Study On The Growth Model Of Chinese Fir Stand Basal Area In Hunan Province Based On Soil Nutrient Effect

With the economic development of the society,the Cunninghamia lanceolata area and stock volume of fir forests once ranked first in my country’s artificial forests and became an important fast-growing timber species in southern my country.Therefore,studying the relationship between Cunninghamia lanceolata soil nutrient elements and the growth of fir is of great significance for maintaining and improving the soil fertility of Cunninghamia lanceolata and providing a theoretical basis for management.In this study,150 Chinese fir plantation plots in Hunan Province were used as the research object,and the nonlinear mixed effect model method was used to construct a site index model of Chinese fir plantation based on the effect of soil nutrients.With site index as the driving variable,and again considering the influence of soil nutrient elements,a growth model of Chinese fir plantation Stand basal area based on soil nutrient effects was constructed to provide a theoretical basis for the management and fertilization of Chinese fir plantations.The main research contents and conclusions are as follows:(1)Constructed a site index model of Chinese fir plantation.This paper selects the four most common theoretical growth equations to simulate the correlation between the average height and age of Chinese fir dominant trees,The simulation results show four theoretical growth equations.The difference in the fitting effect of the dominance average height-age relationship is small.Among them,the Gompertz model with dominance average height(HT)and age(T)is the optimal basic model,and its determination coefficient R2=0.5481 is the highest,Mean absolute error(MAE=1.0108)and root mean square error(RMSE=1.3028)are the smallest.Using the method of variance analysis,taking the average height of dominant trees as the dependent variable,and taking organic matter(Om),total nitrogen(Tn),available phosphorus(Ap),and quick-acting potassium(Ak)as the independent variables of the four soil nutrient element factors.Significant factor screening shows that Tn,Ap,Ak are significant influencing factors,and the order of significance is Ap>Tn>Ak.The selected Ak,Ap,and Tn are combined to classify the soil nutrient element types(SNET).After the combination,150 samples can be divided into 54 SNETS.Due to the excessive number of initial soil nutrient element types,it is not conducive to the effectiveness of the mixed model.application.In order to simplify the mixed effects model and further improve the simulation accuracy of the model,this paper adopts K-means clustering and clustering 54 initial soil nutrient element types into 4 soil nutrient element type groups(SNETG)based on the clustering accuracy≥90%.Based on this,SNETG is added as a random effect to each parameter of the optimal basic model for simulation.The simulation results show that compared with the basic model,SNETG is added as a random effect to the parameter a for the best simulation effect,R2 Increased from 0.5481 to 0.6745,MAE decreased from 1.0108 to 0.8431,RMSE decreased from 1.3028 to 1.1107.Finally,the site index is obtained according to the high stand advantage corresponding to the base age(20 years)on the optimal model curve.(2)Constructed a model of the section area of Chinese fir plantation.The 4 most common basic models with site index representing site quality and 4 models with the average height of the dominant trees representing the site quality were selected for simulation respectively.The simulation results show that the basic model with site index representing site quality is in the simulation.,Schumacher model 3.11 is the optimal basic model,with R2=0.6851 the highest,MAE=3.7174 and RMSE=4.8657 the smallest.In the model simulation where the average height of the dominant trees represents the site quality,the Schumacher model 3.15 is the optimal basic model,with R2=0.7429 the highest,MAE=3.2808,RMSE=4.4009 the smallest.Using analysis of variance method,taking forest Stand basal area as the dependent variable and 4 soil nutrient element factors as independent variables,the significant factors were screened,and Om and Ak were significant influencing factors,and the order of significance was Om>Ak.Combining the selected Ak and Om can divide 150 samples into 25 initial SNETS.Due to the excessive number of initial soil nutrient element types(SNET),it is not conducive to the effective application of the mixed model.In this paper,K-means clustering is adopted,and the clustering accuracy is≥90%as the standard,and 4 soil nutrient element type groups(SNETG)are obtained.And on this basis,using the nonlinear mixed-effects model method,SNETG is added as a random effect to each parameter of the optimal basic model for simulation.Among them,the mixed-effects model simulation results of the site index representing the site quality show that:compared with the basic In the model,the effect of the mixed effect model with SNETG added is further improved.The R2 added to the parameter w is increased from 0.6851 to 0.7863,the MAE is reduced from 3.7574 to 3.0738,and the RMSE is reduced from 4.8957 to 4.0033,the effect is the best.The simulation results of the mixed-effects model with the average height of the dominant forest stands representing the site quality show that compared with the basic model,the mixed-effects model with SNETG is added to further improve the effect,in which the R2 added to the parameter a increased from 0.7429 to 0.8106,and the MAE increased from 0.7429 to 0.8106.3.2808 is reduced to 2.8269,RMSE is reduced from 4.4009 to 3.7649,the effect is the best.According to the initial soil nutrient element type(SNET)applied to the optimal basic model 3.11 and 3.15,the random effect parameter value of the fitting can be obtained:the random effect parameter value(0.2056)and(0.0610)of the soil nutrient element type 3 are both the highest,namely Soil nutrient element type 3 has the best simulation effect as a random effect to build a mixed effect model of forest Stand basal area.From this,it is obtained that the organic matter Om content is greater than 40g·kg-1 and the available potassium Ak content is 50-100 mg·kg-1,Organic matter and available potassium have the most beneficial effects on the growth of the Stand basal area.(3)The study proved the scientificity and rationality of constructing a growth model of Chinese fir plantation by adding random effects,and also proved that soil nutrient elements are closely related to the growth of Chinese fir plantation.The simulation results can be used for the growth and harvest of Chinese fir plantations.Provide a theoretical basis for guiding fertilization in terms of estimation,management,and production.