Prairie
Page 21
Prairie fires were also set by Native people, who used burning as a tool to hold back the brush and maintain the grasslands as pasture for bison. But with the end of the buffalo ecosystem and the introduction of agriculture, prairie fires were suppressed and trees began to make a slow but steady advance. This expansion of woody vegetation has now been documented in almost every ecoregion across the Great Plains, from the Aspen Parklands of central Alberta (where a 60 percent increase in the area of brush was noted between 1907 and 1966) to the Flint Hills of Kansas (where patches of woodland protected from burning have enlarged by 250 percent since the 1850s) to the dry grasslands of west Texas (where the area covered by mesquite has more than tripled locally in just over a century). At the same time, various species of junipers are intruding on western rangelands, ponderosa pines have expanded from the Niobrara River valley into the Nebraska Sand Hills, and dense stands of eastern red cedar dominate the once-grassy slopes of Iowa’s Loess Hills. These examples could be multiplied many times over.
Extrapolating from local experience to calculate the effects on the Great Plains as a whole has not proven to be simple. Estimated area of presettlement woodlands minus acreage lost to clearing and other development plus postsettlement expansion of woody growth equals the sum of many unknowns. Still, the available evidence strongly suggests that the Great Plains region is now more heavily wooded than it was two hundred years ago. The big, unanswered question is, why? Some researchers have suggested that the changes began back in the days of the cattle barons of the nineteenth century, when the prairie was badly damaged by overgrazing. As the hold of the grass was weakened, patches of bare soil were exposed, creating openings where woody plants could grow. Once the brush was established, it was able to expand. Or perhaps the cattle not only broke the sod but also sowed the seeds by ingesting the fruits of trees and shrubs (notably, honey mesquite) and distributing them in their droppings. Alternatively, it is possible that global, rather than local, forces have been at work. Recent evidence suggests that rising concentrations of carbon dioxide in the atmosphere, due to the burning of fossil fuels, may be encouraging the growth and expansion of trees and shrubs by giving them a physiological advantage over nonwoody plants. But whatever other factors have been involved, the suppression of the natural fire regime has certainly played a central role in encouraging the intrusion of woody plants into the grasslands.
In the Riparian Zone
The transformation of grasslands into woodlands is far from trivial. It represents the difference between ground squirrels and tree squirrels, meadowlarks and robins, sharp-tails and ruffed grouse, between the stripped-down economy of the drylands and the richer options of a more sheltered, and sheltering, wooded environment. Of all the habitats on the Great Plains, none is more biologically productive—or more subject to disturbance—than the sinuous stands of trees and shrubs that line the major creeks and rivers. Although these riparian forests occupy only about 1 percent of the region, they provide living space for somewhere between 60 and 90 percent of the species of vertebrates (reptiles, amphibians, mammals, and birds) that occur on the prairies. For example, researchers working in northeastern Colorado were amazed to discover that 82 percent of all the local birds could be found down by the river. This finding helps to account for the delight of walking along a prairie river on an early morning in spring, as a dozen different bird songs shimmer above the canopy. For anyone who has experienced this pleasure, it will come as no surprise to learn that, in both numbers of species and numbers of individuals, these woodlands are home to some of the richest avian communities in North America.
A pair of yellow warblers tend their nest.
Arthur Savage photo
But the resources of the riparian zone do more than provide habitat for an abundance of wild animals and birds. Over the years, they have also proven highly attractive to humans, from the seminomadic people of the Plains Woodland tradition, who gardened along creeks in Colorado some 2,000 years ago, to the village-dwelling Mandans and Hidatsas, who, as recently as the 1880s, were growing corn, beans, and squash in plots along the Missouri River and its tributaries. With the intrusion of industrial society, the demands on the riparian zone have grown ever-more intense, as we look to the river valleys for farmland, rangeland, roadways, reservoirs, and river-view subdivisions. When these localized impacts are overlain on the global effects of fire suppression and climate change, the riparian community can be expected to respond in conflicted and complex ways.
Even the most innocent changes can trigger profound effects. For instance, what could look less stressful than a herd of cud-chewing cattle bedded on a riverbank, watching the water slip quietly past? Yet wherever they are present, cattle present a major threat to the health of riparian ecosystems. Unlike the native-born bison, which were well adapted to life on the dry plains, cattle evolved in more temperate environments and are drawn to treed valleys in search of water, shade, and forage. Although moderate grazing, by a small number of cattle for a short period of time, may not cause noticeable harm, heavy grazing inevitably leaves deep scars. Too many hooves in too little space soon pound the place to death, as the animals foul the water, destabilize the banks, and trample or chew the upcoming crop of woody plants. Traditionally written off as sacrifice areas by the cattle industry, riparian woodlands have only recently been appreciated as a rich resource for wild animals and plants. Thanks to this new awareness, range managers are promoting the use of artificial water points, streamside fencing, and other strategies to protect these “hot spots” in the living landscape.
The sparky black-billed magpie is a familiar resident of woodlands across the plains, from the riparian zone to farm shelterbelts and urban plantings. A bright-eyed opportunist that eats both plant and animal foods—including songbird eggs and young—the magpie is despised by people who do not understand the role of predation in the ecosystem.
Unfortunately, damage caused by other means is often more difficult to correct. In the central United States, for example, corridors of hardwood forest once spread across the broad floodplains, or bottomlands, of rivers and creeks, including (among others) the Milk, the Marias, the Wildhorse, and the mighty Missouri. The riparian forests of the Missouri River, for example, opened out in the Dakotas and, like the river itself, grew broader and more majestic as they flowed south, expanding from a width of about half a mile (800 meters) in the north to a span of almost 19 miles (30 kilometers) as they neared the river’s mouth. Today, this irresistibly flat and fertile plain has been mostly converted to farms. Between the Nebraska state line and the Mississippi River, more than 80 percent of the lands that were forested before settlement are now planted to crops like corn and sorghum.
Farther upstream in the Dakotas, the outcome has been similar, though the means have been more complex. Here, too, forests have been lost to clearing: for instance, almost 60 percent of the native woodlands between the Dakotas’ Garrison and Oahe dams are now under cultivation. As for the remainder, most of the once-wooded floodplain now lies under the waters of lakes Sakakawea and Oahe and other impoundments. Although the shores of the reservoirs are fringed with patches of brush, these fragmented woodlands are neither as extensive nor as diverse—nor as rich in species of birds—as the original forests were. And even in places where remnants of the native forest have survived (for instance, along undammed stretches of the river west of Williston, north of Bismarck, and east of Yankton), things are not looking good. The problems first surfaced in the mid-1970s, when an analysis of cores from trees in the Garrison-to-Bismarck stretch of the river, downstream from the Garrison Dam, showed that they were not growing as quickly as they had twenty years earlier, before the dam was constructed. Somehow or other, the impoundment of the river was affecting the health of riparian ecosystems over distances of as much as 60 miles (100 kilometers).
Box elder/ Manitoba maple
How could a stationary wall of earth and concrete exert this kind of influence? The answer was that it
was doing what it had been designed to do: controlling the flow of the river and preventing flooding. By depriving the riparian forests of a well-timed influx of water from spring floods, the dam had cast a shadow over them. In response, all the major species of trees in the forests, including green ash, box elder (or Manitoba maple), and American elm, were suffering from reduced rates of growth. And then there were the cottonwoods.
The dominant trees of the riparian zone across the prairies—and often the only species on the arid plains of Alberta, Montana, and other points west— cottonwoods are dependent on the energy and flux of flood for their very survival. Each spring, a typical grove of cottonwoods pumps out billions of cottony, wind-borne seeds, most of which end up drifting forlornly around the country. Only a tiny fraction find the conditions that they need for germination and growth: bare soil, full sun, and plentiful moisture. Where better to meet these requirements than in the wake of a spring flood? As the river recedes from the floodplain, a terrain of newly deposited sandbars and clean-scoured banks is exposed, creating ideal conditions for cottonwood seeds to sprout. (That is why cottonwoods often grow in bands and arcs of same-age trees, each representing a historical moment of opportunity.) But if spring floods are constrained and seed beds are not produced, the trees lose their only chance at reproduction for an entire year.
As Wallace Stegner once pointed out, “western history is a series of lessons in consequences,” and the consequences for cottonwoods have, for the most part, not been good. Because their rate of reproduction is reduced, the trees can’t compensate for natural mortality, and many populations are collapsing. (Cottonwoods are old by age thirty, and few last longer than a century.) The surviving stands of cottonwoods along the Missouri, Bighorn, Milk, South Saskatchewan, and several other rivers are all dying off, and there are few ambitious young saplings to take their places. As early as 1981, for instance, just a couple of decades after the Waterton and St. Mary rivers of southeastern Alberta were dammed, the number of cottonwoods along their banks had declined by 23 percent and 48 percent, respectively. By contrast, along the neighboring Belly River, which was not restrained by dams, the cottonwoods continued to reproduce and sustain themselves.
With large, heart-shaped leaves that whisper and rattle in the wind, plains cottonwoods are among the most talkative of trees.
Riparian forests and other mixed woods provide habitat for ruffed grouse. By beating his wings rhythmically against his body, this male will thump out a “come-hither” call to attract breeding females.
Arthur Savage photo
Where the trees are dying off, they are being replaced, in the west, by grassy banks or, farther east, by species of trees that are not dependent on seasonal flooding. These include both native trees such as ash, maple, and elm and invasive, introduced “weeds” such as tamarisk (salt cedar) and Russian olive. But even if other trees do grow up to fill the gap, the absence of cottonwoods is a loss to the roughly one-third of prairie birds that rely on them for food and shelter. Because (like other members of the poplar tribe) they support an exceptionally rich fauna of insects, they are an especially valuable resource for insect eaters such as warblers and woodpeckers. And because of their propensity for heart rot (another poplar attribute), they are an important source of nesting habitat for woodpeckers, chickadees, tree-nesting ducks, and other cavity nesters. Cottonwoods also produce fat, juicy leaf buds that serve as a vital wintertime food for fat, juicy browsing birds such as sharp-tailed and ruffed grouse, which teeter improbably in the upper branches.
Bufflehead at nest
The Case of Braided Rivers
Inevitably, on a planet that always has a surprise or two up its sleeve, there is a twist to this story—a special case in which the same suite of human actions has brought about an equal but opposite effect. Along certain rivers in the west-central plains, notably, the Arkansas in eastern Colorado and the Platte system of Wyoming, Colorado, and Nebraska, the curtailment of spring floods has had the unexpected effect of promoting the growth of cottonwood forests. To reproduce successfully, cottonwoods need bare, flood-scoured seed beds for germination. In succeeding years, as young saplings, they also need a respite from flooding. A so-called meandering river like the Missouri throws up banks and sandbars as it carves and recarves its course, thereby creating slightly raised beds on which some of the saplings, some of the time, find safe haven. But a shallow, braided river like the Platte runs and ripples across its streambed and, under natural conditions, is likely to scour out any seedlings in its channel. Even if a cottonwood seed does manage to put down roots, it will probably be swept away by the river long before it can reach adulthood.
The sun sets over the braided—and increasingly brushy— channels of the Platte River in Nebraska.
Or at least this is how things used to work, before the dams went in. According to land surveys from the middle to late 1800s, the banks of all three Platte rivers—the north and south forks and the Platte River itself—were originally covered in grass, with only scattered clumps of brush and little or no cottonwood forest. But by the 1950s, the waters of the Platte system had been impounded behind several dozen dams, and peak flows were dramatically reduced. In response to these changes, each of the three rivers dwindled into progressively narrower channels, thereby abandoning much of their streambeds and exposing a series of beaches, or natural levees, on which trees and bushes could become established. By the time the channels stabilized in the late 1960s, the streams had been reduced to between one-fifth and one-twentieth of their historic widths, and their once-open beds were scrawled with willows and lined with a gallery of towering cottonwoods.
Once set in motion, the wheels of ecological change have just kept on rolling. Because the rivers are now in balance with their flows and are no longer narrowing, they have effectively stopped producing nursery sites for cottonwood trees. It follows that these forests will soon suffer the same fate as the cottonwoods everywhere else: they will die off and be replaced by mixed hardwood forests. This coming transition is already signaled by the advance of Russian olive throughout the Platte drainage, a silvery foreshadowing of future change. But despite these continuing adjustments, the forests that have sprung up along the Platte system appear to be here to stay, at least for as long as the rivers are constrained.
Plains cottonwood
Go West, Young Bird
From a bird lover’s point of view, it may be hard to understand why anyone would object to the expansion of woodlands along the Platte or anywhere else on the Great Plains. The more trees have invaded the grasslands over the last hundred-odd years, the more species of birds have moved in, either as summer residents or as visiting migrants. No matter how the expansion of woody growth was triggered—whether through overgrazing or climate change or the suppression of fire and flood—the result has been a diversification of the local environment. The addition of woody vegetation brings a complex vertical dimension to the living world that extends from the ground up, through the shrub layer, into the sheltered midcanopy and the treetops. At every level, trees and shrubs provide habitats to a particular community, or guild, of breeding birds that is adapted to specific conditions of light, humidity, and shelter. In addition, woodlands also offer a range of options along the radius of each wooded stand, from the shadowy stillness of the interior to the bright and breezy margins. For an ovenbird, a secluded forest interior. For a catbird, a shrubby edge. For an oriole, the airy heights of the upper branches.
Life responds to opportunity, and the expansion of woody vegetation across the plains has opened up new opportunities for dozens of species of birds that were seldom, if ever, seen here by early settlers. If the first generations of prairie naturalists could peer down from heaven, binoculars in hand, they would be astonished by the flood of recent arrivals and range extensions. On the northern plains, for example, the western kingbird (a flashy gray-and-black flycatcher with a lemon-yellow front) was originally restricted mainly to wooded river valleys in the dr
ainage of the Missouri and the South Saskatchewan rivers. But since the late 1800s, the birds have ventured out of the valleys and, discovering that the world has changed, have expanded through North Dakota and across the plains and parklands of the Canadian prairies. Like land birds crossing an ocean, they have hopped from island to island of wooded habitat, laying claim to a country that in the past had denied them access.
The change that made all the difference for western kingbirds was not so much the expansion of natural islands of brush as the sudden appearance of artificial forests. Ever since the first Arbor Day was celebrated in Nebraska, in 1872, the people of the Great Plains have eagerly bent to the task of cultivating what one prairie arbori-enthusiast referred to as “missionaries of culture and refinement.” By which he meant woody plants. Over the decades, the settlers and their descendants have planted literally hundreds of millions of trees and shrubs, including so many shelterbelts that if they were placed end to end, they would reach around the equator and partway back again. Between 1935 and 1958, for instance, a combined total of 32,000 miles (52,000 kilometers) of these rural windbreaks were planted on the grasslands of Canada and the U.S., providing a literal hedge against soil erosion. Meanwhile, in both rural and urban settings, the little house on the prairie had become the little house in the woods, as tree-loving people altered the landscape to create habitat for themselves. As these plantings matured, they also provided habitat for western kingbirds and other species of birds, which used them as stepping-stones in their progress across the grasslands.