Development of growth models for dominant Castanopsis species in subtropical broadleaf secondary forests in Chongyi County, Jiangxi Province

To reveal the differences in growth rhythms and ecological strategies among three dominant Castanopsis species in the subtropical broadleaved secondary forests of Chongyi County, Jiangxi Province, and to provide a theoretical basis for classified management, structural regulation, and sustainable management of this region and similar subtropical secondary forests, as well as methodological references for multidimensional tree growth simulation under small-sample conditions.

Three dominant Castanopsis species-C. fargesii, C. faberi, and C. carlesii-in the subtropical secondary forests of Chongyi County were selected for analysis. Sample trees were subjected to stem analysis to obtain precise time-series data of annual radial growth. Five classic growth functions (Logistic, Gompertz, Hossfeld IV, Korf, and Richards) were applied to fit and compare the growth processes of DBH, height, and volume for each species. The Richards model, identified as the best-fitting function in most cases, was used to construct a system of nonlinear compatible growth equations. These equations were then estimated using the seemingly unrelated regression (SUR) method. Additionally, logarithmic transformation was introduced to correct heteroscedasticity in allometric growth models, thereby improving model stability. Based on the modeling results, growth rate and maturation patterns of the three species were further compared and analyzed.

Among the five tested models, the Richards function provided the best fit across multiple growth dimensions. The SUR-based compatible model revealed significant residual correlations between DBH and volume (r=0.875 1), and between DBH and height (r=0.682 0), validating the necessity of joint modeling. After logarithmic transformation, the allometric models exhibited a substantial reduction in mean systematic error (18.7% to 56.7%), while maintaining high R² values and stable fitting performance. Growth pattern analysis showed that C. faberi and C. carlesii had stronger early growth potential and faster maturation, making them suitable for timber-oriented management. In contrast, C. fargesii exhibited a slower growth rate but demonstrated strong ecological adaptability, making it ideal for deployment in ecological conservation forests.

Under limited sample conditions, this study successfully developed nonlinear compatible models for DBH, height, and volume of three dominant Castanopsis species, and effectively identified interrelationships among growth dimensions using the SUR method. The introduction of logarithmic transformation significantly improved the performance of the allometric models by reducing heteroscedasticity. Furthermore, through a comprehensive analysis of current annual increment and mean annual increment, the study further reveals significant differences among the three Castanopsis species in growth rates of diameter at breast height, tree height, and stem volume, as well as in maturation patterns. C. faberi and C. carlesii exhibit a fast-growing strategy, whereas C. fargesii shows a stable and conservative strategy, reflecting interspecific differences in resource utilization and environmental adaptation.