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 under ground by a mass of coarse, fibrous roots that reaches as much as a good 12 feet (or 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 almost 6 feet (2 meters) down!
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 .2 inches, or 5 millimeters, of rainfall.
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 wheatgrass (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.
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