Correct Response: B. Annual average precipitation in the prairie of the upper Midwest is sufficient to support a grassland ecosystem and isolated trees in some favored locations. The fairly low average annual precipitation and its often erratic nature can create extremely hot and dry summer conditions that give grasses a competitive advantage over tree species. For this reason, precipitation is the primary climate characteristic that controls the types of vegetation that can successfully compete in the prairie. The strong winds that often blow across the region (A) increase evaporation in summer, further reducing available soil moisture, but it is the relatively low precipitation that makes this a problem. The grass fires in the region (C) result from the very dry summer conditions caused by low and erratic rainfall as well. The wide variation in temperature over the course of the year (D) does not prevent trees from growing in the area.
