| The structure dynamics within natural forests are impacted by several factors,such as species composition,site quality,and competitive conditions.Understanding the changes across various scales is crucial for predicting the long-term development trends in stand dynamics and achieving sustainable forest management.This study used Betula albosinensis,a vital specie in the Qinling Mountains,as the research object to explore how new predictors,alternative models and modeling approaches can accurately capture and simulate tree height-diameter relationships and stand structural dynamics under different initial conditions.The main results are summarized as follows:(1)The study effectively enhanced the accuracy of the height-diameter model by incorporating dominant height and basal area as predictors,and employing forest stratum as a categorical variable.The improved model also performed well on the validation dataset(R~2=0.804,MAE=0.881 m,MAPE=7.5%,and RMSE=1.162 m),indicating its ability to accurately estimated tree height.Additionally,it was found that dominant height can represent site quality,determining the upper limit of tree height.It had a more significant impact on the forest sub-story than the main storey.The effect of basal area on tree height was phased,reaching its maximum when the basal area was around 30 m~2/hm~2 and decreasing in forests with higher basal areas.(2)The study aimed to explore the differences between moment estimation and hybrid estimation.It also examined the effectiveness of three fitting methods:maximum likelihood estimation(MLE),cumulative distribution function(CDF),and modified CDF(MCDF)in predicting the diameter distribution of B.albosinensis.The results showed that fitting with CDF under the framework of hybrid estimation ranked highest and resulted in the best diameter distribution fit.Besides,both mean diameter and stand density had a significant influence on diameter distribution.Stands with larger mean diameters and lower density had a diameter distribution closer to normal.Conversely,its distribution tended to right-skewed or reverse J-shaped.(3)Simulatng the dynamics in diameter and height distribution of B.albosinensis forests under varying stand conditions.The findings revealed that as forest age increased,diameter distribution became more symmetrical,displaying lower skewness and kurtosis.Better site quality and higher initial stand density accelerated this trend,but the effect of density diminished at 800 trees/hm~2.Additionally,lower initial stand density preserved more sub-story trees in later stages of forest development.While superior site quality intensified asymmetric competition among tree species,exacerbating the decline of smaller trees.Height distribution patterns mirrored those of diameter distribution,with site form index and stand density index exhibiting a negative correlation with distribution curve kurtosis.Furthermore,site quality directly limited the maximum of tree height distribution.In summary,this study has quantified and simulated the tree height-diameter relationships and stand structure dynamics of B.albosinensis.The results will inform efforts to optimize stand structure,support sustainable management,and advance the conservation of B.albosinensis natural forests in the Qinling Mountains. |
