Testing the performance of a Dynamic Global Ecosystem Model: Water balance, carbon balance, and vegetation structure

While a new class of Dynamic Global Ecosystem Models (DGEMs) has emerged in the past few years as an important tool for describing global biogeochemic al cycles and atmosphere-biosphere interactions, these models are still lar gely untested. Here we analyze the behavior of a new DGEM and compare the r esults to global-scale observations of water balance, carbon balance, and v egetation structure. In this study, we use version 2 of the Integrated Bios phere Simulator (IBIS), which includes several major improvements and addit ions to the prototype model developed by Foley et al. [1996]. IBIS is desig ned to be a comprehensive model of the terrestrial biosphere; the model rep resents a wide range of processes, including land surface physics, canopy p hysiology, plant phenology, vegetation dynamics and competition, and carbon and nutrient cycling. The model generates global simulations of the surfac e water balance (e.g., runoff), the terrestrial carbon balance (e.g., net p rimary production, net ecosystem exchange, soil carbon, aboveground and bel owground litter, and soil CO2 fluxes), and vegetation structure (e.g., biom ass, leaf area index, and vegetation composition). In order to test the per formance of the model, we have assembled a wide range of continerital- and global-scale data, including measurements of river discharge, net primary p roduction, vegetation structure, root biomass, soil carbon, litter carbon, and soil CO2 flux. Using these field data and model results for the contemp orary biosphere (1965-1994), our evaluation shows that simulated patterns o f runoff, NPP, biomass, leaf area index, soil carbon, and total soil CO2 fl ux agree reasonably well with measurements that have been compiled from num erous ecosystems. These results also compare favorably to other global mode l results.