Why don't trees grow on the Great Plains?

July 8, 2005

Dear Cecil:

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?

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, say--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:

  • It's too dry. Not true on average, as I say, but--key distinction--true episodically, a matter to which I'll return.
  • The soil won't support trees. A plausible but completely wrong idea that caused many early settlers to bypass some of the most fertile land in the world to reach the distant forests of Oregon. (Granted, the prairie was a bear to cultivate prior to John Deere's invention of the self-scouring steel plow in 1837.) It's now reasonably well established that prairie soil is not the cause but the result of prairie vegetation.
  • The trees were blown down by the wind. We'll pass silently by this conjecture, attributed to newspaper editor Horace Greeley, except to say that the prevailing westerlies do have something to do with the matter.
  • The Indians burned down the forest. Seemingly another dumbass idea, but actually an important part of the truth.

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:

  • Drought. Notwithstanding relatively plentiful average rainfall, the prairie peninsula suffers from severe drought 50 to 200 percent more often than the surrounding forests.
  • Dry season. In contrast to forest regions, which have relatively uniform precipitation throughout the year, the prairie peninsula is noticeably drier in late fall and winter.
  • High ratio of evaporation to precipitation. A key insight of Transeau's, this one gets a little technical, but the main idea is that despite abundant rain, plants dry out faster in the prairie peninsula due to wind, temperature, and so on.
  • Flat terrain. The prairie offered few natural barriers and particularly--you see where I'm going with this--few natural firebreaks.
  • Lightning. After Florida and the Gulf Coast, the prairie peninsula has electrical storms more often than any other region in the U.S.
  • 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.
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