and enlivened by patches of golden beans, blazing stars, and prairie smoke, or three-flowered avens. The prairies of historic times had finally been created.
This new grassland was big and bold, but it was also much less varied than the mixed landscape of tundra, grass, and forest that it had displaced. And perhaps this in itself is enough to explain the disappearance of the Ice Age megamammals, which required a rich and varied supply of foods that grasslands alone could not provide. Yet if the new habitats were insufficient to sustain mammoths, they appeared perfectly suited to the bison, which soon emerged as the dominant grazing animal on the open range.
As the great herds drummed across the prairie, they of course had no conception of how their homeland had been shaped. The evidence of that drama lay unnoticed beneath their hooves—long-buried evidence of onrushing seas, rising mountains, silt-burdened rivers, and towering cliffs of ice. But the bison were untroubled by the traumas of the past, as they flowed across the horizon toward the present.
CHAPTER 3
THE GEOGRAPHY OF GRASS
I am the grass. Let me work.
CARL SANDBURG, “GRASS,” 1918
GRASSES ARE THE heart and soul of the prairie, the living link between the physical resources of the Great Plains—sunshine, rain, and soil—and almost every other aspect of the ecosystem. At first glance, grasses may look simple or even primitive. In fact, they are highly evolved organisms, especially adapted to cope with extreme climatic uncertainties, including frequent drought. From probable origins in the African region of the supercontinent Pangaea some 60 million years ago, grasses have migrated to every continent except Antarctica and have diversified into about 10,000 species throughout the world. Of these, approximately 140 species, in 41 genera, naturally occur in the Great Plains Grasslands. That’s nearly twelve dozen distinctly different native grasses! Some of them, like the magnificent big bluestem, or turkey foot (so called for its large, three-lobed seed head), grow up to 10 feet (3 meters) tall. Others, like the stick-in-your-socks specialist needle-and-thread grass, seldom exceed 3 feet (a meter) in height. At the low end of the scale are species like the diminutive blue grama, which grows close to the ground and rarely raises its elegant, eyebrow-shaped seed heads more than a few hand widths above the soil.
Yet despite these obvious differences, the prairie grasses all share one crucial ability. They are tuned in to the climate, able to dial their metabolisms down when conditions are unfavorable for growth and speed them up when the weather improves. Far from being passive stalks blowing idly in the wind, prairie grasses are lean, mean growing machines, designed to make the most of limited and unreliable resources.
MANAGING MOISTURE
ONE KEY TO the prairie grasses’ success is their ability to conserve water. Like most plants, grasses take in water through their roots and lose it as water vapor through tiny mouth-shaped valves, or stomata, in their leaves. The larger the surface of the leaf and the more stomata it bears, the greater the risk that the plant will lose too much moisture through evaporation, causing it to collapse. Grasses are protected from this trauma by having a reduced number of stomata and by the design of 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
