Greenhouse Gas Budget of a Cool-Temperate Deciduous Broad-Leaved Forest in Japan Estimated Using a Process-Based Model

Abstract

A terrestrial ecosystem model, called the Vegetation Integrative Simulator for Trace gases model (VISIT), which fully integrates biogeochemical carbon and nitrogen cycles, was developed to simulate atmosphere–ecosystem exchanges of greenhouse gases (CO2, CH4, and N2O), and to determine the global warming potential (GWP) taking into account the radiative forcing effect of each gas. The model was then applied to a cool-temperate deciduous broad-leaved forest in Takayama, central Japan (36°08′N, 137°25′E, 1420 m above sea level). Simulations were conducted at a daily time step from 1948 to 2008, using time-series meteorological and nitrogen deposition data. VISIT accurately captured the carbon and nitrogen cycles of this typical Japanese forest, as validated by tower and chamber flux measurements. During the last 10 years of the simulation, the model estimated that the forest was a net greenhouse gas sink, having a GWP equivalent of 1025.7 g CO2 m−2 y−1, most of which (1016.9 g CO2 m−2 y−1) was accounted for by net CO2 sequestration into forest biomass regrowth. CH4 oxidation by the forest soil made a small contribution to the net sink (11.9 g CO2-eq. m−2 y−1), whereas N2O emissions were a very small source (3.2 g CO2-eq. m−2 y−1), as expected for a volcanic soil in a humid climate. Analysis of the sensitivity of GWP to changes in temperature, precipitation, and nitrogen deposition indicated that warming temperatures would decrease the size of the sink, mainly as a result of increased CO2 release due to increased ecosystem respiration.