North America's tallgrass prairie was shaped for millennia by people setting fires
The prairie looks like untouched wilderness, but it was partly made by people. For 10,000 to 20,000 years, Indigenous peoples burned the tallgrass every year, and evidence suggests humans, not lightning, lit most of those fires. Grasses survived because most of their mass lives underground; the trees that might have shaded them out did not.
Ecologists place prairies in the temperate grassland biome alongside Argentina's Pampas and the Eurasian steppe: moderate rainfall, temperate climate, and grasses, herbs and shrubs instead of trees. The word itself is French for meadow, from the Latin pratum. Theodore Roosevelt explained that American backwoodsmen, emerging from dense forest onto vast meadows of long grass, had no name for such a sight and borrowed the one French settlers already used.
In North America, prairie spreads across much of the continent's Interior Plains, spanning three countries and taking in the whole of the Great Plains. Rainfall sorts it into bands. The dry west holds shortgrass prairie, receiving roughly 12 inches of rain a year; the east has tallgrass prairie on richer soils with more than 30 inches; mixed-grass prairie lies between. Local drainage creates wet prairies in low hollows and dry ones on slopes. Canada's Palliser's Triangle, dominated by dry prairie, was once written off as unfarmable but now ranks among the nation's key agricultural regions thanks to irrigation.
The land took shape over deep time. Uplift of the Rocky Mountains near Alberta threw a rain shadow over the Canadian plains. Then, beginning about 110,000 years ago, advancing glaciers scraped and levelled the terrain, and as they retreated around 10,000 years ago they left behind till that, along with wind-blown loess, became the parent material of prairie soils.
Fire and grazing kept it open. Bison, elk and white-tailed deer roamed the grasslands before European colonisation. Burning removed trees and dead growth and released nutrients from ash. It killed the tissue of woody plants but not prairie species, up to 75 percent of whose biomass can lie below ground, with roots reaching more than 20 feet deep. Without such disturbance, trees creep in and shade out the understory.
Source: Prairie
