It has widely been documented that nitrogen (N) enrichment stimulates plant growth and modifies plant functional traits in the terrestrial ecosystem. However, it remains unclear whether there are critical transitions or tipping points for the response of plant growth or traits to N enrichment, and how these responses differ to different N forms. We chose the native, perennial clonal grass, Leymus chinensis in Inner Mongolia steppe, and conducted a field experiment, in which six N addition rates (0, 2, 5, 10, 20, and 50 g N m^–2 year^–1) and five N compound types [NH₄NO₃, (NH₄)₂SO₄, NH₄HCO₃, CO(NH₂)₂, slow-release CO(NH₂)₂] are considered. Here, we found that the different N compound types had no significant effect on the growth of L. chinensis. N addition rate significantly increased plant aboveground biomass and leaf nitrogen concentration, whereas decreased leaf dry matter content. The tipping point for N-induced aboveground biomass increase was at 10 g N m^–2 year^–1, and the changes in functional traits were at N addition rates of 20 g N m^–2 year^–1. Our findings suggested that the responses of aboveground biomass and functional traits to N addition were asymmetric, in which responses in aboveground biomass were more sensitive than that in functional traits. The differential sensitivity of aboveground biomass and functional traits of L. chinensis occurred to N deposition highlights the importance of functional traits in mediating ecosystem functioning in the face of N deposition, regardless of which chemical forms dominate in the deposited N.

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.

Assessing the effectiveness of protected areas is crucial for refining conservation policies, yet it remains challenging in remote, biodiversity-rich tropics due to logistical constraints. To address this challenge, we developed a remote sensing-based Pressure–State–Benefit (PSB) framework comprising four dimensions: human activity, ecosystem pattern, ecosystem quality, and ecosystem services. Using the Hainan Tropical Rainforest National Park (HTRNP) as a case study, we evaluated conservation effectiveness at the park scale by comparing ecological changes before (2015–2020) and after (2020–2025) its establishment. We further compared the Core Protection Zone (CPZ) and the General Control Zone (GCZ) to reveal differences in conservation outcomes under distinct management regimes. The establishment of HTRNP effectively curbed cropland and built-up land expansion. During 2020–2025, landscape patterns improved, characterized by enhanced connectivity and reduced fragmentation. Concurrently, fractional vegetation cover increased at an accelerated rate, indicating improved ecosystem quality, while key ecosystem services showed upward trends. Notably, the CPZ maintained superior ecological conditions with steady improvements under strict protection, whereas the GCZ, being more sensitive to human disturbances, exhibited fluctuating outcomes. This study demonstrates the effectiveness of HTRNP in enhancing regional ecosystem conditions and highlights the importance of zoned management. Furthermore, the proposed PSB framework offers a transferable approach for comprehensively assessing conservation effectiveness in other protected areas.