to ten thousand years. In the early days of this regime, the climate was considerably warmer and drier than it is today and even more prone to drought. But sometime in the last few thousand years, the system took a turn toward cooler, moister norms, so droughts have gradually become less frequent.
In fact, it seems that the twentieth century was the wettest in two thousand years. This conclusion is based on studies of microscopic fossils found in lake beds across the northern plains, in Alberta, Saskatchewan, and North Dakota. By extracting core samples from lake bottoms and studying the fossils that are found at different depths, researchers are able to estimate the salt content of the water at different times in the past. Since salinity increases as water levels drop, these findings give them a measure of past droughts. At Humboldt Lake in central Saskatchewan, for example, the fossils bear witness to a severe drought that persisted unbroken for more than seventy years. On the southern plains and in the desert United States, researchers have uncovered evidence of prehistoric droughts that lasted for three centuries.
Could the prairie climate revert to its fierce old habits? Yes, and it may already be happening. In 2017, for example, Amarillo, Texas, went 126 days without measurable precipitation, far surpassing a long-standing record. To make matters worse, average annual temperatures are on the rise—already up by 3.4°F (1.9°C) in some localities over the last hundred-plus years—as the prairies ride the leading edge of global warming.
Prairie grasses are not all equally capable of coping with drought. In general, tall grasses, including big bluestem and other shoulder-high species such as switchgrass and Indian grass, require the most moisture, while short grasses like blue grama, galleta, and the stubby little buffalo grass are the most resistant to drought. Midheight species, including needle-and-thread grass, rough fescue, and western wheat-grass (a.k.a. bluejoint, for its bluish leaf nodes), tend to fall somewhere in between. But all prairie grasses can contend with drought more successfully than can most deciduous trees—which is why the prairies are prairies instead of forests. The grasslands are an expression of the drought-prone prairie climate and a living response to the geography of the midcontinent.
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 cropland 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.
Like most of the world’s great grasslands, the Great Plains of North America lie squarely in the middle of a large continental landmass. 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 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 (–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
