Prairie

by Candace Savage

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  > MYRMECOPHILOUS, OR

  ANT-LOVING, BUTTERFLIES

  The gossamer-winged butterflies of the family Lycae-nidae are among the most familiar of insects, small, darting glimmers of metallic blue or coppery orange that flutter from flower to flower and spend the summer feeding on nectar. Yet behind all this sweetness and light lies a surprising story of danger, pay-offs, and protection.

  Many species of gossamer-wings are reared by ants. Take the Edwards’ hairstreak, for example, a tawny little butterfly with a vivid blue-and-orange tail and a faded orange streak on the underside of its hindwing. Edwards’ hairstreak is found on the eastern plains, from far-southeastern Saskatchewan to north-central Texas, and eastward across the continent to the Atlantic. So far as is known, it is found only in places that are home both to oak trees and to a particular species of red-and-black ant called Formica integra. The oak trees are important as egg-laying sites (in wounds and under rough bark) and as a source of food for the caterpillars. As the flat, slug-like caterpillars creep up the tree trunk to feed on leaf buds, ants scurry along with them and fight off attacks by predators and parasitic insects.

  At first, the butterfly larvae have little to offer the ants in return for their services as bodyguards. But as the caterpillars develop, they acquire glands that secrete honeydew, a sugary solution that the ants eat. It’s at about this stage that the ants increase their contribution as well, by constructing shelters for their “herd.” These are anthill-like cones of dirt and debris—4 to 10 inches (10 to 25 centimeters) in diameter and 2 to 8 inches (5 to 20 centimeters) tall—that appear at the base of caterpillar-infested oaks. The caterpillars, by now nocturnal, feed in the branches at night, with an ant guard at their side, and retreat to the safety of the shelter at first light. As many as 114 caterpillars have been found in a single “byre,” together with dozens of attendants.

  In midsummer, the caterpillars form pupae, usually in or near the byre, and the first adults emerge in late June or early July. Although some gossamer-winged butterflies are able to reach adulthood either with or without the assistance of ants, Edwards’ hairstreaks appear to be entirely dependent on this alliance.

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  > AMAZING ASPENS

  Of all the trees that grow on the prairies, none can bounce back from disaster more quickly than the trembling, or quaking, aspen. (“Stand still for half a hour,” one biologist was heard to complain, “and those darned things grow right up your pant legs.”) The key to their remarkable vitality lies in their roots, which have a grasslike ability to put up new shoots. The root system is typically shallow but extensive, with lateral runners that can reach 100 feet (30 meters) from the trunk and sinkers that descend from them to a depth of 6 to 10 feet (2 to 3 meters). The entire length of the runners is studded with growth points, as many as thirty in a thumb width, every one of which is capable of producing a new stem and, potentially, a new aspen. Such growth occurs in the first few years after a devastating fire or drought, as saplings shoot up by the thousands and then are gradually thinned out through the survival of the fittest.

  Although each stem in the grove eventually develops a more-or-less separate system of roots, there is a very real sense in which a grove of aspen is a single plant. Because all the trees in the stand have risen from the same parent stock, they carry identical DNA—they are in fact clones—and share such characteristics as protein content, sprouting rate, and insect resistance. Each bluff also responds to the seasons at its own genetically programmed pace, so related trees tend to green up and drop their leaves in synchrony. Since aspens are monecious (they produce pollen and seeds on separate male and female plants), all the members of a particular clone are usually also of the same sex. The largest known aspen clone—and the largest organism currently alive—is a stand of 47,000 male stems in the Wasatch Mountains of Utah.

  Aspens do produce seeds, but these flights of cottony fluff only rarely take root and produce new plants. The species basically maintains itself without the genetic mixing and innovation that come through sexual reproduction. Instead, each grove sticks with its own time-honored formula, producing shoots that transmit the parent DNA unchanged into the future. Although individual stems seldom survive longer than a couple of centuries, some clones are thought to have sustained themselves for between 1 million and 3 million years.

  Trembling aspen

  Aspen twig with bud scars

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  A stand of trembling aspen provides a rich tangle of habitats, from the lush understory to the upper branches.

  Where the Trees Are

  Green ash

  American elm

  Until recent times (before agricultural settlement began), the limits of life on the Great Plains were largely set by weather. The invisible contours of the climate—especially large-scale gradients of temperature, precipitation, evaporative demand, and wind—were mapped onto the landscape as visible differences in vegetation. The same west-to-east trend in precipitation that governed the transition from short-grass through mixed-grass to tall-grass prairie had a parallel influence on the height, density, and vigor of woody vegetation. In the dry short-grass ranges of the western plains, for instance, the brush tended to be scrubby, sparse, and tough, dominated by slow-growing, drought-hardy species such as sagebrush and juniper. And in the extreme southwest, where low precipitation was compounded by searing heat, the land was invaded by shrubs from the desert chaparral, including honey mesquite, creosote bush, and the ferociously barbed cat’s claw mimosa. Even with this intrusion, as little as 3 percent of the western plains was wooded.

  Moving eastward into the mixed-grass zone, the shrubbery gradually became more lush, until the draws were thick with bushes such as chokecherries, hawthorns, and wild roses, and even the grassy uplands were studded with clumps of snowberry and silverberry (also known, respectively, as wolf-berry and wolf willow). By the same token, the banks and bottomlands of the major rivers, which had been covered with grass farther west, became shrubby and then heavily treed as they cut east through the mixed grasslands. By the time they reached the tall-grass prairie, these streamside, or riparian, woodlands had taken on the character of full-fledged forests, with willows and cottonwoods along the shore, hardwoods such as green ash and American elm (with their understory of shrubs) on the lower slopes, and bur oak at the top of the banks, where they faced off against the bluestems and other grasses. These serpentine strips of riparian woodlands contributed to the estimated 15 percent of the tall-grass region that, under natural conditions, was covered by woody plants.

  The globular fruits of the wild prairie rose provide nutritious food to a wide range of animals and birds, including deer, elk, sharp-tailed grouse, prairie chickens, rabbits, skunks, and squirrels.

  If one left the river valleys and headed still farther east—beyond the basins of the Red and Missouri rivers, toward the eastern hardwood forests—the country went through a passage of ecological uncertainty, in which it was never quite clear if the trees or the grasses were in ascendancy. Still tracking along the same west-to-east increase in precipitation, the tall grasses were increasingly invaded by stands dominated by hickories and oaks until these groves eventually coalesced into a dense deciduous forest. This grassland-to-forest transition occupied a broad arc that swept south from Minnesota to Missouri and then (under the sway of moist weather rolling in from the gulf) swung to the southwest, following the curve of the Ozarks in the direction of Texas. There, at the southern terminus of the transitional zone, lay the Cross Timbers Forest, a picturesque savanna of weather-beaten oaks and junipers interspersed with glades of grass that once sprawled over almost 20 million acres (8 million hectares) in southwestern Kansas, Oklahoma, and central Texas. Although most of the trees were logged off in the early 1900s, significant fragments of the primordial Cross Timbers remain, including gnarled cedars that date back to the sixteenth century.

  A comparable transition zone, or ecotone, is also found on the
prairies’ northern shore, where mixed grasslands lap up against the boreal forest. Just as the west-to-east increase in precipitation encourages the growth of trees, so the south-to-north decline in average temperatures reduces the evaporative demand and creates conditions that are hospitable to forests. Somewhere around 52 degrees north latitude (about the latitude of Saskatoon), the climate strikes a balance between the requirements of certain midheight grasses— rough fescue, for example, and western porcupine grass—and those of various trees and shrubs, notably, trembling aspens. The result is a pleasantly varied landscape of rolling, grassy meadows and contoured groves, or bluffs, that is known as Aspen Parklands. Extending perhaps 60 miles (100 kilometers) from north to south, the parklands stretch all the way across the Prairie provinces, from the foothills of central Alberta to the Canadian shield, and south through Manitoba into northwestern Minnesota. There the broken stands of aspens shade imperceptibly into the savannas of hickory and oak that extend, with remarkably few interruptions, south to the Cross Timbers.

  Thus, for the most part, prairie woodlands sort themselves along the gradient of the four cardinal directions, north, south, west, and east, in accordance with their ability to withstand water stress. On a finer scale, they tuck themselves into the prairies wherever the lay of the land either improves the supply of water to the roots or (through shelter and shade) reduces the evaporative demand on the foliage. That is why, for example, the north-facing slope of a coulee is often densely tangled with brush, while the exposed southern slope supports nothing but a carpet of forbs and grasses.

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  > OLD-GROWTH SPECIALISTS

  For most of us, the phrase “old-growth forest” conjures up images of towering redwoods and massive firs in the misty gloom of the Pacific rain forest. We’re much less likely to think of scraggly old juniper or oak trees, out on the burning plains, that have taken four hundred years to reach the height of a two-storey building. Yet stunted and bent as they are, these venerable prairie warriors deserve not only the respect due to age but also consideration for their increasing rarity.

  Take, for example, the twisted trees of the Texas Hill Country. Originally an open savanna of mixed woods interspersed with meadows of grass, the region was heavily logged in the late 1800s. Then, thanks to fire suppression, woody plants pushed back in, often in the form of dense, monotypic, pollen-spewing stands of the native “cedar,” or Ashe juniper. Despised by ranchers, developers, and allergy sufferers alike, Ashe juniper was soon listed as Public Enemy No. 1, to be attacked with all possible vengeance.

  Yet Ashe juniper has its uses, most notably to a bird called the golden-cheeked warbler. A close relative of the more widespread yellow warbler, this species can be recognized by appearance (dark body, yellow face with black eye stripe), by song (“bzzzz layzee dayzee”), and by location (remarkably, it nests only in central Texas). Most at home in extensive tracts of mixed woodlands, the golden-cheek feeds mainly on caterpillars and other insects found on deciduous trees. But it also has an absolute and specific requirement for mature Ashe junipers, aged twenty or thirty and up, that have begun to develop shredding bark. The female warbler uses these fibers, bound together with spiders’ silk, to build her soft, gray, perfectly camouflaged nest. As shaggy old junipers have become scarce, so have the golden-cheeks, which were granted emergency protection under the Endangered Species Act in 1990. Predation by blue jays from encroaching urban forests (Austin, San Antonio, Waco) and parasitism by cowbirds are compounding the problems, which do not offer easy solutions.

  Golden-cheeked warbler

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  Ice Age Relics

  White spruce

  Ponderosa pine

  The tensions between supply and demand also help to account for another little-noticed feature of the prairie landscape. You’ll be scooting across the flatlands when suddenly the way ahead is obstructed by an abrupt rise. As the road climbs the escarpment, the view tilts toward the light and a phalanx of bristling conifers appears against the sky. Why, you may well wonder, would an evergreen forest sprout on the top of a rocky ridge, smack dab in the middle of the prairies? Yet this phenomenon is repeated in dozens of places across the Great Plains, from the white-spruce forests of the Cypress and Black hills to the ponderosa pines of Pine Ridge to the junipers of the Caprock Escarpment, among many others.

  These mysterious scarp woodlands are relics from the past. At the end of the last Ice Age, the climate was cool and damp, and a dense coniferous forest stretched across the breadth of the continent. A dark mantle of spruce extended from what are now the Canadian prairies south through the Dakotas into the central states, while pine woods appear to have flourished on the southern plains. But as the chill of the glaciation gradually lifted, the climate eased into a drier and warmer phase, marked by more frequent droughts, and the boreal forest was forced to retreat to the north. Within remarkably short order—a blink of the cosmic eye—the forests of the Great Plains either surrendered directly to grasses or else gave way first to deciduous trees and then to prairie.

  By the time the transformation was over, coniferous forest could only be found on the crowns of the tallest breaks and ridges. High enough to catch the rain and snow, and cooler than the grasslands below, these uplands created a microclimate in which the trees could retain a toehold. The thin mineral soils of the ridges—more suitable for conifers than for grasses or other plants—probably also gave the trees an advantage. But the factor that made the biggest different to the scarp forests was their top-o’-the-world location. Historically speaking, the greatest threat to prairie woodlands, apart from prolonged drought, was the fierce heat of grass fires. Where better to find refuge than atop a natural firebreak, an outcropping of safety in a world of flame? The present distribution of scarp woodlands therefore probably represents the limits of wildfires past, a kind of high-tide line beyond which the danger did not pass.

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  > FOSSIL FORESTS

  Relics of the late Ice Age forest are strewn across the Great Plains, rather like flotsam from an ecological shipwreck. In the Sweet Grass Hills of Montana, for example, stands of Rocky Mountain Douglas-fir are found alongside hybrid (white x Engelmann) spruce, growing just where they were left stranded thousands of years ago. The main populations of all three species are now far to the north and west, in the mountains and the boreal forest. Similarly, scientists working in Wyoming and Nebraska have discovered relict stands of hybrid poplars (cottonwood x balsam) in parts of the Niobrara Valley, where the two species had not been in contact since the late Pleistocene.

  The distribution of these living fossils has helped researchers to get a sense of the extent of the prehistoric forest. For instance, the fact that there are moose in the Turtle Mountain/ Pembina Hills region of Manitoba and North Dakota has been taken as evidence that the northern plains were once covered by extensive woodlands. By the same token, the flora and fauna of the Black Hills, with their disjunct populations of white spruce, paper birch, moose, black bears, and other boreal species, speak of a time long past when coniferous forest flowed out of the Rocky Mountains and across the Great Plains.

  But the late Ice Age is not the only geological episode that has left its mark on the present. There was also an era, roughly 5,000 years ago, when the climate was both moister and warmer than it is now. Under these conditions, southern-adapted species, including a round-eared, silky-haired rodent called the eastern woodrat, were able to migrate north. (More commonly known as a pack rat—from its habit of stuffing shiny objects into its nest—the woodrat looks like a handsome, oversized mouse and is not closely related to the introduced Norway rat.) Although most of the population subsequently retreated southward, a hardy remnant still persists on the wooded banks of the Niobrara River and its tributaries in north-central Nebraska, 100 miles (150 kilometers) from any other members of the species.

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  Trees on the Move

  For the woody plants that grew down on the grasslands,
by contrast, there was no chance of escape. Despite a variety of adaptations for withstanding occasional burns—corky, fire-resistant bark (bur oak); seeds that are stimulated by heat (ponderosa pine); roots that put out new shoots to compensate for fire damage (many poplars and oaks)—few species of trees can survive frequent, intense fires. Grasses, by contrast, are basically born to burn. Not only do they produce a tinder-dry thatch of dead foliage that lights with the slightest spark, but they are equipped to rise from their own ashes. The buds, or meristems, from which they put up new growth are tucked down at the surface of the ground, where they are protected from serious harm. But woody plants, which are inclined to reach for the light, hold their buds on the tips of their branches, where they are exposed to the flames and are sorely vulnerable to fire damage.

  Eastern red cedar

  In the days of the buffalo prairie, frequent grass fires conspired with severe drought and occasional intense grazing to limit the spread of woody plants. In the sometimes-lengthy interludes between die-backs, trees and shrubs were often able to take advantage of cool, moist weather to extend their reach onto the grassy plains. But no sooner were the trees established than some random bolt of lightning would set the prairies aflame, killing shrubs and most trees except in the humid coulees and river valleys. On the tall-grass prairies, in particular, where catastrophic droughts were relatively infrequent, trees and shrubs might have taken over completely if it hadn’t been for the erratic but inevitable return of lightning.