Forest structural mediation of hydrological processes and ecosystem multifunctionality: mechanisms, trade-offs, and adaptive management

Forests provide multiple ecosystem services, including water regulation, water supply, soil retention, carbon sequestration, habitat provision, biodiversity conservation, and timber production. These services arise from coupled processes operating across vegetation, litter, soil, and watershed compartments. Changes in climate, land use, forest structure, or management can therefore alter several services simultaneously, but the direction of change depends on the process pathway, environmental context, and scale of assessment. Here, we synthesize advances in the quantitative assessment of interactions between forest hydrological functions and other ecosystem services, emphasizing how environmental drivers and management reorganize forest structure, how structural states alter hydrological partitioning and water-carbon coupling, and how these process changes propagate to ecosystem-service outcomes. We first establish a process-based framework covering canopy interception, throughfall, stemflow, litter interception, infiltration, evapotranspiration, soil-water storage, deep drainage, groundwater recharge, lateral flow, baseflow, and surface runoff. We then evaluate field experiments, watershed observations, process-based and ecosystem-service models, spatial statistics, machine learning, and causal inference, with explicit attention to their assumptions and scale limitations. Quantitative evidence from global thinning syntheses and classical catchment experiments is used to distinguish relatively consistent responses from context-dependent ones. A long-term Korean pine forest-management experiment on Changbai Mountain is included as an illustrative case study: the treatments persisted for 27 years, whereas the hydrological measurements summarized here represent observations made in 2023 rather than a continuous 27-year hydrological time series. The case illustrates how post-treatment structural states are associated with differences in precipitation partitioning and soil-water processes, but it is not presented as a formal causal mediation test. We further identify knowledge gaps involving belowground hydraulic redistribution, nonlinear thresholds, temporal legacies, cross-scale interactions, causal attribution, and model uncertainty, and propose a research and management framework linking long-term experiments, remote sensing, process-based models, causal inference, and multi-objective optimization.