Predicting Climate-Driven Habitat Suitability and Identifying Environmental Associations of Ancient Trees of Four Dominant Ficus Species on Hainan Island, China

Hainan Island supports abundant tropical ancient Ficus trees owing to its distinctive hydrothermal conditions and long-term human–environment interactions, yet the environmental factors associated with their distributions and responses to future climate change remain insufficiently understood. This study focused on the ancient trees of four dominant Ficus species on Hainan Island—Ficus microcarpa, F. altissima, F. benjamina, and F. virens—using systematically surveyed occurrence records and 59 candidate variables representing climate, soil, topography, anthropogenic activity, and typhoon disturbance. Two parameter-optimized MaxEnt frameworks were developed. The Environmental Association Model (EAM) incorporated climatic, soil, topographic, anthropogenic, and typhoon-related variables to identify factors associated with the current distributions of these ancient trees, whereas the Natural-Environment Projection Model (NEPM) included climatic, soil, and topographic variables to estimate potential habitat suitability under current conditions and SSP126 and SSP585 scenarios for the 2050s and 2070s; only climatic variables were updated in future projections. Under the EAM framework, population density was the key variable associated with the modeled distributions of all four ancient-tree groups, accounting for 65.3%, 46.1%, 39.6%, and 26.3% of the model contribution for ancient F. microcarpa, F. altissima, F. benjamina, and F. virens, respectively. Climatic and soil variables showed species-specific effects; topographic variables contributed mainly to ancient F. altissima and F. virens, whereas typhoon impact intensity contributed only slightly to ancient F. benjamina. Under current climatic conditions, total suitable habitat ranged from 7771.31 to 10,489.97 km2. Ancient F. microcarpa had the largest total suitable area, ancient F. benjamina the smallest, and ancient F. altissima the largest highly suitable area. Highly suitable habitats were concentrated mainly in northern and northwestern Hainan, with scattered central patches. Under future scenarios, the total and highly suitable habitats of ancient F. benjamina generally expanded, particularly under SSP585. In contrast, those of ancient F. microcarpa, F. altissima, and F. virens generally contracted, with substantial losses of highly suitable habitat; ancient F. virens faced the greatest habitat-loss risk. SSP585 intensified contraction for ancient F. microcarpa and F. virens, whereas ancient F. altissima showed more complex, scenario-dependent changes. Sensitivity analyses using alternative regularization settings supported these overall trends. These findings support conservation zoning, habitat restoration, germplasm preservation, and climate-adaptive management of ancient Ficus trees on Hainan Island.