Seasonal variation in leaf properties and ecosystem carbon budget in a cool-temperate deciduous broad-leaved forest: simulation analysis at Takayama site, Japan

Seasonal changes in gross primary production (GPP) and net ecosystem production (NEP) in temperate deciduous forests are mostly driven by environmental conditions and the phenology of leaf demography. This study addresses another factor, temporal changes in leaf properties, i.e., leaf aging from emergence to senescence. A process-based model was used to link the ecosystem-scale carbon budget with leaf-level properties on the basis of field observation and scaling procedures; temporal variations in leaf thickness (leaf mass per area, LMA), photosynthetic rubisco (V_cmax) and electron-transport (J_max) capacity, and dark respiration (R_d) were empirically parameterized. The model was applied to a cool-temperate deciduous broad-leaved forest at Takayama, in central Japan, and validated with data of net ecosystem CO₂ exchange (NEE=–NEP) measured using the eddy-covariance method. NEP of the Takayama site varied seasonally from 3 g C m^–2 day^–1 net source in late winter to 5 g C m^–2 day^–1 net sink in early to mid-summer. A sensitivity experiment showed that removing the leaf-aging effect changed the seasonal CO₂ exchange pattern, and led to overestimation of annual GPP by 6% and annual NEP by 38%. We found that seasonal variation in V_cmax affected the seasonal pattern and annual budget of CO₂ exchange most strongly; LMA and R_d had moderate influences. The rapid change in V_cmax and R_d during leaf emergence and senescence was important in evaluating GPP and NEP of the temperate deciduous forest.