Implementing thinning practices can enhance the growth of plantation forests and improve soil health. Nevertheless, the impacts of thinning applications on soil quality, large-diameter timber production of Castanopsis hystrix, and ecosystem multifunctionality are poorly understood. Therefore, we chose two sample plots, unthinned (control) and thinned, to investigate productivity and ecosystem multifunctionality after thinning for six years. Results revealed that thinning significantly reduced the soil’s bulk density, enhanced large-diameter timber growth, and undermined ecosystem multifunctionality in the short term compared to control (unthinning) treatment. Compared to the control, the thinning treatment considerably enhanced the soil organic carbon (0–30 cm soil layer) and tree diameter at breast height (20–30 cm), and enhanced shrub leaf nitrogen (N), shrub root N, herb aboveground N, Gram-positive bacteria (0–10 cm soil layer), and Gram-positive bacteria (20–30 cm soil layer) contents by 29.61%, 65.29%, 44.61%, 274.35%, and 323.44%, respectively. Furthermore, the thinning application could improve the N and P resorption efficiency more than control. Furthermore, compared with control, thinning treatment maximized decomposition and nutrient cycling function by 11.81% and 143.40%, respectively. Moreover, total PLFA content significantly impacts carbon stocks, wood production, and water regulation functions. In conclusion, this study underscores the considerable potential of thinning in augmenting large-diameter timber production by stimulating the positive effects of forest stands. These findings provide valuable insights for ecosystem multifunctionality elevation and the judicious application of thinning to improve forestry productivity, facilitating sustainable development in the forestry sector.

The ecological restoration of tropical islands, such as the Xisha Islands, is critical for sustainable development but is hindered by extreme environmental conditions and nutrient-poor coral sand soils. This study assessed the adaptive strategies of two introduced landscape species, Acacia auriculiformis and Nerium oleander, by comparing their leaf physiological and biochemical traits across three treatments: native coral sand (SS2), coral sand amended with garden soil (SS1), and a garden soil control (GZ). Results revealed differentiated physiological adaptation strategies: N. oleander exhibited a ‘conservative tolerance’ strategy, characterized by maintaining higher levels of soluble proteins and the non-enzymatic antioxidant GSH, whereas A. auriculiformis employed an ‘active defense’ strategy, significantly upregulating its enzymatic antioxidant system (SOD activity increased by up to 58.80% in coral sand compared to the control). Soil amendment was crucial for improving plant performance by fundamentally altering the soil’s physicochemical properties and nutrient status. Specifically, amending coral sand with garden soil (SS1 vs. SS2) resulted in a threefold increase in both soil organic carbon (from 3.81 to 11.63 g kg⁻¹) and water content (from 0.04% to 0.12%), while also increasing available phosphorus by over 50% and reducing the extreme soil alkalinity. This amelioration of the soil environment directly enhanced plant antioxidant capacity and overall growth performance. These findings provide a scientific basis for plant introduction on tropical islands, demonstrating that success depends on matching species-specific adaptive strategies with appropriate soil improvement techniques.