Integrating Hydrology and Plant Physiology in Forest Carbon Accumulation Modeling

Forests serve as a major carbon storage in the Earth system, and accumulated biomass represents the net result of continuous carbon exchange through photosynthesis and respiration. Conventional yield-based inventory methods, operating at annual or multi-year temporal scales, are limited to capture underlying physiological processes. In this light, we develop a process-based model that simulates carbon uptake, respiratory losses, mortality, biomass growth, and evapotranspiration at daily temporal resolution. Photosynthetic and respiratory fluxes are dynamically regulated by temperature, solar radiation, vapor pressure deficit, and soil water availability, where the latter is computed from a rainfall–runoff model representing catchment-scale soil storage and water balance. Rather than prescribing evapotranspiration, the model allows it to adjust with vegetation growth, such that increasing biomass expands transpiration capacity under prevailing environmental conditions. In parallel, biomass accumulation reflects the portion of carbon retained by vegetation following atmosphere–biosphere carbon exchange, completing the coupled representation of carbon–water–vegetation interactions. Taking a forested catchment in South Korea as an example, a 100-year simulation reproduces expected patterns of forest development, from rapid carbon accumulation in early stages to reduced net carbon gain in mature forests due to physiological aging. Although generally consistent with inventory estimates, the model additionally reveals hydrologic and climatic controls in plant growth, particularly moisture limitation, which annual inventory approaches cannot diagnose. The framework enables physically grounded and continuous prediction of forest carbon accumulation, supporting more realistic carbon accounting, ecosystem monitoring, and climate policy evaluation.