Accurate knowledge of individual tree ages is critical for forestry and ecological research. However, previous methods suffer from flaws such as tree damage, low efficiency, or ignoring autocorrelation among residuals. In this paper, an approach for estimating the ages of individual trees is proposed based on the diameter series of Cinnamomum camphora (Cinnamomum camphora (L.) Presl), Schima superba (Schima superba Gardn. et Champ.), and Liquidambar formosana (Liquidambar formosana Hance). Diameter series were obtained by stem analysis. Panel data contains more information, more variability, and more efficiency than pure time series data or cross-sectional data, which is why diameter series at stump and breast heights were chosen to form the panel data. After choosing a base growth equation, a constraint was added to the equation to improve stability. The difference method was used to reduce autocorrelation and the parameter classification method was used to improve model suitability. Finally, the diameter increment equation of parameter a-classification was developed. The mean errors of estimated ages based on the panel data at breast height for C. camphora, S. superba, and L. formosana were 0.47, 2.46, and −0.56 years and the root mean square errors were 2.04, 3.15 and 2.47 years, respectively. For C. camphora and L. formosana, the estimated accuracy based on the panel data was higher at breast height than at stump height. This approach to estimating individual tree ages is highly accurate and reliable, and provides a feasible way to obtain tree ages by field measurement.

Seedling growth is an essential stage for the future growth of trees, and Paulownia is treated as a rapidly growing hardwood tree species with wide industrial application. There is a knowledge gap in the modeling and analysis of the seedling growth pattern of Paulownia hybrid clones under field nursery conditions. A total of 300 saplings belonging to 25 Paulownia hybrid clones (named PT1–PT25, 12 saplings per clone) were observed 16 times after flat stubble treatment, yielding 4800 records. After accounting for mortality under field nursery conditions, 118 saplings survived, resulting in 1888 valid records for model construction. Seven theoretical growth equations with clone-specific dummy variables were fitted using nonlinear mixed-effects models (NLMM) with an AR(1) correlation structure to account for the hierarchical structure of the repeated measurements. Among the seven candidate models, Hossfeld IV was selected as the optimal model for both ground diameter (AIC = 8250.62, RMSE = 1.47 mm, conditional R² = 0.9892) and tree height (AIC = 15586.26, RMSE = 10.48 cm, conditional R² = 0.9916). However, because the Hossfeld IV model lacks an analytical third derivative, the standard logistic model was adopted to calculate the biologically meaningful fast-growing period boundaries. PT24 exhibited the highest growth potential among all clones, with asymptotic maximum values of 86.76 mm for ground diameter and 490.00 cm for tree height. However, its low survival rate (25%, n = 3) limits its practical recommendation. In addition, height-to-diameter (H/D) ratio analysis revealed considerable variation among clones, ranging from 3.89 cm∙mm⁻¹ (PT22) to 8.06 cm∙mm⁻¹ (PT3). The H/D ratio showed no significant correlation with ground diameter A (r = 0.286, p = 0.166), but a significant positive correlation with tree height A (r = 0.660, p < 0.001). PT24 maintained a moderate H/D ratio (6.92) despite its superior growth, indicating favorable stem stability. PT6 (91.67% survival, moderate growth) represents a more robust alternative for breeding programs prioritizing establishment success. This study demonstrates that a dual-model strategy provides a robust framework for evaluating clone-specific growth potential and supports informed breeding decisions in Paulownia improvement programs.