Spatiotemporal Vegetation Dynamics and Quantity-Structure Relationships on a Tropical Island: A Case Study of Hainan, China

Vegetation serves as a critical ecological barrier on tropical islands, but conventional assessments often emphasize greening magnitude while overlooking persistence, structural quality, and scale-dependent attribution. In this study, we reconstructed a high-precision fractional vegetation cover (FVC) dataset for Hainan Island, China, covering the period from 2000 to 2024 using Google Earth Engine (GEE). We then combined trend analysis, emerging hot spot analysis (EHSA), the coupling coordination degree model (CCDM), RESTREND, and Geodetector to examine vegetation change from complementary perspectives. The results show that FVC increased overall and gradually shifted toward a more stable state over time. EHSA further revealed a distinct core-periphery pattern, with persistent hot spots concentrated in the central mountainous region, persistent cold spots distributed along the coastal urban belt, and oscillating hot spots occurring within agricultural transition zones. Regarding quantity-structure coupling, FVC and the aggregation index (AI) generally improved together across the island; however, some agricultural ecotones exhibited weaker structural improvement despite increasing vegetation cover, suggesting potential risks of homogenization and structural simplification. In the broad attribution analysis, vegetation recovery was primarily associated with the combined influence of climatic and human-related improvement. In the factor-specific analysis, land cover and slope showed the strongest explanatory power, and their interactions with other variables further enhanced spatial differentiation. These results demonstrate that greening magnitude alone is insufficient for evaluating vegetation change on tropical islands. Structural coordination and scale-dependent attribution should also be considered when interpreting ecological improvement and related management implications.