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Long-term ecological research under the International Biological Program and several other programs at research stations within the former USSR collected a large amount of data on phytomass, productivity and element cycling, together with climatic and soil regimes for various types of grassland ecosystems. This provides a unique opportunity to assess the performance of CENTURY across a wide environmental gradient from the luxuriant highly productive meadow-steppes of Central Russia to the ultracontinental steppes of Central Asia and the arid ephemeral grasslands in the Middle-Asian republics of the former USSR. The model simulations across this broad environmental gradient proved that the CENTURY ecosystem level model reproduced the seasonal, the mid-term, and in some cases the long-term dynamics of the live and dead aboveground phytomass of the grassland ecosystems in highly different natural-climatic zones of the former USSR. The r(2) for the comparison of observed and simulated live phytomass varies from 0.41 to 0.98 and the ratio of the absolute mean error of live phytomass to the peak seasonal live phytomass varies from 10 to 20%. Tne means and variation limits of the model are close to that of field data. The results suggest that many of the model discrepancies are a result of the fact that the model does not consider year-to-year changes in plant species composition. The measured Russian phytomass data for all of the sites are available on the World Wide Web. (C) 1997 Elsevier Science B.V.