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 in temperature to compensate for the difference. Less moisture is simply less.
In the days before the prairies were plowed and settled, this east-west moisture gradient found expression in the natural vegetation. As the average precipitation declined toward the west, the vegetation diminished in step, gradually reducing in height like a living bar graph. Somewhere around 100 degrees west longitude (give or take a few degrees), the tall-grass species dwindled away, leaving the wind to ripple through knee-high stands of mixed-grass prairie. To the west and southwest, the midheight grasses in turn gave way to a carpet of ground-hugging grasses, as a dry land clothed itself in drought-resistant plants.
The boundaries between tall-, mixed-, and short-grass prairies are not as tidy as they look on the map. In the patchwork quilt of the grasslands, each of the major blocks of vegetation is composed of many smaller blocks. And just as the overall picture is determined by large-scale climatic patterns, so each of these distinctive patches is a response to local variations in microclimate. Conditions are different on a south-facing slope than on the north and on lowlands than on hilltops, and these subtle differences are reflected in the vegetation. Moving uphill, from humid bottomlands to the drier crest, reproduces the moisture gradient of moving from east to west. And so, a sea of tall-grass prairie is broken by islands of mixed (or even short) grasses that grow on uplands and arid slopes. Meanwhile, out on the short-grass prairie, blue grama and its diminutive associates follow the opposite trend, ceding ground to midheight or tall grasses in moist valley bottoms.
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