Prairie
Page 9
Taking full advantage of a well-timed spring rain, Texas bluebonnets and Indian paintbrush blaze along a roadside near Llano, Texas.
Long-term patterns of precipitation not only determine whether the land will grow trees or grass but also establish the limits that distinguish one “type,” or ecoregion, of grasslands from the next. When precipitation and other variables are averaged over the long term, the underlying order of the prairie climate begins to emerge. In the textbooks, these hidden patterns are revealed through charts and maps, but out on the prairie, they are written as gradients of grass.
Sometimes, the dialogue between the vegetation and the climate is intriguingly complex. For instance, summer precipitation on the prairies depends, in large part, on air masses that blow in from the south, carrying moisture from the gulf. Because of their southerly origins, these winds naturally have a greater influence on the southern plains (where they “reside”) than in the north (where they merely “visit”). So it isn’t entirely surprising to discover that the southern plains receive significantly more moisture than the northern prairies do. If, for example, Amarillo can hope to get 20 inches, or 500 millimeters, of moisture in a normal year, Lethbridge typically has to make do with only three-quarters as much. With this difference in mind, one might expect the prairies of northern Texas to be lusher than those of southern Alberta or Saskatchewan. Instead, the reverse is the case.
The trick is that the south-to-north gradient in precipitation is canceled out by an equal but opposite north-to-south gradient in evaporation. Because the average annual temperature increases from north to south, so does the rate at which moisture is lost through evaporation. Whatever the southern plains gain as rain, they lose as water vapor. As a result, the “effective precipitation”— the amount of water that is available to growing plants—is about the same in southern Alberta as in northern Texas. This helps to explain the long, gradual transition from the semiarid climate of the Northwestern Short/Mixed Grasslands to the sun-frazzled conditions of the Southern Short Grasslands. (A map showing these ecoregions can be found.)
Meanwhile, there is yet another climatic gradient that also helps to shape the vegetational profile of the Great Plains. This is an east-to-west decline in average annual precipitation. The tropical air that brings summer rains to the prairies typically swings up from the Gulf of Mexico, through the central United States, and off toward the east. As a result, its influence is stronger on the tall grasslands of the eastern plains than on the short-to-mixed grasslands farther west. If Winnipeg receives about 20 inches (500 millimeters) of moisture on average, Lethbridge gets 20 percent less (just 16 inches, or 400 millimeters). And if Kansas City can count on almost 40 inches (1 meter) of precipitation in a normal year, Amarillo can only expect to receive about half as much—and this time there is no reverse gradient in temperature to compensate for the difference. Less moisture is simply less.
Raindrops cling to the translucent petals of a harebell, a flower that is native not only to the northern Great Plains but to parts of Europe as well, where it is famous as the bluebell of Scotland.
In the days before the prairies were plowed and settled, this east-west moisture gradient found expression in the natural vegetation. As the average precipitation declined toward the west, the vegetation diminished in step, gradually reducing in height like a living bar graph. Somewhere around 100 degrees west longitude (give or take a few degrees), the tall-grass species dwindled away, leaving the wind to ripple through knee-high stands of mixed-grass prairie. To the west and southwest, the midheight grasses in turn gave way to a carpet of ground-hugging grasses, as a dry land clothed itself in drought-resistant plants.
The boundaries between tall-, mixed-, and short-grass prairies are not as tidy as they look on the map. In the patchwork quilt of the grasslands, each of the major blocks of vegetation is composed of many smaller blocks. And just as the overall picture is determined by large-scale climatic patterns, so each of these distinctive patches is a response to local variations in microclimate. Conditions are different on a south-facing slope than on the north and on lowlands than on hilltops, and these subtle differences are reflected in the vegetation. Moving uphill, from humid bottomlands to the drier crest, reproduces the moisture gradient of moving from east to west. And so, a sea of tall-grass prairie is broken by islands of mixed (or even short) grasses that grow on uplands and arid slopes. Meanwhile, out on the short-grass prairie, blue grama and its diminutive associates follow the opposite trend, ceding ground to mid-height or tall grasses in moist valley bottoms.
Western wood lilies scarcely put in an appearance during dry years but burst into a riot of color when the rains return.
Arthur Savage photo
Whether considered locally or regionally, the transition zones between the different “types” of prairie are dynamic, migrating upslope and down-slope, east and west, following changes in the climate. During periods when the weather is unusually wet, the tall and mixed grasses expand their influence and range, tracking the sweep of abundant rain. In times when the prairies are stricken by drought, the tall grasses are forced into retreat and the drought-resistant short grasses extend their reach. (During the Dirty Thirties, for example, the short-grass community pushed as much as 150 miles, or 240 kilometers, to the east, temporarily usurping the tall-grass prairie.) Thus, the grasslands keep time with the climate, mirroring its ebbs and flows and dancing to the erratic beat of the rainfall.
> PRAIRIE FIRE
Climate is the major factor that determines the extent of the Great Plains grasslands. To put it in technical terms, grasses hold sway wherever the evaporative demand (the amount of moisture that the atmosphere would draw away if it could) is slightly greater than the precipitation (the amount of moisture that is actually out there, in the ecosystem). But there is one important exception to this rule of thumb. The lush tall-grass prairies that fringe the eastern margin of the plains receive abundant moisture, more than enough to keep pace with evaporation. Theoretically, then, the region ought to support trees. And, in fact, wherever fragments of native tall-grass prairie have survived untilled, they have been aggressively invaded by stands of aspen, oak, and dogwood during the last 150 years.
The missing link—the force that maintained the tall-grass prairie in the past but that has since been all-but-extinguished—is fire. Prairie fire was the terror of the early settlers, a rapacious enemy that raced across the plains, leapt over firebreaks and rivers, and at times sped from horizon to horizon in a single day. But alarming as it was to humans, wildfire was a friend and ally of the tall grasses. Not only did it clear away the thick thatch of dead vegetation that prevented new shoots from breaking through, it also killed trees, the true “terror” of the prairies. When a tree burns, the growth points on its twigs and branches are likely to be injured, so the plant is hampered in its ability to produce new shoots. But a grass protects its growing tips under the ground, where the fire cannot reach, and rises from the flames like the proverbial phoenix.
Before the agricultural era, most parts of the tall-grass prairie were probably set ablaze once every three to ten years, either by lightning or by the deliberate action of Native people, who used fire to green up the prairie and bring in animals. But however prairie fires started, they always had the same effect: they were the guardians of the tall-grass prairies.
Controlled burning is used to maintain the health of tall grasslands, like this one in Badlands National Park, South Dakota.
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SECRETS OF THE SOIL
. . . the prairies are nothing but grass as the sea is nothing but water.
WILLIAM LEAST HEAT-MOON, PRAIRYERTH
THE GRASSLANDS OF the Great Plains tug at our senses, challenging us to overcome our limitations as mere humans. On the one hand, the landscape calls us to enlarge our horizons and soar across its spacious expanse. But on the other, the prairie seduces us with details, quietly drawing our attention to the little things th
at matter in this big, round world. The flick of a ground squirrel diving into its burrow, the wheeling shadow of a hawk, the ripple and run of its wingspan across the bending grass. If we are willing, the prairies will even lead us beyond these small sightings to truly minuscule events, beckoning us into an alien but all-important world that lies almost entirely beyond the reach of our ordinary awareness. For the typical life-form on the grasslands is not a bison or a gopher or even a blade of grass. It is some weird little creepy-crawly, far too small to be seen by the unaided eye, that lives and dies just beneath our feet in the deep prairie soil.
Recent estimates suggest that the total weight, or biomass, of the invisible organisms that live in prairie soils is greater than the mass of all the visible, aboveground animals put together. If all the microflora and microfauna from grassland soils could be piled onto one arm of some Great Cosmic Scale, with all the insects, reptiles, mammals, and birds from the upper world heaped onto the other pan, the balance would tip sharply toward the soil organisms. Together with the plant roots around which they live, these little creatures are the powerhouse of the prairie, responsible for anywhere between 60 and 90 percent of all the biological activity in the Great Plains grasslands.
Entering the Underworld
Waterbear
The soil has been described as the “poor person’s tropical rainforest,” one of the richest and most diverse ecosystems anywhere on Earth. The most numerous organisms in this busy world are also the smallest—thousands upon thousands of different kinds of single-celled organisms, including algae, fungi, bacteria, and protozoans. Although a few of these creatures may be familiar from Biology 101—the ooze-along, blob-shaped amoebae, for example, and the whip-tailed flagellates—many (if not most) of the soil microbes are still completely unknown, never having been studied, named, or even seen by anyone. A single teaspoon of dirt typically holds around 5 billion of these fantastically varied little animals and plants, all of them fiercely engaged in the business of life and death. In their billions and their trillions, they expend as much energy as if several thousand people were living on every acre, or hectare, of the prairies.
Adding to the hubbub is a lively assemblage of somewhat larger (though still mostly microscopic) and more complex animals. The bestiary includes plump, lumbering, eight-legged little critters called tardigrades, or waterbears, that waddle through the film of water around soil granules, searching for food. When they find plant roots or fungi, the “bears” drill into the cells with their sharp snouts, or stylets, and suck out the nourishing fluids. Another, more important group of plant eaters, the springtails spend their lives bumbling blindly through the dirt, munching on bacteria, fungi, dying roots, and the occasional dead waterbear or worm. Jumpy and alert, springtails are equipped to leap into action at the first tremor of danger. As their name suggests, they have spring-loaded tails that when all is well they carry tucked up under their abdomens. But when danger is detected, the tail fills with fluid and suddenly snaps down, sending the springtail catapulting to what, with luck, is safer ground. (The light-colored specks of “dust” that sometimes somersault away from compost when the pile is turned are actually springtails taking flight from the disturbance.)
About one square yard of prairie can support anywhere from 5,000 to 15,000 springtails in the top foot of soil. (That’s about a square meter dug to the depth of 30 centimeters, or enough to make a couple of wheelbarrow loads.) But these statistics pale compared with the totals for another major group of soil dwellers. The soil is crawling with microscopic mites, those scurrying, scuttling, eight-legged cousins of spiders and ticks. The same small patch of prairie soil that is hopping with springtails may simultaneously be home to up to 60,000 mites, representing dozens, or even hundreds, of species. (Although 48,000 kinds of mites have so far been identified around the world, the final catalog of species is expected to reach half a million or more.) While many mites make their living peaceably by feeding on microbes and plant roots, others are voracious predators—the tigers of this invisible world. Spinibdella cronini, for example, is a largish red mite (about the size of the period at the end of this sentence) that is common in prairie soils and that preys on soft-bodied mites and springtails. Its modus operandi is to seize its prey by surprise, tie it down with silk, and then suck the life out of it.
The opulent, fragrant blossoms of the gumbo, or tufted, evening primrose are typically found on dry, eroded slopes, usually where shales laid down in the Cretaceous Period are near the surface. These insect-friendly flowers open in early morning and close at night.
Pity the poor springtails. For “giant” mites are not the only menace in their shadowy world. They must also be prepared to flee from pseudoscorpions, jaunty little monsters that stalk through the soil with their venomous pincers armed and ready for action. Danger can also come swimming through the darkness in the form of carnivorous worms, innocuous-looking creatures (mostly smaller than the letter i) that either swallow their prey whole or attach themselves to the victim’s body and scrape at it until the internal organs can be drawn out through the puncture. These silent predators are members of an enormous tribe of roundworms, or nematodes, that are the most numerous multicellular animals on the grasslands. Hundreds of species have been found at sites across the Great Plains, including those that feed on bacteria, protozoans, fungi, algae, mites, other nematodes, and the roots of higher plants, or some combination of the above. A heap of soil that contains thousands of spring-tails and mites typically also holds between 3 million and 5 million nematodes. Across the broad expanse of the grasslands, their numbers spin out of control, quickly mounting toward the astronomical.
Springtails
Pseudoscorpion
Predatory mite
Nematode trapped by nematode-eating fungus
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> CHEMICAL WARFARE
Nematodes are not only the most numerous mul-ticellular animals on the grasslands. They are also the most important herbivores. While aboveground plant eaters nibble on stems and leaves, herbivorous nematodes eat their way through the roots, attacking silently from beneath. One study conducted on a conservation reserve determined that the unassuming little worms worked their way through more plant matter per unit area than either bison or prairie dogs. And on pasturelands, the standing population of nematodes has been known to consume twice as much vegetation as a herd of cattle.
Fortunately, some plants are able to protect themselves against the attack of the killer worms. A number of common prairie forbs, including pasture sage (or fringed sagewort) and nodding onion, produce natural nematicides that ooze out of the roots and create a worm-free zone for themselves and their neighbors. Such protection is presumably one of the benefits of a diverse plant community in which the roots of different species are intertwined.
But not all plants are so community minded. Some use chemical warfare not to fight against pests but to resist the encroachment of other plants. For example, a dainty-looking short-grass species called prairie three-awn produces root exudates that kill nitrogen-fixing algae and bacteria. (These are soil organisms that capture nitrogen gas from the air and convert it into a water-soluble form in the soil, ready for plants to use.) Because prairie three-awn can get by without much nitrogen, this action creates ideal conditions for its own growth and keeps out competing species that need nitrogen-rich soil.
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Growing Soil
All soil is full of life, whether under prairie, forest, tundra, or desert. Yet compared with other biomes, the temperate grasslands are remarkable for the extent to which they concentrate their resources in the underworld. The reason is simple: it’s safer down there. The prairie climate is grueling— too hot, too cold, too wet, too dry, too wild and variable—but go a little way beneath the surface and the environment becomes surprisingly equable. During the growing season, for instance, when the thermometer can easily spike from 50˚F to 105˚F (10˚c to 40˚c), the temperature 20 inches (half a meter) down in the so
il remains pleasantly cool and virtually constant.
The claret cup, or mound hedgehog-cactus, is a dryland specialist that prefers well-drained gravelly or sandy soil. A native of the southwestern deserts, it can also be found in suitable sites on the southern plains.
In a region where moisture is often the limiting factor for life, the soil also provides a buffer against drought. Long after the surface of the ground has been wicked dry by the sun and wind, the soil retains hidden reserves of moisture. Not only does water pool in the spaces, or pores, between the soil granules, but it is also glued to the surface of the particles by powerful chemical bonds. Thus, even apparently dry dirt often retains a thin film of water that is tightly bound to the soil grains. To tap this precious resource, grasses put down dense networks of thin, fibrous, probing, branching, fast-growing roots, all seeking moisture and nutrients. On average, grasses produce three or four times more roots by weight than they do leaves and stems, giving them a root-to-shoot ratio that is ten times as high as that of a forest.
The sheer volume of roots produced by prairie grasses is mind-boggling. In the mid-1930s, for example, one determined agrologist undertook to excavate and measure all the roots produced by a small patch of tall-grass prairie in Nebraska. From half a square yard, or meter, of ground, he extracted about 150 miles (250 kilometers) of fibrous roots—enough, laid end to end, to reach from Lincoln, Nebraska, to Sioux City, Iowa. The tally included 11 miles (17 kilometers) of green needlegrass roots, 13 miles (21 kilometers) of big bluestem, 23 miles (37 kilometers) of little bluestem, and 109 miles (176 kilometers) of Kentucky bluegrass. And that was without factoring in the silvery fuzz of absorbent root hairs that coat root fibers from base to tip. In another heroic experiment, a single rye plant (the domesticated cousin of several native prairie grasses) was found to have produced 7,000 miles (11,000 kilometers) of roots and root hairs in one four-month growing season.