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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
Persons of the urban smarty-pants persuasion are now thinking: Duh. Everybody knows that if you have a little rain, you can grow little plants; if you have a lot of rain, you can grow big plants. The Great Plains are dry, so of course all that grows there is grass.
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.
So while the popular portrayal of blinking pioneers emerging from the forest primeval to behold an uninterrupted sea of grass is a bit exaggerated, the change in vegetation was sufficiently abrupt that many were moved to wonder: What gives? Some guesses:
The real story, or so it now seems, emerged piecemeal over a century and may rightly be regarded as one of the triumphs of the science of ecology. The question was squarely framed and partly answered in a classic 1935 paper entitled “The Prairie Peninsula,” by botanist Edgar Transeau. Numerous others have made important contributions since, as summarized in a 2003 paper by weather scientists Stanley Changnon, Ken Kunkel, and Derek Winstanley. The chief factors:
Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.