Prairie. Candace Savage
Читать онлайн книгу.
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 (–18°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 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!)
Prairie grasses ride this climatic roller coaster with composure. Species that couldn’t stay the course fell into extinction in ages long past, leaving the modern community of hardy survivors. Each of these successful species has been further refined over the past several thousand years, producing subspecies, or varieties, that are finely attuned to local conditions. A native grass from Alberta, for example, typically achieves maturity in a matter of weeks, fitting its life cycle to the abbreviated growing season of the northern plains. But a clump of the same species from Missouri or Oklahoma is programmed to take its time, pacing its activities to the more leisurely schedule of southern climes. These kinds of local, genetic differences have been detected in a wide range of native grasses, including blue grama, its cousin sideoats grama, the compact and graceful June grass, switch-grass, and both big and little bluestem. Natural selection, that master gardener, has been at work on them.
The one climatic factor that presents a continuing challenge for prairie grasses is the moisture supply. Over the Great Plains as a whole, precipitation is more variable than it is almost anywhere else on the continent, with years that are both much wetter and much drier than the long-term norm. (One study of precipitation records in western Kansas, for example, showed that in most months the amount of moisture received was either significantly below or significantly above average. Only “normal” values were truly abnormal.) When the rains are generous, the prairie flourishes and blooms; but when drought sets in, the grasses—indeed the whole ecosystem—are severely tested. More than any other single factor, the limits to growth on the prairies are set by precipitation.
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 later, 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.
WHY SO DRY?
THE GREAT PLAINS are subject to drought partly because they lie in the lee of the western mountains. Without this elevated barrier, westerly winds from the Pacific could sweep across the plains and bring moisture to the dry lands from Airdrie to Abilene. But with the Coast Ranges, the Cascades, the Sierras, and the Rockies all standing in their path, the Pacific westerlies are forced to rise, cool, and drop their moisture as they pass. By the time the winds flow down over the plains, they are almost devoid of rain. As they move across the western grasslands, they pick up humidity and carry it to the well-watered eastern forests. (Sometimes, as the winds swoop over the mountains, they whip themselves up into disturbances—known as Alberta Lows or Colorado Cyclones, depending on where they occur—that sweep across the plains, carrying a splattering of rain or a dusting of snow.)
Once the westerlies get past the mountains, there is nothing to stop them—except for the invisible resistance of other air masses. The prairies’ open spaces are a playground for the winds, drawing in not only mild, dry air from the Pacific but also colder and even drier air from the tundra and polar seas. As this Arctic air floods south, it meets moist, tropical air flowing north from the western Gulf of Mexico. A typical weather diagram for the Great Plains would show Arctic air pushing down from the north, tropical air swinging in from the south, and wrung-out Pacific air wedged between them like the point of an eastward-facing arrow. Arctic air dominates in the winter, sometimes forcing itself all the way to Texas as a stinging “blue norther” and, occasionally, pushing on south to the isthmus of Tehuantepec at the very tip of Mexico. In the spring, the balance of power is reversed, as the gulf air mass gains strength and surges north, sometimes carrying tropical heat and humidity all the way up to the Canadian prairies. The rained-out Pacific westerlies, with their meager stock of moisture, make themselves felt throughout the year, especially during the dry months of fall and winter. See Map 5: Major Air Masses Affecting the Great Plains.
In spring and summer, in particular, this picture is complicated by an influx of warm, dry-as-bone air that blows across the southern plains from the southwestern deserts. The interplay of these “four strong winds” produces the distinctive precipitation patterns of the Great Plains Grasslands. For instance, winter is a relatively arid season across most of the plains not only because cold air cannot hold much moisture but also because of the strong seasonal influence of dry air from the north and west. As a rule, less than one-third of the year’s precipitation falls between October and March, when these air masses exert their strongest influence. The other roughly 70 percent of the year’s moisture is received during the April-to-September growing season. Without this well-timed gift, the Great Plains would be a prickly expanse of cactus and other desert plants.
Much of the all-important spring-and-summer rainfall is generated when tropical air surges north and runs into Pacific and Arctic systems moving across the prairies to the east and south. Where the air masses collide, the lighter, warmer air from the tropics is forced up, cooling as it climbs, condensing to form clouds, and ultimately losing its moisture as general rain showers. Violent thunderstorms also frequently develop along these collision zones, or fronts, as the unstable tropical air rises up into towering, super-energized cumulonimbus clouds that glower over the landscape before releasing their humidity as hail or pounding downpours.
Because the contending air masses often meet in mid-continent, frontal thunderstorms are most common in the middle of the plains, in and around Colorado, Wyoming, South Dakota, Nebraska, and Kansas. But storms can also develop locally, without the clash of opposing weather systems to set them off. All it takes is a mass of warm, moist air and something to send that air spiraling up through the atmosphere. This lift-energy usually comes from the summer sun, which blazes down through cloudless prairie skies to heat the ground. Heat then radiates out of the soil into the surface air, causing it to rise, rotate, and mount upward to form a rain-filled thunderhead. In the dry western plains, one-third of the year’s precipitation can fall in a single hour from one of these spectacular cloudbursts.
GLOBAL “TELECONNECTIONS”
BY AND LARGE, the prairie climate is reliably unreliable. As the rival air masses interact with each other over the plains, they keep the atmosphere in a state of more-or-less-constant flux, so that the weather oscillates