Leaf Physiological Plasticity and the Adaptability of Introduced Landscape Plants on a Tropical Coral Island

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.