Kucharik, C., C. Barford, M. El Maayar, S.C. Wofsy, R.K. Monson, D.D. Baldocchi (2006). A multiyear evaluation of a dynamic global vegetation model at three AmeriFlux forest sites: Vegetation structure, phenology, soil temperature, and CO2 and H2O vapor exchange. Ecological Modelling 196, 1-31

Abstract

We utilized eddy covariance observations of daily, seasonal, and inter-annual carbon dioxide (CO2) and water vapor exchange between the atmosphere and three mid-latitude forest stands (Walker Branch, Tennessee; Harvard Forest, Massachusetts; and Niwot Ridge, Colorado) to evaluate the Integrated BIosphere Simulator (IBIS), a Dynamic Global Vegetation Model (DGVM). Measurements of leaf area index (LAI), soil moisture, soil temperature, runoff, soil carbon, and soil surface CO2 effluxes were also compared with model output. An experimental approach was designed to help attribute model errors to the vegetation dynamics and phenology schemes versus other simulated processes such as soil biogeochemistry, plant physiology, and ecosystem respiration. The global to continental scale phenology sub-models poorly represented the timing of budburst and evolution of canopy LAI at deciduous forest sites. Biases of early season green-up of 6 weeks and delayed senescence were noted. Simulated soil temperatures were overestimated during the summer on average by 2-5º C, and underestimated by a similar magnitude during the winter. Ecosystem respiration was overestimated during the growing season, on average, by 20 – 60 g C m-2 mo-1, and underestimated during the winter by 10 – 20 g C m-2 mo-1 across all sites. Simulated soil respiration did not capture observed mid-summer peak rates and was generally lower than observed in winter. The overall comparison of simulated net ecosystem production (NEP) to observations showed a significant underestimate of growing season NEP of 25 – 100 g C m-2 mo-1, and an overall positive bias of 10 – 40 g C m-2 mo-1 during the winter. It was apparent that the excellent agreement between annual average NEP observations and IBIS simulations in “fixed vegetation” mode resulted from offsetting seasonal model biases. The magnitude of simulated variation in seasonal and inter-annual carbon exchange was generally dampened with respect to observations. The parameterization, and in some cases the formulations (particularly of ecosystem respiration and phenology) of this global scale biosphere model, are limiting its capacity to capture the dynamics of changing water and carbon exchange rates at individual sites. Model parameterizations and formulations were originally constrained and generalized for application to a wide range of global climate and soil conditions and biome types, likely contributing to model biases. This problem applies to other DGVM’s and similar types of biosphere models, and will likely become increasingly relevant as investigators begin to apply their models at higher spatial resolution.