The Immense Grasslands of North America
Some water becomes part of the structure of plants, some moves through the plant slowly and evaporates into the air, some sinks into the soil to join the huge underground reservoirs beneath the Earth’s surface. Once in the ground, it is absorbed by the probing roots of plants, to be returned again to the air through transpiration. (The root systems of plants far smaller than trees can be extraordinary networks for the gathering of water.
In The Web of Life, John H. Storer cites a study in which a single plant of winter ryegrass was grown in a box that contained less than two cubic feet of dirt. During the course of four months, the plant grew twenty inches above the ground and developed fifty-one square feet of visible surface. But beneath the surface, 378 miles of roots and 6,000 miles of root hairs had formed to support the plant.)
Throughout most of the Northern Hemisphere, the western land areas are favored by prevailing winds; they gather the rains that come off the oceans, blown by storms circling the globe from west to east. Perhaps the greatest geographic misfortune of the United States is the fact that the Cascade and Sierra Nevada mountains run from north to south so close to its western coast. These towering ranges catch most of the life-giving water that sweeps in off the Pacific, with the result that the lands on their eastern slopes are semiarid, suitable mostly for grazing.
Almost the whole western half of the United States lies in the “rain shadow” of mountains where the land can be farmed only if it is irrigated by water trapped on the mountain peaks.
Although it appears on no political map of the United States, one of the most important geographic boundaries we have is the twenty-inch rainfall line that runs north and south, virtually through the middle of the country. It has been called the “disaster line” because of the devastating droughts that have occurred to the west of it.
East of the line is a countryside of humid agriculture and deciduous and coniferous forests; west of it is a wide swath that extends from the Canadian border all the way to Mexico — several hundred thousand square miles — where there are almost no trees.
As with trees, so with humans: A source of water has always been a primary concern. That vast area between the twenty-inch rainfall line and the western slopes of the coastal ranges was known to generations of pioneers as the “Great American Desert.” For the same reasons that few trees grew there, no major settlements were established until most of the country was well populated.
Throughout the world, prehistoric communities grew up around a good source of water, near a river, lake, or spring, and it is worth reminding ourselves that the deserts of the Near East are spotted with ruins of what were once great civilizations. Originally situated where they had ample supplies of water, they vanished when humans and their animals destroyed the vegetation in the hills where the water originated.
The above is excerpted from The Secret Life of the Forest by Richard M. Ketchum.