Prairie. Candace Savage
Читать онлайн книгу.
the climatic system seems to get stuck. “Wet spells,” for example, when the rain refuses to stop. “Dry spells” of months—or years—when the clouds seem dry as parchment and the air fills with dust.
These persistent weather patterns also tend to be widespread, affecting significant parts of the Great Plains for prolonged periods. The droughts of the 1930s, for example, occasionally flared out to singe the entire continent, but they were at their most intense across the Great Plains Grasslands. Some parts of the High Plains in Oklahoma and Texas experienced eight consecutive years of drought, between 1933 and 1940. Little more than a decade later, the central and southern plains—from the Mississippi to the Rockies and from Colorado to Texas—were again stricken by a severe drought that persisted from 1952 to 1957. The Canadian Prairie provinces were hit hard in 1961. Then, in the late 1980s, a three-year drought parched the entire northern plains and fueled disastrous forest fires in Yellowstone National Park. During the growing season of 1988, when the crisis was at its worst, many parts of the prairies were hotter and drier than they had been at any time during the Dirty Thirties.
Yet five years later, some of these same areas were in full flood, as torrential rains pounded the western Midwest and sent both the Missouri and the Upper Mississippi rivers spilling over their banks. By the time the waters receded, twenty-six people were dead.
Why do the prairies suffer these violent climatic spasms? Part of the answer to this question may lie halfway around the world, in a region somewhere between Australia and Peru. There, in the equatorial waters of the South Pacific Ocean, weather patterns that will eventually affect the prairies begin to brew. Recent research suggests that there is a link between the surface temperature of the South Pacific and the amount of precipitation that ultimately falls on the Great Plains, particularly during the winter and early spring.
So far, no one knows exactly how all the complex linkages in this world-wide “teleconnection” work. And global influences, however stupendous, are not the only factors involved. Often, extreme conditions linger on the Great Plains long after the systems that triggered them have dispersed. A wet spell seems to breed more wet weather; a dry spell appears to breed more drought. But how could weather patterns possibly perpetuate themselves? The answer turns out to be surprisingly obvious. When precipitation is plentiful, water accumulates in the soil. As plants draw on this moisture to grow, they release water vapor into the air. This water vapor, in turn, combines with humidity that has evaporated directly from the earth, and these exhalations rise together to form clouds. Thus rain in the soil begets rain showers. What’s more, both rainfall and evapotranspiration (the release of water vapor from plants) have a cooling influence that helps to moderate temperatures and keep the evaporative demand within comfortable limits.
After a prolonged dry spell, by contrast, the cycle grinds to a stop. Plant growth slows and the rate of transpiration declines. So too does cooling evaporation from the soil. The ground and the surface layers of air sizzle in the sun, as a hot, dry land gets hotter and drier. (A case in point is the drought of the 1930s, which seems to have been intensified and prolonged by farming methods that left the soil exposed to the parching wind and robbed the system of what little moisture it held.) Eventually humid air from the south or the west returns to the scene, bringing welcome relief and restoring the climate to its own eccentric sense of normalcy.
GRADIENTS OF GRASS
THESE PROLONGED EPISODES of drought have been the making of the Great Plains Grasslands. Drought sucks moisture out of the soil, beginning at the surface and gradually burning farther down. If the dry spell is brief, the deep stores of moisture remain untapped, but if the evaporative demand persists, even the subsoil becomes parched and cracked. As a result, deeply rooted trees can cope without rain for several years by drawing water from underground, but they are doomed to defeat when drought reaches their root zone. Meanwhile, the grass lies patiently around the dying trunks, ready and able to spring back to life when the new rains finally come.
Long-term patterns of precipitation not only determine whether the land will grow trees or grass but also establish the limits that distinguish one “type,” or ecoregion, of grasslands from the next. When precipitation and other variables are averaged over the long term, the underlying order of the prairie climate begins to emerge. In the textbooks, these hidden patterns are revealed through charts and maps, but out on the prairie, they are written as gradients of grass. See Map 6: Average Annual Precipitation on the Great Plains.
Sometimes, the dialogue between the vegetation and the climate is intriguingly complex. For instance, summer precipitation on the prairies depends, in large part, on air masses that blow in from the south, carrying moisture from the gulf. Because of their southerly origins, these winds naturally have a greater influence on the southern plains (where they “reside”) than in the north (where they merely “visit”). So it isn’t entirely surprising to discover that the southern plains receive significantly more moisture than the northern prairies do. If, for example, Amarillo can hope to get 20 inches, or 500 millimeters, of moisture in a normal year, Lethbridge typically has to make do with only three-quarters as much. With this difference in mind, one might expect the prairies of northern Texas to be lusher than those of southern Alberta or Saskatchewan. Instead, the reverse is the case.
PRAIRIE FIRE
Climate is the major factor that determines the extent of the Great Plains Grasslands. Technically speaking, grasses hold sway when the evaporative demand (the amount of moisture that the atmosphere would draw away if it could) is slightly greater than the precipitation (the amount of moisture that is out there, in the ecosystem). But there is one important exception to this rule. The lush tall-grass prairies that fringe the eastern margin of the plains receive abundant moisture, more than enough to keep pace with evaporation. Theoretically, the region ought to support trees. And, in fact, wherever fragments of tall-grass prairie have survived, they have been aggressively invaded by stands of aspen, oak, and dogwood during the last 150 years.
The missing link is fire. Prairie fire was the terror of the early settlers, but it was a friend and ally of the tall grasses. Not only did it clear away the thatch of dead vegetation that prevented new shoots from breaking through, it also killed trees, the true “terror” of the prairies. When a tree burns, the growth points on its twigs and branches are often injured, so the plant cannot easily produce new shoots. But a grass protects its growing tips under the ground and rises from the flames like the proverbial phoenix.
Before the agricultural era, most of the tall-grass region probably burned every three to ten years, set ablaze by lightning or by Indigenous people, who used fire to green up the prairie and bring in animals. But however the flames were ignited, they had the same effect: they renewed and sustained the tall-grass prairies.
The trick is that the south-to-north gradient in precipitation is canceled out by an equal but opposite north-to-south gradient in evaporation. Because the average annual temperature increases from north to south, so does the rate at which moisture is lost through evaporation. Whatever the southern plains gain as rain, they lose as water vapor. As a result, the “effective precipitation”—the amount of water that is available to growing plants—is about the same in southern Alberta as in northern Texas. This helps to explain the long, gradual transition from the semiarid climate of the Northwestern Short/Mixed Grasslands to the sun-frazzled conditions of the Southern Short Grasslands. (As climate change imposes hotter, drier conditions on the southern plains, this gradient is expected to become even more dramatic.) See Map 4: Ecoregions of the Great Plains.
Meanwhile, there is yet another climatic gradient that helps to shape the vegetational profile of the Great Plains. This is an east-to-west decline in average annual precipitation. The tropical air that brings summer rains to the prairies typically swings up from the Gulf of Mexico, through the central United States, and off toward the east. As a result, its influence is stronger on the tall grasslands of the eastern plains than on the short-to-mixed grasslands farther west. If Winnipeg receives about 20 inches (500 millimeters) of moisture on average, Lethbridge gets 20 percent less (just 16 inches, or 400 millimeters). And if Kansas City can count on 40 inches (1,000 millimeters) of precipitation in a normal year, Amarillo can only expect to receive about half as much—and this time there is no reverse gradient