How the prairie is different now from how it was in the past. When a moderate rainfall area had wetter years, many trees began growing. When there were drier years, fires often occurred and killed off the small trees. ... Therefore, prairies with grasslands won out in the open areas.
As the Rocky Mountain along the west grew higher and higher due to plate tectonics, rain- bearing winds from the Pacific Ocean were blocked and prevented from reaching the leeward side of the mountains. As a result, the tree could not grow near east of the mountains, which led to the formation of the Prairies.
