| Forest soil is a crucial component of the forest ecosystem,as it directly impacts the growth of forest trees and the quality of forest stands.Additionally,it plays a determining role in the potential productivity of crops.The spatial structure of stands provides a visual representation of stand structure through various aspects,including spatial distribution patterns,competition,and the extent of species intermixing.It serves as a driving force behind tree growth and forest succession.It influences the nutrient content of the soil and the rate of nutrient return to the stand by affecting the growth of undergrowth shrubs and grasses and the composition of the composition of forest litter.Therefore,optimizing the spatial structure of stands is an important measure to promote the improvement of soil nutrient content.Optimizing the spatial structure of stands is important for maintaining and improving soil nutrients.This study took the Chinese fir public welfare forests of five age groups(young forest,middle age forest,near-mature forest,mature forest and over-mature forest)in Fushou Forest Farm,Pingjiang County,Hunan Province as the research object.The spatial structure of the stands were characterized by the total mingling degree,uniform angle index,spatial density index,storey index,open degree and angle competition index.The soil nutrients were reflected by the soil organic matter,total nitrogen,total phosphorus,available nitrogen,available phosphorus and available potassium.The spatial structure and soil nutrient differences of the five age groups were compared and analyzed.On this basis,the horizontal spatial structure and the vertical spatial structure of the stand were used as exogenous latent variables.The soil total nutrients and available nutrients were used as endogenous latent variables.And a total of 12 indicators of stand spatial structure and soil nutrients were used as observed variables.The Structural Equation Modelling(SEM)was used to determine the path coefficients of the effect of stand spatial structure on soil nutrients.The mechanism of the effect of stand spatial structure on soil nutrients was revealed,and suggestions for optimizing the management of stands in Fushou Forest Farm were put forward.The main research conclusions are as follows:(1)Analysis of the characteristics of stand spatial structure.The results showed that the young stand was in a moderately mixed state,the middle age stand was in a weakly mixed to moderately mixed state.And the near-mature forest,mature forest and over-mature forest were in a weakly mixed state.The uniform angle index of each age group was greater than 0.5,which was in the horizontal spatial distribution pattern of cluster distribution.The mean spatial density index of all age groups was around 0.5,and the stand density was moderate and showed a trend of gradually decreasing spatial density with age.The stand stratification was not clearly differentiated among the age groups and the stand layer was single,but the young and middle-aged stands were slightly more complex than the near mature,mature and over-mature stands.In the five age groups,except for the over-mature stands,there was less openness and poor light transmission conditions in the stands.However,the angle competition index of each age group was relatively small,indicating that the target tree was in a dominant position in the stand,and was less affected by the upper cover and the lateral extrusion.(2)Analysis of soil nutrient characteristics.The results of soil nutrient content analysis of 0-60cm in different age groups showed that the contents of soil organic matter,total nitrogen and available potassium in all age groups were at a medium level.The content of soil total phosphorus and available phosphorus showed a scarcity state.The content of soil available nitrogen in the middle age forest was in a deficient state,but in the other four age groups was in a medium state.Soil nutrient indicators showed different patterns of variation in different age groups,but the contents of soil nutrient indicators were lower in the middle age stands and higher in the mature and over-mature stands.Age had a significant impact on soil organic matter and total nitrogen(p<0.05).From the different soil layers(0-15cm,15-30cm,30-45cm,45-60cm)of the soil profile,the variation regularity of each index was different with the soil layer.The soil depth had a significant impact on the content of soil organic matter,total nitrogen and available nitrogen(p<0.05).Most of the soil nutrient indexes showed that the nutrient content of the surface soil was higher than that of the deep soil.(3)The fitness of structural equation model.The Cronbach’s alpha values of the models of horizontal spatial structure,vertical spatial structure,total soil nutrients and available soil nutrients of latent variable stands were 0.929,0.855,0.836 and 0.896.The KMO values were 0.754,0.716,0.656 and 0.741,respectively.The model reliability and validity test results were in line with the reference value requirements.The evaluation results of the internal structural fitness of each measurement model were good.The model fitness test results showed that the CMIN/DF value was 1.959,the CFI,TLI and IFI values were 0.921,0.910 and 0.922,respectively.The RMSEA value was 0.098,and the model combination reliability CR was 0.9703.The results were within the reference value range,and the model fitness was good.(4)Effects of stand spatial structure on soil nutrients.The structural equation model was used to analyze the influence of stand spatial structure on soil nutrients.The results showed that the influence coefficients of stand horizontal spatial structure on soil total and available nutrients were 0.37 and 0.30 respectively.The influence coefficients of the vertical spatial structure of the stand on the soil total and available nutrients were 0.27 and 0.22,respectively.The influence of horizontal spatial structure on soil nutrients was greater than that of vertical spatial structure on soil nutrients,and the influence of forest spatial structure on total soil nutrients was greater than that of available nutrients.The spatial density index was the most important factor that affected the horizontal spatial structure of the stand.Both the open degree and the angle competition index had great impact on the vertical spatial structure.The organic matter was the most important factor that affected the total soil nutrients,and the available phosphorus was the most important factor that affected the soil available nutrients.Both the horizontal spatial structure and vertical spatial structure of exogenous latent variables had positive causal relationship with endogenous latent variables of soil total nutrients and available nutrients.(5)Measures for optimizing the spatial structure of stands to improve soil nutrients.In the young forest,the trees that were too densely distributed should be selectively cut down,and the light-loving broad-leaved native trees should be replanted.The trees in the middle age forest should be pruned,and the light cutting could be properly carried out.And the trees that like light and had certain shade tolerance could be selected for replanting.The near-mature forest should be renewed and felled,and the shade-tolerant broad-leaved native trees could be replanted in the open space.The mature forest could be thinned and replanted with some shade tolerant native trees.In the over-mature forest,the trees that had reached the rotation period could be properly selected for cutting.Furthermore,for all age groups,it is recommended to use phosphorus-based fertilizers during afforestation and replanting activities to supplement phosphorus deficiencies in the soil. |
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