Objective This paper explores the effects of thinning intensity on stand growth and soil properties of Toona sinensis plantations, aiming to screen efficient thinning models suitable for constructing multi-layered, uneven-aged T. sinensis forests and to provide theoretical support for improving the quality of near-mature T. sinensis plantations.

Method A 21-year-old monoculture T. sinensis plantation in Guangxi Yachang Forest Farm of southern China was subjected to three thinning intensities: TPH (retaining 1050 tree/ha), TPM (750 tree/ha), and TPL (450 tree/ha), with unthinned stands as the control (CK). Precious tree species such as Zelkova schneideriana and Phoebe bournei were under-planted in a 1:1 ratio under the thinned and control stands. Differences in stand growth, soil physicochemical properties, and enzyme activities under different treatments were systematically compared.

Result (1) Thinning significantly promoted the increase of DBH, tree height, and stand volume of Toona sinensis. The TPH treatment was most conducive to the increase of tree height and volume growth, while TPL most strongly promoted DBH growth. (2) Growth of under-planted seedlings differed significantly among treatments (P < 0.05), with ground diameter and seedling height increments ranking as TPM > TPH > TPL > CK. (3) Thinning significantly affected soil physicochemical properties. TPH performed best in improving soil pH, organic matter, total nitrogen, total phosphorus, available potassium, and available phosphorus; TPM showed the greatest advantage in increasing total potassium; while CK performed best in reducing bulk density and increasing moisture content and available nitrogen. (4) Soil enzyme activities were also significantly influenced. TPH most markedly enhanced catalase, urease, and sucrase activities, whereas acid phosphatase activity was highest in CK.

Conclusion The TPH regime effectively coordinates tree growth, understory regeneration, and soil improvement in T. sinensis plantation. It is identified as a preferable management mode for establishing mixed uneven-aged multi-storey stands, improving stand quality, and supporting multifunctional sustainable management of T. sinensis plantations.