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The central grassland region occupies the center of North America in the United States, Canada and Mexico and is a unique resource for the continent. While there are no other areas with comparable features, the largest similar grassland areas occur in Europe and Asia. The uniqueness of the region derives from its size, its relative flatness, and the smoothness of its physical gradients. The smooth gradients in precipitation and temperature are the reasons why most gradients in ecosystem properties are also smooth. The west-east gradient in precipitation and the north-south gradient in temperature result in corresponding gradients in plant community types, net biomass production by plants, soil carbon storage, and nitrogen availability to plants. One of the most striking features of the present condition of the central grassland region is that a huge fraction of the original native grassland have been replaced by cropland.