Abstract:Nitrogen deposition is a major driver shaping the structures and functions of forest ecosystems worldwide. When nitrogen inputs exceed ecosystem critical loads (CLs), significant changes in the diversity and abundance of understory herbaceous plants can occur. This study aims to systematically compile and integrate critical load data for understory herbaceous plants in response to nitrogen deposition across three mycorrhizal types: arbuscular mycorrhiza (AM), ectomycorrhiza (ECM), and mixed arbuscular-ectomycorrhizal forests (AM+ECM), in forests. By establishing a dedicated and standardized database, this work facilitates comparisons of herbaceous plant responses to nitrogen inputs among different mycorrhizal types in forests and provides a scientific basis for assessing the impacts of nitrogen deposition on forest microbe-plant systems. On the basis of the published literature and the global nitrogen deposition critical load database developed by Wilkins et al., relevant data were systematically collected, screened, and standardized to construct the Database of Critical Loads of Nitrogen Deposition for Understory Herbaceous Plants across Different Mycorrhizal Types in Forests. All critical load values were consistently derived via the threshold indicator taxa analysis (TITAN) method. A rigorous quality control workflow was applied, including cross-validation of mycorrhizal types, outlier detection and treatment, and data standardization. The database contains 3 592 standardized records. The core data table includes the following fields: Latin name of herbaceous plant species, forest alliance, mycorrhizal types (AM, ECM, or AM+ECM), species-level critical load values (zenv.cp) estimated by TITAN with corresponding bootstrap uncertainty intervals (5th, 10th, 50th, 90th, and 95th percentiles), response direction (increase or decrease in abundance), purity and reliability metrics, community-level change points (CCP), and associated environmental metadata. The database covers the three major mycorrhizal types as well as graminoid and non-graminoid herbaceous functional groups in forests. This database represents the first large-scale, standardized database explicitly focusing on the relationships among mycorrhizal types, understory herbaceous plants, and nitrogen deposition critical loads in forests. Its standardized structure, transparent metadata, and stringent quality control procedures ensure its reliability for future research and applications, including nitrogen deposition risk assessment, comparative analyses of mycorrhizal functions, ecological model parameterization, and the formulation of biodiversity conservation strategies.
