Mangroves grow in high-salinity environments with low soil water potential (Ψ_s), where high light intensity and strong winds increase the vapor pressure deficit (VPD), causing physiological drought and high transpiration demand (Δw), which limits carbon dioxide (carbon gain) for photosynthesis. This study explored how mangroves optimize their carbon-gain-to-water-loss ratio (water-use strategies) to maximize carbon gain during both dry and rainy seasons. We also calculated the relative costs of key leaf traits and compared them with those of terrestrial forests under the carbon gain optimization model. The results revealed that (1) with increasing Δw, terrestrial forests primarily adjusted leaf hydraulic conductance (K_leaf), while mangroves altered the difference in water potential (ΔΨ); (2) as Ψ_s decreased, π_tlp of both terrestrial forests and mangroves increased; (3) terrestrial forests developed a more balanced distribution of leaf trait costs between osmotic pressure (46.7 ± 0.2%) and stomata (43.3 ± 1.2%), whereas mangroves had the highest cost in osmotic pressure (49.04 ± 0.03%) and the lowest cost in stomata (11.08 ± 3.00%) during the rainy season; and (4) although mangroves showed differences in trait values between dry and rainy seasons, their responses to drought stress remained consistent. These findings provided new theoretical insights into how mangroves maintain high carbon gain and water-use efficiency under extreme environmental conditions, which is important to improve mangrove conservation efforts and contribute to climate mitigation policies.

Madhuca hainanensis is a rare, endemic tree species of Hainan Island, with considerable ecological and economic value. Its natural regeneration is severely limited by habitat fragmentation and environmental stress. To investigate its adaptive across environmental gradients, we established experimental plots in the Jianfengling area of Hainan Tropical Rainforest National Park, encompassing elevation (400–1000 m) and canopy closure (30%–90%) gradients. Sapling growth and health were monitored for one year, alongside measurements of soil physicochemical properties and leaf photosynthetic pigment content. The results indicate that elevation was the primary factor influencing growth, with saplings at lower elevations exhibiting higher increments in height, diameter, and crown spread. While canopy closure was not statistically significant, moderate openness (30%–50%) at low elevations favored growth, whereas high-elevation, heavily shaded conditions constrained development. Sapling health declined over time, particularly in high-elevation and high-canopy-closure plots, and the interaction between elevation and canopy closure amplified physiological stress. Redundancy analysis revealed that elevation and canopy closure jointly explained ~36%–38% of the variance in growth and health, with chlorophyll a, carotenoids, and soil available phosphorus also contributing to sapling performance. These findings indicate that M. hainanensis is highly sensitive to light and elevation-related environmental gradients, and that low-elevation sites with moderate canopy openness are optimal for restoration and cultivation. This study provides a scientific basis for in situ conservation, wild reintroduction, and management of this threatened endemic species.

In tropical regions, the establishment of large-scale exotic plantations has addressed the demand for timber resources but has also disrupted the structural stability of native vegetation and altered soil nutrient cycling, thereby impairing ecosystem functions. Identifying effective restoration strategies for these plantations is crucial for sustainable forest management and ecological security. This study examined Acacia mangium Willd., Cunninghamia lanceolata (Lamb.) Hook., and Pinus caribaea Morelet. plantations in Hainan Tropical Rainforest National Park under three treatments: plantation control, girdling, and natural secondary forest. Vegetation surveys and soil analyses were conducted to explore the relationships between community structure, soil physicochemical properties, and enzyme activities. Diversity indices, Pearson correlations, and redundancy analysis were used to assess plant–soil relationships. The results showed that girdling significantly accelerated succession in C. lanceolata and P. caribaea plantations, increased species diversity, and enhanced the dominance of native species. Shrub-layer diversity indices (H_shrub, D_shrub, E_shrub) were the main drivers of soil properties and enzyme activities, while tree-layer effects were weaker. Girdling regulated soil nutrients and biological activity primarily via changes in community structure. These findings highlight the importance of optimizing shrub-layer structure and enhancing diversity for tropical plantation restoration. Combining forest type conversion with moderate interventions can promote coordinated plant–soil development over time.