Model Construction and Dynamic Analysis of the Seedling Growth of Paulownia Hybrid Clones Under Field Nursery Conditions

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.