Ecological Modelling 220 (2009) 2009–2023 Contents lists available at ScienceDirect Ecological Modelling journal homepage: www.elsevier.com/locate/ecolmodel A hierarchical analysis of terrestrial ecosystem model Biome-BGC: Equilibrium analysis and model calibration Weile Wang a,b,∗ , Kazuhito Ichii c , Hirofumi Hashimoto a,b , Andrew R. Michaelis a,b , Peter E. Thornton d , Beverly E. Law e , Ramakrishna R. Nemani b a California State University, Monterey Bay, Seaside, CA, USA b NASA Ames Research Center, Moffett Field, CA, USA c Faculty of Symbiotic Systems Science, Fukushima University, Japan d Oak Ridge National Lab, Oak Ridge, TN, USA e Department of Forest Ecosystems and Society, Oregon State University, Corvallis, OR, USA article info Article history: Received 12 June 2008 Received in revised form 23 March 2009 Accepted 27 April 2009 Available online 18 June 2009 Keywords: Terrestrial ecosystem Biome-BGC Hierarchical analysis Equilibrium analysis Model calibration abstract The increasing complexity of ecosystem models represents a major difficulty in tuning model parameters and analyzing simulated results. To address this problem, this study develops a hierarchical scheme that simplifies the Biome-BGC model into three functionally cascaded tiers and analyzes them sequentially. The first-tier model focuses on leaf-level ecophysiological processes; it simulates evapotranspiration and photosynthesis with prescribed leaf area index (LAI). The restriction on LAI is then lifted in the following two model tiers, which analyze how carbon and nitrogen is cycled at the whole-plant level (the second tier) and in all litter/soil pools (the third tier) to dynamically support the prescribed canopy. In particular, this study analyzes the steady state of these two model tiers by a set of equilibrium equations that are derived from Biome-BGC algorithms and are based on the principle of mass balance. Instead of spinning- up the model for thousands of climate years, these equations are able to estimate carbon/nitrogen stocks and fluxes of the target (steady-state) ecosystem directly from the results obtained by the first-tier model. The model hierarchy is examined with model experiments at four AmeriFlux sites. The results indicate that the proposed scheme can effectively calibrate Biome-BGC to simulate observed fluxes of evapotranspira- tion and photosynthesis; and the carbon/nitrogen stocks estimated by the equilibrium analysis approach are highly consistent with the results of model simulations. Therefore, the scheme developed in this study may serve as a practical guide to calibrate/analyze Biome-BGC; it also provides an efficient way to solve the problem of model spin-up, especially for applications over large regions. The same methodology may help analyze other similar ecosystem models as well. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Climate change due to anthropogenic increases in greenhouse gases has lead to concerns about impacts on terrestrial ecosys- tems, and has generated an imperative for the understanding of, and the ability to predict, the role of terrestrial ecosystems in the global carbon cycle (IPCC, 2007). In response to this call, a variety of biogeochemical ecosystem models have been developed since the 1980s, including CASA (Potter et al., 1993), CENTURY (Parton et al., 1993), TEM (Raich et al., 1991; McGuire et al., 1992), BGC (Running and Coughlan, 1988; Running and Gower, 1991), and many others. These models are driven by surface climate variables, ∗ Corresponding author at: c/o Ramakrishna R. Nemani, Mail Stop 242-4, NASA Ames Research Center, Moffett Field, CA 94035, USA. Tel.: +1 650 604 6444; fax: +1 650 604 6569. E-mail address: weile.wang@gmail.com (W. Wang). and employ algorithms to simulate important ecosystem processes such as the exchange of water between the surface and the atmo- sphere through evaporation and transpiration, the assimilation and release of carbon through photosynthesis and respiration, and the decomposition of organic matter and the transformation of nitro- gen in soil. As such, they provide an important means to simulate regional and global carbon/water cycles, and to assess the impacts of climate variability and its long-term change on these cycles (e.g, Randerson et al., 1997; Cramer et al., 1999; Schimel et al., 2000; Nemani et al., 2003). Early versions of biogeochemical models usually have simple structures; as models evolve to create more realistic simulations, their later versions become increasingly sophisticated. For exam- ple, in Forest-BGC, the first member of the BGC family, leaf area index (LAI) of the vegetation canopy is prescribed, and carbon allo- cation is solely controlled by external parameters (Running and Coughlan, 1988). In the latest BGC model (Biome-BGC, version 4.2), in contrast, LAI is dynamically simulated and updated at daily scales 0304-3800/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ecolmodel.2009.04.051