showed that the SD has a significant influence on the other main stand structure factors.The results of the stand structure characteristics in these three low-efficiency forest indicated that the five main forest structure factor which SD,the CD,the LAI,the TCI and the CA have a polarization distribution trend with the increase of the level of low-efficiency,while the distribution trend of TH is decreasing.Therefore,unreasonable configuration of stand structures is the main reason for causing low efficiency of R.pseudoacacia forest.Additionally,the soil and water conservation function of R.pseudoacacia forest is from bad to worse under the influence of the climate warming and drying,soil moisture content declining,and low soil fertility.?5?The results of optimal configuration of stand structures in a low-efficiency R.pseudoacacia forest showed that the optimal configuration of stand structures of mild low-efficiency R.pseudoacacia low-efficiency R.pseudoacacia forest as follows:SD=1529 plant·hm-2,CD=0.66,TH=9.86 m,TCI=2.14,AS=0.62;The severe low-efficiency R.pseudoacacia forest as follows:SD=1459 plant·hm-2,CD=0.61,TH=9.39 m,TCI=2.03,LAI=2.13.The ecological benefits of soil and water conservation of the mild,moderate and severe low-efficiency R.pseudoacacia forest are expected to increase by about0.86 times,3 times and 6 times,respectively,compared with the pre-optimization.The structural optimization model equation of stands with different low efficiency types was verified,and the results of response surface analysis showed that the structural optimization model equation could be used to estimate the benefit value of soil and water conservation?APE<10%?.Among the above stand structure factors,the SD is easy to be directly regulated,while the TH,the CD,the LAI,the TCI and the AS are not easy to be directly regulated.The results of the regression model equation of the main stand structures and SD of these three low-efficiency R.pseudoacacia forests showed that the main stand structure factors can be optimized by adjusting the SD.?6?The appropriate stand density obtained under guided by the soil and water conservation function was verified through the response relationships between soil water resources and soil nutrient resources of R.pseudoacacia forest and stand density.Accordingly,the optimal stand density of R.pseudoacacia forest was determined to be controlled within 1400?1700 plant·hm-2 on the Loess Plateau in western Shanxi,China.The density of R.pseudoacacia forest should be regulated to this range to ensure a sustainable soil and water conservation function. |
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