Chance and Change: Ecology for Conservationists / Edition 1 available in Hardcover, eBook
Chance and Change: Ecology for Conservationists / Edition 1
- ISBN-10:
- 0520211553
- ISBN-13:
- 9780520211551
- Pub. Date:
- 06/21/1998
- Publisher:
- University of California Press
- ISBN-10:
- 0520211553
- ISBN-13:
- 9780520211551
- Pub. Date:
- 06/21/1998
- Publisher:
- University of California Press
Chance and Change: Ecology for Conservationists / Edition 1
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Overview
Product Details
| ISBN-13: | 9780520211551 |
|---|---|
| Publisher: | University of California Press |
| Publication date: | 06/21/1998 |
| Edition description: | First Edition |
| Pages: | 209 |
| Product dimensions: | 6.00(w) x 9.00(h) x 0.90(d) |
About the Author
Read an Excerpt
Chance and Change
Ecology for ConservationistsBy William Holland Drury, Jr.
The University of California Press
Copyright © 1998 Regents of the University of CaliforniaAll right reserved.
ISBN: 0-520-21155-3
Chapter One
Geological Activity and the Response of Vegetation on Upland Slopes* * *
In the earlier chapters, I identified the power and pervasiveness of geological processes along the seashore and riverbanks, where their effects are obvious. In this chapter I will show that similar although less evident processes are active in the uplands, breaking stones, churning sediments, and moving earth downslope. These processes are active at many levels of intensity and frequency as they mold the form and content of what geologists call unconsolidated surficial deposits. Because of this multiplicity of levels these processes strongly influence the distribution and abundance of plants.
The Mark of Ice on the Land
Now I shift to what are recent events on a geological time scale that, however remote they are to us, still influence plant distributions. Continental ice sheets left the mark of their heavy hand on the landscape of the northeastern United States and Canada. The ice polished rock surfaces on the shoulders of hills and scoured out valleys, cutting off the lower reaches of tributaries. Like a giant bulldozer it pushed cubic miles of surficial unconsolidatedmaterial out of northern uplands to the edges of the low-lands. The ice left morainic tills and outwash at its edges and piled up the fishing banks from Newfoundland to Cape Cod. It seems impossible to exaggerate the extent and importance of the ice's impact, but its very existence has been acknowledged for barely twice my lifetime.
Glacial Scour
Ice alters topography by gouging out preglacial valleys. It grinds hard against the sides and bottoms of valleys, plucks angular rock fragments off bedrock, fixes them in its grasp, and uses them as tools to grind other outcrops as it moves over them.
Matthes described ice scour on the granitic mountains of Yosemite National Park (1950). Similar effects, on a less grand scale, are evident in Acadia National Park in Maine. Massive granite is resistant and ice moves upslope over it, polishing the upslope surfaces. On the lee side, the ice freezes onto exposed blocks and carries them away. Hence, granitic mountains show spectacular, angular cliff faces and rounded domes.
More friable bedrock (metamorphic and sedimentary rocks) is shattered by frost action. The frost rives peaks and ridges that stand out above the ice and heaves the rubble downslope. The riving of broken rocks carves jagged peaks and sharp ridges. Mountains of such friable materials show isolated jagged peaks and great sweeping lines of broken rock on convex slopes. The rapid erosion of exposed peaks and ridges reinforces the tendency of mountain tops in glaciated regions to be at similar levels.
The ice under the glacier presses heavily as it moves, deepening and altering valleys from V-shaped (convex slopes) to U-shaped (concave) with sheer valley sides.
The Drift
Rock debris left by the ice or the streams flowing off it is known as drift. The great bulk of the drift was removed from northern uplands and pushed to the exposed coastal shelf between the ice and the deep sea. Ice came and went many times, and traces of different ice sheets are evident as loose, younger till and weathered-compacted older tills. Some remnants were plastered on valley sides and in valley bottoms under the ice. These are ground moraines. Some were moved to the edges of the ice where the ice stood temporarily. These make up lateral and end moraines.
Glacial deposits tell you how they were formed, more than what they were beforehand. Till, the muddled mixture of coarse stones, sand, and stuff as fine as clay, is a major component of glacial deposits. Compacted tills packed into lower slopes are usually partially covered by material that slumped off the valley sides, creating the gentle "run-out" of lower slopes in glaciated topography.
The ice may override debris, pushing up rounded, elongate hills that look like overturned boats, called drumlins. Drumlin fields are scattered along the coasts of Nova Scotia, around Boston, and in Wisconsin, where preglacial topography apparently jarred the flow of ice. Other topographic signs of glacial landscape include undulating topography of round hills and closed valleys, small lakes, and sand ridges. The landscapes along the glacial border from Minnesota and Iowa to Pennsylvania and Long Island are barely subdued versions of those in the north, where the ice is today a few miles away. Moraines left by earlier advances are muted, as frost heaving and slump have moved materials off the ridges into low places.
The prairie potholes, west and southwest of the Great Lakes, are examples of ground moraines. The low rolling hills are pockmarked with undrained hollows. Where underlaid by tills, the hollows have "perched" water tables, making for multitudes of lakes, ponds, and marshes. Rainwater percolates through lenses and sheets of sand extending for miles under the till, and it emerges as artesian springs that maintain the water table in the lakes and marshes. These ephemeral wetlands make highgrade nesting grounds for waterfowl and waders. One pothole may attract half a dozen species of nesting ducks and several sandpipers.
When the last ice melted away from the coast twelve thousand to thirteen thousand years ago, the land was still pressed down by the weight of the ice, and the ocean flooded far up the hillsides and into New England valleys. Breaking waves built cobble beaches two hundred feet above the present sea level on granite ridges on Mount Desert Island and along glacial moraines fifteen miles from the sea farther east. At this time, rivers flowed turbid with clay and silt, which settled out, plastering much of midcoast Maine with marine clays containing the shells of blue mussels and Arctic species of clams. As the coast rebounded, the sea retreated, but it still fills the lower reaches of preglacial valleys. The rocky ridges separating valleys are now peninsulas and the hilltops are islands.
Frost heaving in the ground around the margins of the ice sheets churned the upper levels of the soil, eliminating the orderly zones of leached and deposited minerals that we associate with soil profiles. British farmers noticed this and called it the warp. New England soils show evidence of churning associated with the intense frost activity. Indeed, frost action created many of the fine-grained topographic features on New England uplands. Most people are unaware of these features' existence, let alone of their pervasive influence on the distribution of trees.
The vegetation of the mountains responds to the effects of solifluction, or frost-moved rubble, and vertical movement of fines on patterned ground, because of the effects the soil structures have on the distribution of moisture. Once your eye is trained to see these responses you begin to see similar ones lower down. The forest trees there also respond to the effects the soil structures have on the distribution of moisture.
Vegetation Responds
The shape and size of a small valley reflects the importance of the size of a drainage basin. First-order streams occupy much smaller drainage basins than do second- or higher-order streams. The importance lies in the frequency of intense, localized downpours of rain in the hills. A single heavy rainstorm may drench the entire drainage basin. Such downpours, like spring freshets that follow a sudden thaw, produce spectacular local floods. The effective forces carving valley forms are periodic downpours, landslides, and floods-all rare events of unusual intensity.
Between downpours, fallen trees, branches, and leaf litter collect on slopes and in channelways. Long-term studies of a forest stand in the drainage basin of a small headwater stream, Hubbard's Brook in the White Mountain National Forest in New Hampshire, have clarified what is happening in the physiology of the forest (Bormann and Likens 1979). Teams of botanists and chemists followed the chemical interactions between the trees, the litter, the soil organisms, and the mineral soil. For decades, organic materials and mineral nutrients cycle between organic debris in the soil and the wood in trees. Leaf and branch litter collects in valley bottoms and may block the stream channel. High winds add branches and occasionally whole trees to the debris on the forest floor. Following fires or cutting, the trees grow back and production of organic matter exceeds respiration for many years. Then botanical processes slow as the canopy is closed. Although, for short-lived botanists, the forest seems to be "at equilibrium," sooner or later geological time takes over. A downpour carried on especially high winds knocks the forest down and the floods move the debris out of the creek beds. During these special events sediments start their travels to the sea.
John Hack, a geomorphologist, and John Goodlett, a forest ecologist, worked together in the headwaters of the Shenandoah Valley, in the heart of the Appalachians, in the 1950s (Hack and Goodlett 1960). Hack and Goodlett's study was inspired by one such spectacular flood on the Little River in Virginia. They found massive new gravel and sand deposits all up and down the river, both in the alluvium and in tongues of debris on the valley sides. The landslides of surficial debris loosened from valley sides and left scars of exposed mineral soil. Equally important, they found conspicuous evidence of previous landslides. By dating the trees on the old debris fans, they showed that major slides had occurred well before the arrival of European colonists.
Hack found several generations of debris slides extending back several hundred years, and Goodlett found several species of forest tree, such as scarlet oak, to be closely associated with the coarse soils at the tips of these "fossilized" avalanche tracks. The chutes created by landslides, twenty to one hundred feet across, were most numerous in the hollows at the heads of valleys over outcrops of Hampshire shale and sandstone, a weak rock. The landslides began just below ridge crests, under outcrops of Pocono sandstone, a resistant rock.
The evident relation between landslides and vegetation had confused earlier foresters who interpreted the correlation as reflecting the effects of logging in the late nineteenth century. It was logical within classical American ecological theory to consider the second growth, which grew up following the logging, to be immature.
To attribute the flood damage to the "inferiority" of the vegetation carries the implication that stable mature vegetation would have prevented it. But Hack and Goodlett pointed to debris from many landslides older than the logging that had been colonized by the same species that grow on the recent debris fans. The debris from the older landslides showed that the suggestion that an undisturbed forest can protect the soil from landslides leads to a contradiction. The two researchers observed that "compound debris fans are unequivocal evidence of flood damage to slopes in the past, prior to the period of logging and probably prior to the time of settlement by the white man. Either the forests mantling the slopes in the past were not all 'thrifty normal stands'-a possibility-or the slope forests, regardless of quality, cannot control huge volumes of runoff" (1960).
Furthermore, if the vegetation is developing toward a regional norm, then progressively older landslides should be vegetated by species closer to the regional norm. But Goodlett found the species on the oldest landslides to be the same as among the seedlings on the newest slides.
Hack and Goodlett found the distribution of forest trees to correspond closely to topographic divisions and contemporary geological processes. Goodlett found that altitude, exposure of the slopes, soil moisture, and kind of bedrock strongly influence the distribution of forest trees. He found that species distributions correlated best with features that affected the availability of water through the season. Details of local conditions were especially important. He found that the moisture-loving and fibrous-rooted yellow birch grew on rubbles along streams and on block fields where snow and ice persist, and that the drought-tolerant tap-rooted oaks may be excluded from the hollows by frost heaving.
Forests of Once-Frost-Active Soils
The continental ice reached the central uplands in Pennsylvania. Goodlett surveyed the forests on residual soils beyond the glaciated area and on the drift within the glacial boundary to see what might be the effects of differences in history (1954). You would expect, a priori, to find stark differences. South of the reach of the ice, you would expect deep residual soils, developed in situ during long periods of chemical weathering. In contrast, where the ice had reached, you would expect the soil to have been scraped off ridge tops, leaving blocky deposits in pockets behind rock shoulders and on valley bottoms.
As he expected, Goodlett found thin till on the hilltops and long, smooth, boulder-strewn slopes in the glaciated areas. But instead of the expected marked differences between the glaciated and residual soils, he found a thin, disturbed, and structureless soil spread over the entire landscape, under the current organic layer. The structureless layer included fine-grained material studded with large pebbles, boulders, and fragments and a large amount of fine sand and silt, which appeared as if wind-blown. Many of the large rock fragments were rounded and derived from the underlying bedrock, while others were angular, derived from bedrock further upslope.
Goodlett found no differences in the tree species composition over a wide range of topography on the two sides of the border, but he did find important differences in species according to present climate, topography, and soil structure. While the general vertical distribution of forests reflects climate, local differences reflect the composition of surficial deposits, especially the abundance of rock fragments that affect the depth to the water table.
In her important book on the forests of the Appalachians, Lucy Braun suggested that the oak-chestnut forests occur on younger surfaces of the Appalachians because the soils on those surfaces are too young to allow the full development of the climax mixed mesophytic forest (1950). Extending the logic to the south, she believed that the mixed mesophytic forests persisted since the Tertiary on undisturbed soils on the highlands of the Cumberland Plateau. Extending the logic to the north, she suggested that the forests within the region of the Wisconsin Drift are immature because the soils have had less than ten thousand years to mature since the retreat of the last ice.
Continues...
Excerpted from Chance and Change by William Holland Drury, Jr. Copyright © 1998 by Regents of the University of California. Excerpted by permission.
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