The great grasslands of the Midwest and Great Plains are defined, both ecologically and aesthetically, by plants that are mostly herbaceous. The mid-continent is a grassland landscape because climatic factors and the influence of fire make the growth of woody plants difficult. Due to the combination of excessive summer heat, persistent winds, and low precipitation, evaporation often equals precipitation.
Trees have shallow, widespread root systems and a huge, persistent biomass to support.
Where rainfall is sporadic or strongly seasonal, trees are limited to areas where moisture is more predictable, such as along water courses and coulees, moist valleys in the rolling glacial moraine country, where trees grow sheltered from the harsh environment around them. These basic terms relating to moisture are used to describe other plant communities as well, such as forests. Moisture regimes are determined by topography and soil type. In general, dry prairies are found at the highest elevations on gravely, sandy, or sandy loam soils. Mesic prairies are seldom inundated by water and are on loamy soils with varying degrees of silt and clay.