Spatial responses of vegetation-soil system to complex factors in a sandy-rocky island chain

Vegetation-soil systems vary spatially across sandy and rocky islands due to the complex interaction of natural and anthropogenic factors. Revealing the spatial responses of vegetation-soil system to these factors is important for island conservation and the understanding of island ecology. An island chain in China, which is characterized by the coexistence of sandy and rocky islands connected by bridges, was selected for the study. The vegetation-soil system was represented using the vegetation health index (VHI), soil health index (SHI), and vegetation-soil system health index (VSSHI). A total of 20 factors were identified in aspects of island basic factors (e.g., area, shape, and isolation), landscape pattern (i.e., composition and configuration), terrain condition (e.g., altitude, slope, and distance to the shoreline), and ecological indices (vegetation, salinity, heat, humidity, and aridity indices). The spatial responses were analyzed at the island (10 islands) and site (111 sites) scales. The results indicated that the vegetation-soil system presented distinctly better conditions on the rocky islands than on the sandy island. The spatial responses showed similar and different characteristics across the dual scales. The similarity was represented by the higher sensitivities of VHI and VSSHI than that of SHI at the two scales, and the difference mainly lay in the decrease in the influences of landscape pattern along the scales from island to site. The factors related to macro aspects (i.e., island area, sea reclamation scale, land use intensity, and vegetation coverage) were the main influencing factors at the island scale. Islands with small area and sea reclamation scale, low land use intensity, and high vegetation coverage presented a high VSSHI. The detailed land surface characteristics that refer to the factors in micro aspects, including aridity degree, soil salinity, vegetation quality, and altitude, became the main influencing factors at the site scale. The VSSHI generally decreased and increased with the increase in the former and latter two factors, respectively. Across different types of islands, anthropogenic activities dominated the vegetation-soil system on the sandy island through intensive sea reclamation and land uses, and natural and anthropogenic factors jointly determined the system on the rocky islands.