Spatial Structure And Intraspecific Competition Of Chinese Fir Plantation In The Middle Subtropical Of China

The reasonable spatial structure and intraspecific competition determine the overall situation of stand function and growth.The study on the stand structure of Chinese fir plantation and its optimization and adjustment can effectively improve the stand quality and provide theoretical basis for scientific management of Chinese fir plantation.In this study,the Chinese fir plantation of different ages at the national field science observation and research station of Huitong Chinese fir forest ecosystem system in Hunan province was studied.Based on the survey data of each tree,the diameter distribution characteristics,spatial structure characteristics and competition intensity of the stand were analyzed.On this basis,based on the idea of multiplication and division,the comprehensive index of forest spatial structure was constructed to made a reasonable choice of logging,so as to realize the optimization and adjustment of forest spatial structure,and the spatial structure before and after logging was compared and analyzed.The main results are as follows:(1)The skewness coefficients of young and middle-age stands were both positive and negative.The skewness coefficient was less than 0,and the diameter distribution was right,indicating that the majority of trees in large diameter class are larger than 0,and the diameter distribution of trees was left,showing that the trees in small diameter class are dominant.The skewness coefficient of near mature forest was greater than 0,the stand was dominated by small diameter trees,and the skewness coefficient of mature forest was less than 0,the stand was dominated by large-scale diameter trees.The kurtosis coefficient of the middle age forest was greater than 0,which means that the diameter distribution of the stand was relatively concentrated,and the diameter distribution curve was in a peak state.The kurtosis coefficient of other stands was less than 0,and the diameter distribution was relatively scattered,showing a low peak state.With the growth of forest age,the average diameter and standard deviation of stand increased.(2)The results showed that with the increase of forest age,the distribution range of tree diameter was gradually expanding.The distribution histogram of each stand diameter scale showed the trend of decreasing gradually from the peak value of the middle diameter scale to the two ends.The curve of the relative diameter of each stand with the percentage of tree number distribution showed "S-type" distribution.Five common function equations(cubic function,quadratic function,linear function,logarithmic function,reciprocal function)were used to fit the relationship.The results showed that the fitting effect of cubic function equation under different forest ages was the best.This showed that the relationship between the number of trees and the relative diameter can be predicted by the cubic function equation.Using the probability density function of normal distribution to fit the diameter distribution of each stand,the results showed that the acceptance rate of the diameter normal distribution of the stand reached 77.78%.(3)We took the target tree and four adjacent trees as the structural units of spatial structure analysis,the spatial structure characteristics of Chinese fir under different forest ages were:The distance scale distribution of the structural units of each stand was unimodal.The distribution of the neighborhood comparison within 0.5 was the most in each stand,which means that most of the target trees were in the mean state,and the stand differentiation was not serious.The results of the three spatial distribution patterns were different,but the whole distribution was random and uniform,and there were the characteristics of aggregation distribution on the specific scale.The vertical structure of each stand was relatively simple,which was divided into three layers:upper,middle and lower.The index of stand layer was between 0.5,and the structure of stand layer was not obvious.The distribution of crowding of each stand was 0.4785-0.8609,and the distribution of crowding of 4-year-old stands of young stands was the smallest,and the distribution of crowding of other stands were all above 0.6,most of which were in relatively dense and very dense state.The average open degree of each stand was less than 0.5,the open degree of young stand was the highest,mature forest was the lowest,and the growth space of the forest in the stand was seriously insufficient.(4)Among the four competitive indexes selected in this study,the best fitting equation between the DBH size of the target trees and Hegyi competitive index(CIHg)and competition index based on crown size(CIw)was power function equation,while the best fitting equation between the target trees and competition index based on intersection angle(CIH and CID)was cubic function.The correlation between DBH size and competitive index of target trees in different forest ages was CID>CIH>CIW>CIHg,and the correlation reached a significant level.This indicated that the fitting effect between CID and DBH based on intersection angle was the best.Finally,CID was selected as the best competition index to analyze the competition intensity of target trees in the forest.(5)With the increased of DBH of the target trees,the lateral compression increased,the upper covering decreased,and the total competition intensity decreased.The results showed that there were significant differences in the competition intensity between the small and large-diameter stages of the target trees at different ages,but not between the different diameter stages at the same growth stage.(6)The optimization results based on spatial structure index showed that the number of each diameter class remained unchanged,and the distribution of the adjusted stand remained random and uniform.The neighborhood comparison of stand,distribution of crowding and competition intensity were significantly reduced,the open degree of forest layer have been improved,and the growth space of forest has been increased.From the perspective of the overall spatial structure comprehensive index,the indexes after all kinds of optimization are significantly higher than those before optimization.The results of this study can not only clearly understand the spatial structure characteristics and competition situation of Chinese fir man-made forest under different forest ages,but also find out the unreasonable aspects of forest structure,and put forward targeted improvement measures.It also provides theoretical basis and practical basis for guiding and optimizing the spatial structure of the stand,improving the competition relationship between the trees,and improving the quality of the trees.