Prairie. Candace Savage
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their leaves, which take the form of narrow blades. What’s more, the surfaces of these reduced leaves are often modified—corrugated with ridges or covered in hairs—so that the wind can’t sweep across the surface and draw out moisture. The roughened surface holds a thin layer of humid air next to the leaf and thus helps to reduce the “evaporative demand,” or drying power, of the atmosphere. Some grasses, including western wheatgrass, June grass, and blue grama, roll up the edges of their leaves during times of drought to help keep their tissues from drying out.
Why aren’t the stomata kept tightly closed to seal moisture inside the leaf? The reason is that the stomata also supply plants with fresh air. Leaves are miracle workers, able to take carbon dioxide from the air and water from the soil, zap them with solar energy, and transform them into food. This process—photosynthesis—not only produces the sugars and other organic molecules that plants need to maintain themselves and to grow but also feeds microbes, worms, insects, fish, birds, and mammals. If plants sealed their stomata, this life-sustaining process would come gasping to a halt for lack of carbon dioxide. But if the stomata are thrown wide open, the plants risk death due to the loss of moisture through their gaping valves.
Prairie grasses resolve this dilemma by strategic scheduling. In the fierce blaze of the midday sun, the stomata close so that water vapor is held in and carbon dioxide is kept out. In this state, the leaf can capture solar energy and store it in energy-rich molecules (a process that requires sunlight but not carbon dioxide). Then, in the cool of the evening, when the evaporative demand drops off, the stomata snap open, letting water vapor trickle out but also permitting carbon dioxide to flood into the leaf. By mobilizing the energy that was stockpiled earlier in the day, the leaf uses this carbon dioxide to manufacture the sugars and other molecules that it needs for growth (a process that can be accomplished in total darkness). The result is that prairie grasses are partially nocturnal; they do most of their growing at night or in the early hours of the morning.
Prairie grasses also have another ingenious way of evading the demands of the sun. Like many other grassland creatures (prairie dogs, ground squirrels, cottontails, badgers, and so on), they take refuge underground. What we think of as “grass”—the aboveground leaves and stems—actually constitutes less than half of the organism. Between 60 and 80 percent of the plant, by weight, typically grows below ground. The roots extend down from the base of the stems like a tangled head of hair, as main roots divide into minor roots and then into root hairs. A 10-foot (3-meter) stand of big bluestem is anchored underground by a mass of coarse, fibrous roots that reaches as much as 12 feet (3.6 meters) into the earth. Blue grama, for its part, seldom lifts its seed heads very far above the ground, but its network of fine, branching roots can sometimes probe the soil for water to a depth of almost 6 feet (1.8 meters)!
These extensive systems of roots push thirstily through the soil, intent on sucking up every available drop of water. But if the soil is very dry, as it is during periods of drought, the roots can’t draw in enough moisture to keep pace with losses from the stomata. Grasses respond by transferring their most valuable resources (including sugars and proteins) from their leaves into their roots and, especially, into their rhizomes—those aggressive, underground stems that are familiar to anyone who has ever battled with quack grass in the garden. Dead to the world above ground—withered and crisp—the plants live frugally below the surface, drawing on their cached supplies and biding their time until the weather improves. When the rains eventually return, as inevitably they do, the grasses explode into action, sending out fresh rhizomes, which in turn put out fresh leaves and roots, to produce a burgeoning network of tender growth. The amazingly resilient blue grama can revive from dormancy, green up, and grow on as little as 0.2 inches, or 5 millimeters, of rainfall.
Indian grass
Needle-and-thread grass
Galleta
Western wheatgrass
YOU CALL THAT A DROUGHT?
The Great Plains Grasslands, and the climate that defines them, have been around for the last eight thousand 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