Modelling plant carbon and nitrogen dynamics of a boreal aspen forest in CLASS — the Canadian Land Surface Scheme

The formulation of the physiological processes of plant carbon and nitrogen that have recently been implemented in CLASS — the Canadian Land Surface Scheme is presented, including plant photosynthesis of sunlit and shaded leaves, root nitrogen uptake, tissue respiration, growth, and senescence. Results from this formulation provide the vegetation parameters (e.g. stomatal resistance, leaf area index, and root length and distribution) for simulating energy and water exchange, and contribute to the carbon and nitrogen biogeochemical calculations and ecosystem CO2 flux estimations in CLASS. The model was parameterized for deciduous trees and run at a time step of 30 min. Modelled results were tested with measurements of leaf CO2 exchange, half-hourly and daily plant CO2 exchange, and annual plant carbon budgets from the Old Aspen (Populus tremuloides) site in the Southern Study Area (SSA-OA) of the Boreal Ecosystem-Atmosphere Study (BOREAS). Model tests at leaf level and half-hourly timescale improve confidence in the predictive capabilities of the model at canopy level and longer timescale. At half-hourly timescale, simulated plant CO2 exchange explained 81% of the variance in the measured CO2 flux derived from eddy correlation measurements and soil chamber measurements in the three comparing weeks in 1994. At a coarser timescale, simulated daily plant CO2 exchange explained 86% of the variance in the measured values during 1994 and 1996. Annual gross primary production (GPP) accumulated from half-hourly values in the model in 1994 and 1996 averaged about 1053 g C m−2 for this aspen site, close to the estimations from field eddy correlation measurements. Of this carbon fixation, 59% was consumed by autotrophic respiration (Ra) for plant growth and maintenance in the model, similar to estimations from chamber measurements. Sensitivity analyses show that the modelled plant GPP, Ra and net primary production NPP (=GPP−Ra) were very sensitive to the changes in temperature, implying that this boreal aspen ecosystem is strongly constrained by temperature conditions.