Читать книгу Prairie - Candace Savage - Страница 38

The ability to survive episodes of drought is a basic requirement for life on the Great Plains.

> PLANTS FIGHT BACK

Prairie plants have come up with many ingenious strategies for coping with water shortage. A few, like the pincushion cactus, are genuinely drought resistant. In other words, they can store water in their own tissues (in their enlarged stems) and draw on it as needed. Others, including many grasses and wildflowers, attempt to evade drought by going dormant and retreating underground, where they linger on in the form of seeds, rhizomes, or tubers. But if some plants favor patient waiting, others put their faith in speed. Instead of trying to sit out the drought, they attempt to avoid it entirely.

Take, for example, the prairie crocus, or pasque flower. An inexhaustible source of pleasure for people on the northern plains, crocuses appear on the trailing edge of winter as tight clusters of furry, pointed buds that push up through the dead grass like so many inquisitive snouts sniffing for spring air. Without pausing to grow leaves, the plants burst directly into bloom, producing ground-hugging whorls of silky, lavender sepals. By the time most other wildflowers put in an appearance several weeks afterward, crocuses are already sporting headdresses of shiny, plumed seeds. Before the growing season has even properly begun, their reproductive task has been completed.

By getting off the mark so early, crocuses are able to draw on a relatively certain supply of water from snowmelt. And although they are exposed to the bluster of winter’s last blast, they are protected from the wind by a coat of hairs that holds in heat and moisture. They also take shelter by crouching close to the ground, well bedded in grass, creeping juniper, and other plants. Thus protected, crocuses speed through their reproductive cycle and avoid the stress of coping with drought in the hot, dry days of July and August.

Weather Matters

To the homesteaders who came to the Great Plains from Europe or eastern North America in the late 1800s and early 1900s, converting the prairies to croplands must have looked like a dream. Except for the trees that crept in along the rivers, the land lay open to the plow, offering little apparent resistance to the farmers’ ambitions. But the settlers’ early optimism was soon blighted by widespread droughts, as the dry summer of 1889 was followed by the dry years of 1890, 1894, 1910, and 1917, and then by the bleak decade of the 1930s. Life on the prairies was not as easy as it had seemed. For what no one at first quite realized was that grasslands are semiarid zones—better watered than deserts but less humid than forests. The farmlands that the settlers had known in Europe and the East had typically been wrested from the forest and, even after the trees were gone, still received enough rainfall to support a natural vegetative cover of broad-leaved woodlands. But the weather on the prairies naturally favored not trees but grass, and that simple fact made all the difference.

left: Pincushion cactus

right: Prairie crocus, or pasque flower

Like most of the world’s great grasslands, the Great Plains of North America lie squarely in the middle of a large continental land mass. As a result, the region is isolated from the influence of all four oceans—north, south, east, and west— and, as it happens, from any other significant body of water. Without the moderating influence of water (slow to heat and slow to cool), the plains are subject to violent oscillations of temperature. In the northern prairies, in particular, the temperature can span 140˚F in the course of a year, from a brittle –40˚F in midwinter and to a stifling 100˚F in summer. (That’s a range of 80˚C, from a low of around –40˚C to a high of over +40˚C.) The effect of these wild seasonal deviations is equivalent to moving up and down the continent every twelve months. Saskatoon, for example, has an average January temperature of 0˚F (–17˚C), well below that of Anchorage, Alaska. But in July, Saskatoon’s average heats up to 66˚f (19˚C), almost on a par with that of Los Angeles. And though the southern plains are spared the worst extremes of winter, they still get taken for quite a ride. The average January temperature in Amarillo, Texas, for example, is a mere 35˚f, or 2˚c, cooler than Vancouver far to the north; but in July, the north Texas plains can be among the hottest places on the continent. (If prairie people are obsessed by the weather, it may simply be because we have a lot of weather to obsess about!)

Indian grass

Galleta

Needle-and-thread grass

Western wheatgrass