Simulations of seasonal and inter-annual variability of gross primary productivity at Takayama with BEPS ecosystem model

In this study we use a modified version of the ecosystem model Boreal Ecosystem Productivity Simulator (BEPS) to simulate gross primary productivity (GPP) derived from measured CO2 flux at the Takayama site, which is characterized by a broadleaf deciduous secondary forest. The period of simulation is from 1998 to 2002. The performance of the model is assessed in its ability to reproduce seasonal and inter-annual variations in the derived GPP. Given appropriate vegetation parameters and meteorological conditions for the site, we find that the model can simulate seasonal changes and inter-annual variability of GPP that are in close agreement with those derived from the eddy covariance CO2 flux measurements. Estimated annual GPPs (in gCm−2) are 1171 (1998), 1024 (1999), 1274 (2000), 1200 (2001), and 1243 (2002). (Positive GPP is defined as ecosystem assimilation of CO2.) Although the inter-annual variability of the seasonal cycle and its upward trend are simulated relatively well, the model tends to overestimate GPP, particularly from 2000 to 2002 when the measured leaf area index (LAI) during the growing season was relatively high. Furthermore, due to certain missing phenological mechanisms in the model, it tends to overestimate photosynthetic absorption during the latter half of the growing season. Also, the model does not resolve the understory of dense evergreen dwarf bamboo at the site, thus failing to catch the start of photosynthetic activity by the understory before the leaf emergence of the woody component takes place in the spring. Results indicate that the observed inter-annual variability in GPP at the Takayama site has been influenced mostly by changes in LAI.