| As the carrier of forest tree growth,forest soil directly affects the growth and distribution of forest plants.The nutrient content of understory soil was affected by the amount of litter and the rate of litter decomposition.Therefore,scientific evaluation of soil nutrient content and determination of measures to improve forest soil nutrients were of great significance for improving vegetation productivity and achieving sustainable forest management.The spatial structure of stands dominates the composition of understory vegetation,which in turn affects the rate of nutrient return of forest litter and changes in soil nutrient supply capacity.Optimizing the spatial structure of stands was an important means to improve soil nutrient content.The relationship between nutrients was very important.Based on the survey data of 12 sample plots in 2018,four typical stands of Chinese fir pure forest,Chinese fir broad-leaved mixed forest,natural secondary forest and bamboo forest in Fushou forest farm,Pingjiang County,Hunan Province were taken as the research object.Structural characteristics and soil nutrient conditions,the multiplication and division method was used to comprehensively evaluate the soil nutrients of each stand,and the relationship between the stand spatial structure and the individual and combined index values of soil nutrients was studied by the grey correlation method,so as to find the influence of soil nutrients.According to the research results,optimization measures were proposed for the four typical stands of Fushou Forest Farm.By optimizing the stand space structure,it provided scientific basis and technical support for improving forest soil nutrients in Fushou Forest Farm.Conclusion as below:(1)Analysis of the spatial structure characteristics of four types of stands.Using the three stand spatial structure indicators of mingling degree,uniform angle index and storey index,the spatial structure of four typical stands in the study area was analyzed.The results show that,the mingling degree of Chinese fir broad-leaved mixed forest and natural secondary forest were 0.53 and 0.71 respectively,belonging to moderate mixed forest.The mingling degree of Chinese fir pure forest and bamboo forests was 0.05,which was close to zero degree of mixing.The uniform angle index of natural secondary forest was 0.51,which belongs to random distribution.The uniform angle index of pure fir and bamboo forests was 0.35,which belongs to uniform distribution,and the angular scale of fir and broadleaf was mixed.The uniform angle index was 0.40,which belonged to uniform distribution.The storey index of Chinese fir pure forest,mixed fir and broad-leaved forest,and bamboo forest were 0.32,0.38,and 0.38,respectively.The forest layer structure was relatively simple,and the storey index of natural secondary forest was 0.67,which had better forest structure.(2)Analysis and comprehensive evaluation of soil nutrient content in four forest stands.Eight evaluation indexes including soil organic matter,alkali-hydrolyzed nitrogen,total nitrogen,total phosphorus,available potassium,available phosphorus,soil bulk density and pH value were selected.On the basis of the analysis of the soil single index,the comprehensive evaluation index of soil fertility was constructed by the principle of multiplication and division,and the soil fertility of each forest was determined,and then analyzed and comprehensively evaluated.The results showed that in the 0-60cm soil layer,the contents of soil organic matter,alkali-hydrolyzed nitrogen,available phosphorus,total phosphorus and total nitrogen were the highest in natural secondary forest,and the pH and bulk density of bamboo forest soil were the highest.The contents of soil organic matter,alkali-hydrolyzed nitrogen and total nitrogen in the four forests showed a gradual decrease with the deepening of the soil layer,the soil pH value of the four forests did not change significantly with the deepening of the soil layer.The contents of total phosphorus and available phosphorus in Chinese fir pure forest,natural secondary forest and bamboo forest decreased gradually with the deepening of the soil layer.The comprehensive evaluation index of soil fertility of Chinese fir pure forest,Chinese fir broad-leaved mixed forest,natural secondary forest and bamboo forest was 0.02～0.58,and the evaluation grades were 1,2,and 3.It belonged to grade 2,the evaluation grade of natural secondary forest belongs to grade 2 and grade 3,the evaluation grade of bamboo forest belongs to grade 1.In general,the comprehensive evaluation of soil fertility of the four forest stands is natural secondary forest>Chinese fir broad-leaved mixed forest>Chinese fir pure forest>bamboo forest.(3)Study on the relationship between stand space structure and soil nutrients.Based on the analysis of the spatial structure and soil nutrient content of the four stands,the grey relational degree method was used to analyze the correlation between the stand spatial structure and the single factor of soil nutrient and the correlation of the comprehensive factor of soil nutrient.The results showed that in the Chinese fir pure forest,the correlation degree between soil total nitrogen(0.8104)and the storey index was the highest,the correlation degree between soil total phosphorus(0.8067)and the correlation degree between soil alkali hydrolyzable nitrogen and uniform angle index reached 0.9208,which had obvious correlation.In general,the main spatial structure factor affecting the soil nutrient index of Chinese fir pure forest was the uniform angle index.In the mixed forest of Chinese fir and broadleaf,the uniform angle index had the highest correlation with soil total phosphorus(0.8547),the mingling degree had the highest correlation with soil alkaline hydrolyzable nitrogen(0.8578),and the storey index had the highest correlation with soil available phosphorus(0.7376).In the natural secondary forest,the uniform angle index has the highest correlation with soil organic matter(0.6410),soil available potassium(0.7446)had the highest correlation with storey index,and soil available phosphorus(0.7759)had the highest correlation with mingling degree.Mingling degree was the main spatial structure factor affecting soil nutrient index of natural secondary forest.In bamboo forests,the correlation between soil organic matter(0.9335)and the forest layer index was the highest,the mingling degree was the highest for soil alkaline hydrolyzable nitrogen(0.6424),and the correlation degree between soil total nitrogen(0.8245)and uniform angle index was the highest,which affected the bamboo forest soil.The main spatial structure factor of the nutrient index was the uniform angle index.The mingling degree was the most important factor affecting the soil nutrient comprehensive factor of Chinese fir pure forest and natural secondary forest,which were 0.9352 and 0.6467,respectively.The uniform angle index of Chinese fir and broad-leaved mixed forest and bamboo forest had the greatest correlation with the comprehensive soil nutrient index value,which were 0.7429 and 0.7268,respectively.(4)Management measures to improve soil fertility.Based on the research results of the relationship between the spatial structure of four typical stands and the value of soil nutrient single index and comprehensive index value,the key spatial structure factors affecting the soil nutrient content of each stand were found out,and the optimization measures of the spatial structure of four typical stands was proposed.In order to improve the overall level of soil nutrients in each stand,the tree species structure and forest layer structure should be adjusted in the pure Chinese fir forest;in mixed fir and broad-leaved forests and bamboo forests,the spatial distribution pattern of trees should be adjusted first;in natural secondary forests,appropriate tending measures(pruning and forest clearing)could be taken to continue to maintain the healthy growth of the stand. |
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