Why don’t trees grow on the Great Plains? If there’s enough rain and sun to grow grass, what’s stopping the forest from taking over, say, Kansas? –workerant, via e-mail
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Except it’s not that simple, you knuckleheads. True, the plains themselves–anything west of Omaha, roughly–are too arid to support trees. But that doesn’t explain the “prairie peninsula.” By this we mean the immense wedge of grassland that extends eastward from the Great Plains through Iowa and Illinois, over parts of Minnesota, Missouri, and Wisconsin, and into western Indiana, with isolated patches in Michigan and Ohio. In terms of average annual rainfall, this area, or at least the eastern end of it, doesn’t differ significantly from the regions to the immediate north, south, and east, which prior to European settlement were dense woods. Trees can and do grow in the peninsula–the Illinois prairie, for example, was originally 30 percent trees, mostly clustered along riverbanks and in scattered groves. The rest, though, consisted of grasses reaching 10 to 12 feet in height, and for that reason the region is classified as tallgrass prairie, the characteristic grassland east of the 98th meridian.
It’s too dry. Not true on average, as I say, but–key distinction–true episodically, a matter to which I’ll return.
Drought. Notwithstanding relatively plentiful average rainfall, the prairie peninsula suffers from severe drought 50 to 200 percent more often than the surrounding forests.
Fire. There seems little question that recurring fire promoted by periodic dry spells was the central formative feature of the prairie. How the majority of fires got started remains a matter of debate. Native Americans evidently torched the prairie frequently to create more desirable grazing land for game. Other blazes were started by lightning, which often struck the highest thing around, namely the trees. Whatever their cause, the fires were certainly dramatic, racing across the prairie at speeds of up to 15 to 20 kilometers per hour and incinerating vast tracts. Forests were slow to recover from the destruction, but prairie grasses, whose seeds and buds remained cool a few inches below the scorched surface, were back the next year. Grasses, in short, thrived because they were better adapted to the stressful prairie environment than trees, surviving everything except civilization’s appetite for arable land.
