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С����Ƶ General Ecosystem Model (С����Ƶ-GEM) is a process-based plot-scale model of C-N interactions in terrestrial ecosystems. The original structure of the model is described in detail in �鲹���ٱ�ٳٱ����et al. (1991). Le Dizès��et al.��(2003) describe the latest version, С����Ƶ-GEM III, in detail.

The С����Ƶ-GEM III simulates, on an annual time step, plot-level photosynthesis and N uptake by plants, allocation of C and N to foliage, stems, and fine roots, respiration in these tissues, turnover of biomass through litter fall, and decomposition of litter and soil organic matter. The model is generally calibrated to run on mean-July maximum and minimum daily air temperature, mean-July daily irradiance, total growing-season precipitation, mean-annual CO₂��concentration, and annual N inputs in deposition. A major feature of the model is that vegetation in the model acclimates to changes in the environment to maintain a nutritional balance between C and N. For example, environmental changes that stimulate photosynthesis (e.g., increased CO₂��or higher irradiance) result in an increase in the relative allocation of C and N to fine-root growth, which, in turn, stimulates N uptake. Similarly, environmental changes that stimulate N uptake (e.g., increased available N) increase the relative allocation of C and N to foliage growth, which stimulates C uptake. The C:N ratio of litter determines how it is partitioned among soil organic matter fractions that differ in relative turnover rates. The rates of decomposition and N mineralization also depend upon soil temperature and moisture.

Later modifications to С����Ƶ GEM include the incorporation of the Aggregated Canopy Model (ACM, Williams��et al., 1997) as the photosynthesis submodel (С����Ƶ GEM III), and the conversion to a daily model including a water budget (С����Ƶ GEM VI).

С����Ƶ GEM versions
GEM Version	Key Properties	Citation
С����Ƶ GEM I	Original annual model	�鲹���ٱ�ٳٱ����et al., 1991
С����Ƶ GEM II	Modified growth equations	�ѳ������Ա���et al., 1997a
С����Ƶ GEM III	Change photosynthesis model to ACM	ACM: Williams��et al., 1997 GEM: Le Dizes��et al., 2003
С����Ƶ GEM VI	Daily model, new growth and allocation submodel, daily water budget.	Unpublished, code available upon request.

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Download the Source Code

The full С����Ƶ-GEM source code, Windows executables and sample input files, are available for non-commercial use.��See the��download page��for available versions.

Publications

Clein, J. S., B. L. Kwiatkowski, A. D. McGuire, J. E. Hobbie, E. B. Rastetter, J. M. Melillo and D. W. Kicklighter. 2000. Modelling carbon responses of tundra ecosystems to historical and projected climate: A comparison of a plot- and a global-scale ecosystem model to identify process-based uncertainties. Global Change Biology Vol. 6, Supplement I:127-140. doi:

Hobbie, J. E., B. L. Kwiatkowski, E. B. Rastetter, D. A. Walker and R. B. McKane. 1998. Carbon cycling in the Kuparuk Basin: Plant production, carbon storage, and sensitivity to future changes. Journal of Geophysical Research 103:29,065-29,073. doi:

Le Dizès, S., B.L. Kwiatkowski, E.B. Rastetter, A. Hope, J.E. Hobbie, D. Stow, S. Daeschner. 2003. Modeling biogeochemical responses of tundra ecosystems to temporal and spatial variations in climate in the Kuparuk River Basin (Alaska). Journal of Geophysical Research D - Atmospheres 108(D2):8165. doi:��(С����Ƶ GEM v3.3.6.5.b)

McKane, R. B., E. B. Rastetter, J. M. Melillo, G. R. Shaver, C. S. Hopkinson, D. N. Fernandes, D. L. Skole and W. H. Chomentowski. 1995. Effects of global change on carbon storage in tropical forests of South America. Global Biogeochemical Cycles 9(3):329-350. doi:

McKane, R., E. Rastetter, G. Shaver, K. Nadelhoffer, A. Giblin, J. Laundre and F. Chapin. 1997a. Climatic effects on tundra carbon storage inferred from experimental data and a model. Ecology 78:1170-1187.�� doi:

McKane, R., E. Rastetter, G. Shaver, K. Nadelhoffer, A. Giblin, J. Laundre and F. Chapin. 1997b. Reconstruction and analysis of historical changes in carbon storage in arctic tundra. Ecology 78:1188-1198. doi:

Rastetter, E. B., M. G. Ryan, G. R. Shaver, J. M. Melillo, K. J. Nadelhoffer, J. E. Hobbie and J. D. Aber. 1991. A general biogeochemical model describing the responses of the C and N cycles in terrestrial ecosystems to changes in CO₂, climate and N deposition. Tree Physiology 9:101-126. doi:

Rastetter, E. B., R. B. McKane, G. R. Shaver and J. M. Melillo. 1992b. Changes in C storage by terrestrial ecosystems: How C-N interactions restrict responses to CO₂��and temperature. Water, Air & Soil Pollution 64:327-344. doi:

Rastetter, E. B., R. B. McKane, G. R. Shaver, K. J. Nadelhoffer and A. E. Giblin. 1997. Analysis of CO2, temperature, and moisture effects on carbon storage in Alaskan arctic tundra using a general ecosystem model, pp.437-451. In: W. C. Oechel, T. Callaghan, T. Gilmanov, J. I. Holten, B. Maxwell, U. Molau and B. Sveinbjörnsson (eds.), Global Change and Arctic Terrestrial Ecosystems. Springer-Verlag, New York.

Rastetter, E.B., B. L. Kwiatkowski, S. Le Dízes, and J.E. Hobbie. 2004. The Role of Down-Slope Water and Nutrient Fluxes in the Response of Arctic Hill Slopes to Climate Change. Biogeochemistry 69:37-62. (С����Ƶ GEMv3.3.6.5.d) doi:

Williams, M., E. B. Rastetter, D. N. Fernandes, M. L. Goulden, G. R. Shaver and L. C. Johnson. 1997. Predicting gross primary productivity in terrestrial ecosystems. Ecological Applications 7:882-894. doi:

This material is based upon work supported by the National Science Foundation under grants #OPP-9318529, OPP-9732281, DEB-9509613, and DEB-0108960 and the Environmental Protection Agency under grants RFQ-RT-00-00107 and QT-RT-00-001667. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation or the Environmental Protection Agency.