How do forest type, environment, and biodiversity jointly shape tropical understory litter characteristics?

Abstract Litter is a key component of forest ecosystems, acting as a critical interface between vegetation and soil. However, the characteristics of litter in tropical forests remain insufficiently understood, particularly regarding the combined effects of forest type, environmental factors, and plant diversity. In this study, we conducted a comprehensive field investigation of litter characteristics across 80 sampling sites on Hainan Island, a typical tropical region in China, encompassing both natural forests and plantations. Our results showed that compared with natural forests, plantations had lower litter water content but higher litter dry biomass and greater litter layer thickness, a pattern driven primarily by management intensity rather than stand age. In terms of topographic variation, litter dry biomass and layer thickness increased and litter water content decreased toward mountainous terrain and slope tops, and concave slopes exhibited markedly higher litter mass than convex slopes, highlighting the role of terrain-determined environment in shaping litter accumulation and spatial redistribution. Community-scale analyses revealed that species richness and other α-diversity indices of trees and shrubs had positive effects on litter dry biomass, while herbaceous species richness was significantly positively correlated with litter water content. Random Forest models identified environmental factors (e.g. mean annual temperature) as primary determinants of litter characteristics, while SHAP interaction analyses further revealed vegetation attributes as the dominant biological mediators with interaction contributions significantly exceeding their individual importance. These findings underscore the necessity of integrating forest type, environment, and biodiversity, as well as their complex interactions, to fully understand litter characteristics in forests.