America's Ancient Forests: From the Ice Age to the Age of Discovery / Edition 1

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At the time of European discovery, the ancient North American forests stretched across nearly half the continent. And while today little remains of this past glory, efforts are underway to bring back some of the diverse ecosystems of that era. America's Ancient Forests: From the Ice Age to the Age of Discovery provides scientists and professionals with essential information for forest restoration and conservation projects, while presenting a compelling and far-reaching account of how the North American landscape has evolved over the past 18,000 years.

The book weaves historical accounts and scientific knowledge into a dynamic narrative about the ancient forests and the events that shaped them. Divided into two major parts, it covers first the glaciers and forests of the Ice Age and the influences of native peoples, and then provides an in-depth look at these majestic forests through the eyes of the first European explorers. Changes in climate and elevation, the movement of trees northward, the assembly of modern forests, and qualities that all ancient forests shared are also thoroughly examined.

A special feature of this book is its self-contained introduction to the early history of Native American peoples and their environment. The author draws on his roots in the Osage nation as well as painstaking research through the historical record, offering a complete discussion of how the cultural practices of hunting, agriculture, and fire helped form the ancient forests.

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Editorial Reviews

From the Publisher
"Despite these misgivings, I think America's Ancient Forests is a much needed text, written by a skilled forest ecologist and his historian. It deserves a place in every restorationist's library alongside Gordon Whitney's From Coastal Wilderness to Fruited Plain and Michael Willams' comprehensive Americans and Their Forests: A Historical Geography." (Ecological Restoration, Vol. 19, No. 4, 11/01)

"What a wonderful synthesis of information from fields as widely varied as botany, ecology, geology, archaeology, anthropology, and history! Thomas Bonnicksen has produced a work that will capture the imagination of anyone interested in the grandeur and beauty of the forests of North America."(Botanical Research Institute of Texas, March 2002)

Provides information for forest restoration and conservation projects, and presents an account of how the North American landscape has evolved. Weaves historical accounts and scientific knowledge into a dynamic narrative about the ancient forests and events that shaped them, discussing the glaciers and forest of the Ice Age and the influence of native people, and looking at the forests through the eyes of the first European explorers. A special feature is an introduction to the early history of Native American peoples and their environment. The author teaches forest science at the College of Agriculture and Life Sciences at Texas A&M University. Annotation c. Book News, Inc., Portland, OR (
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Product Details

  • ISBN-13: 9780471136224
  • Publisher: Wiley
  • Publication date: 2/7/2000
  • Edition description: New Edition
  • Edition number: 1
  • Pages: 608
  • Product dimensions: 6.42 (w) x 9.43 (h) x 1.29 (d)

Table of Contents


The Great Cold.

Glacial Ages.

Climate and Ice.

Land of the Great Cold.

End of the Ice Age.

Ice Age Forests.

Life Near the Ice.

The Spruce Forest.

Western Forests.

Southern Forests.

Finding the Lost Prairies.

Creatures of the Ice Age.

The Birth of Modern Forests.

Trees Begin to Move.

Pioneer and Settler Trees.

Spruce Migration.

Trees Abandon the Great Plains.

Trees Advance in the Midwest and East.

Trees Advance in the West.

The Great Drought.

The Next Ice Age.

Ancient People in a New World.

First Footprint.

Passage South.

Setting the West.

The Way East.

Trail to Florida.

Journey to Another Continent.

Taming a Wilderness.

Mammoth Hunters.

Ice Age Extinctions.

The Holocene.

Bison Hunters.


Decline and Return of the Bison.

Settlement and the Seasonal Round.

Harvesting the Forest.

Nourishment and Healing.

Temporary and Portable Shelter.

Plank Houses and Canoes.

Enhancing Nature's Bounty.

Wild Gardens.

Agriculture and Forests in the Southwest.

The Hohokam.

The Anasazis.

Agriculture and Forests in the East.

Early Mound Builders.

The Adena.

The Hopewell.

The Mississippians.

Warfare and Forests.

Historical Times.

Fire Masters.

A World of Fire.

Hissing, Roaring Flames.

Fire People.

Country Very Smoky.

Their Fires are Left Burning.

The Ominous Smoke Signal.

Firing the Forests of Their Enemies.

Fire Hunters.

Circles of Fire.

Their Wings are Scorched.

That Necessity May Drive Them.

Green and Fair Pasturage.

To Render Hunting Easier.

Just Set Your Teepee Up There.

They Knew Where to Burn.

Little Hair (Pelillo).

To Dry and Cook.

Straight and Slender.

Burned Places in the Forest (Go-ley-day).

Keeping the Country Open.

A Pleasant Meadow.

Prairies and Open Grounds along the Coast.

Little Knots of Deer.

To Prepare the Ground.

They Cleared the Way with Fire.

Because the Woods Were Not Burnt.


Timeless Qualities of Ancient Forests.



Shifting Mosaics.

Mutual Dependence.

The Spanish Explorer's Forests.

Southern Pine Forests.

Piñon-Juniper and Juniper Woodlands.

Pacific Oak Woodlands.

Valley Woodlands.

Foothill Woodlands.

Coastal Woodlands.

Northern Woodlands.

Coast Redwood Forest.

Forests of the Colonies.

Oak-Chestnut Forest.

Eastern White Pine Forest.

Beech-Maple Forest.

Red Spruce—Fir and Balsam Fir Forests.

Southern Red Spruce—Fir Forest.

Northern Red Spruce—Fir Forest.

High Mountain Balsam Fir Forest.

Forests of the Fathers.

White Spruce Forest.

Great Lakes Pine Forests.

Jack Pine Forest.

Red and White Pine Forests.

Oak-Hickory Forest.

Oak Savannas.

Bottomland and Protected Forests.

Oak Woodlands.

The Trapper's Forests.

Ponderosa Pine Forest.

Lodgepole Pine Forest.

Pacific Douglas-Fir Forest.

Giant Sequoia Forest.

Notes and Citations.



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First Chapter

Note: The Figures and/or Tables mentioned in this chapter do not appear on the web.
Ice Age Forests

This is the forest primeval.
Henry Wadsworth Longfellow (1807-1882)

Forests only exist in human minds. Groups of animals and plants that we call forests come together for a short time; then each species goes its separate way when conditions change. Constant warming and cooling of the climate, and the ebb and flow of glaciers, caused the disassembly of old forests and the reassembly of new forests. Some species thrive in glacial cold while others do best in the warm periods between glaciations. So different forests dominated the land under different climates.

Glacial or cold-weather species such as spruce, fir, and bristlecone pine expanded their territories during cold periods and drew back when it became warmer. Interglacial or warm-weather species such as oak, hickory, and ponderosa pine spread northward when the glaciers withdrew and retreated toward the south when the glaciers returned. Since each species advanced and retreated at its own pace, the trees shifted and sorted themselves into unique forests while moving around the landscape. The most recent sorting process began when the climate warmed shortly after the Wisconsin glaciers reached their maximum size about 18,000 years ago. Until then, North America's Ice Age forests (Fig. 2.1) dominated the landscape, reaching an uneasy stability that endured for thousands of years.


Life perished beneath the glaciers, but it flourished along the edge of the ice cliffs at the front of the glaciers and throughout the rest of North America during the Ice Age. The landscape beyond the glaciers would not be familiar to modern eyes. It was an alien world of modern and extinct animals living among well-known plants mixed in unusual ways. They could co-exist back then because summers were cooler; so differences between seasons were less extreme than today. These exotic mixtures of plants and animals formed a complex patchwork of communities. As a result, Pleistocene communities had a greater diversity of species, higher numbers of animals, more large animals, and larger animals than any that existed from then until now. Many of these primeval communities fell apart after the ice sheets melted, although most of the species survived and reassembled to form the less diverse landscapes that we recognize.

Half the continent escaped the massive ice sheets, even in the far north. Alaska's coastal and interior lowlands extending up the Yukon River into Canada were ice free and full of life during the Pleistocene. Plants and animals took advantage of lower sea levels by crossing land bridges to invade what are now islands. Many remained trapped there when the sea rose again. Some animals on these islands, such as mammoths, became smaller from generation to generation because food was scarce, and then they gradually disappeared. For instance, dwarf Columbian mammoths that stood only 5 1/2 feet tall survived on Santa Rosa Island, off the coast of southern California, until 10,000 years ago. Even more startling, dwarf woolly mammoths survived on Wrangle Island off the north coast of Siberia until as recently as 3700 years ago. Plants and animals also migrated back and forth across the newly exposed land that joined Alaska to Siberia, and they invaded the exposed coastal shelves along the edge of the continent. They also clung to mountain summits that were tall enough to protrude through the ice sheets, called nunataks, and on windswept ridges separating glaciers that flowed into the Pacific Ocean.

Northern nunataks were an unlikely place for life to survive, but it did. They were cold dry islands of rocky ground surrounded by a sea of ice that stretched to the horizon. Trees could not live in such a harsh environment. However, scattered over the stony surface grew louseworts, small plants with a featherlike leaf, while other plants, such as sandworts, hugged the ground to escape the bitter wind. A nunatak poking through an ice sheet near the Pacific Ocean was still cold and bleak, but warmer and wetter than those farther inland. Here on the fog-shrouded coast the same species of ferns thrived that now cover extensive areas high up in western mountains.

Subzero temperatures that averaged about 21 degrees F. near the advancing ice sheets kept all but the surface soil permanently frozen as permafrost. Trees became stunted because they could not push their roots deeply into the frozen ground. Constant freezing and thawing of the thin soil above the permafrost tilted the stunted trees in all directions. In a few protected areas where soils avoided freezing, pockets of spruce managed to prosper near the base of the massive glaciers, such as in parts of Minnesota. In most areas near the ice, however, young trees failed to replace those that died. So forests gradually gave way to grasses, sedges, and shrubs.

A narrow band of treeless land sparsely carpeted with low-growing plants lapped against the southern edge of the ice sheets and covered ice-free areas of Alaska as well. Tundra covered much of this land. It consisted of grasslike sedges, with their triangular shaped coarse leaves, lesser amounts of grass, herbs, lichens, and mosses, and widely scattered small shrubs such as sage. Other areas were covered by cold steppes that contained mostly sagebrush and grass, and very little sedge. Many bare areas also dotted the cold steppes because the dry soils could not support a thick cover of plants. Stringers of willow also wound their way along streams and rivers through the cold steppes and tundra.

Tundra and cold steppes intermixed to form a mosaic of patches, but tundra dominated the soggy soils below the southeastern edge of the glaciers. Tundra also extended down the crest of the Appalachian Mountains to Georgia, although white spruce forest interrupted it here and there. However, cold steppes became more important than tundra west of the Great Lakes where the climate was drier than in the East, and in places that escaped moving ice.

In the West, tundra and cold steppes dipped south into the valleys between the glacier-draped mountains of the Rockies. They also pushed down into the Columbia Basin between the Rocky Mountains and the Cascade Range, and into the Great Basin and the Bonneville Basin of northern Nevada and Utah. Tundra and cold steppes also descended the Puget Trough, which separates the Cascade and Coast Ranges in Washington and Oregon. However, scattered patches of Englemann spruce and lodgepole pine grew within the tundra and steppes, which formed open forests or parklands in some places.

Cold steppes originated in central Asia, but they expanded and nearly encircled the globe during the Ice Age. Even today, more than 90% of the species of flowering plants in cold steppe and tundra communities occur in both Asia and North America. During the Ice Age they extended from Spain and England, across Europe, Russia, and Siberia. They joined similar communities in Alaska when the shallow waters of the Bering Strait drained away creating an enormous flat land bridge between the continents called Beringia.

Rivers and streams colored milky-gray with powdered rock flowed from the glaciers and spread across Beringia and the lowlands of Alaska, creating oases of marshland within a bone dry landscape. They left the ground covered with sand and silt as flowing water curved back and forth across the land. Frigid winds picked up the debris and filled the air with a choking pall of dust, piling up sand dunes as they went. This vast, dry plain, with only an occasional clump of poplar trees to provide protection from frigid winds, extended to the northern edge of the Cordilleran and Laurentide Ice Sheets that covered most of North America.

These two ice sheets backed away from each other at least three times during the late Pleistocene (Wisconsin). This opened a narrow ice-free corridor from Alaska into the center of the continent. This corridor first opened at a time that we cannot date, but it opened again between 28,000 and 23,000 years ago. It opened for the last time between 14,000 and 12,000 years ago after the glaciers began their final retreat. Each time the corridor opened, the Bering land bridge also lay exposed, which created an unobstructed passage from Siberia into the lower United States. Cold steppe and tundra communities worked their way through this passage and down into the middle of the continent. This doorway to the south slammed closed when the ice sheets again expanded, pinching off pieces of these communities below the glaciers. These trapped communities of tundra and cold steppes gradually spread along the southern edge of the glaciers.

As trees shifted toward the south to escape the glaciers, they sorted themselves into wide bands of forest below the tundra and cold steppes. These forest bands stretched in an east-west direction across the continent. Boreal or northern trees that could tolerate bitter winters and short cold summers such as spruce, tamarack, and fir stayed in the North near the tundra and cold steppes. The cold glacial air pushed other trees such as jack pine and red pine out of the northern states, and oak and hickory out of the Midwest, into warmer weather farther south. These trees then pushed southern pines and magnolia to the shore of the Gulf of Mexico where they huddled by the warmth of coastal waters. The trees intermingled as they moved, which also created odd mixtures of species that have no modern counterpart.

Frigid air flowing off the ice sheets and down the Mississippi River Valley cut the bands of Ice Age forests in half. The forest bands retained much of their identity in the East and Midwest, but rows of mountains chopped them into a complex mosaic in the West. Thus, rumpled mountains, jagged peaks, and tundra and cold steppe-covered basins, in the Rocky Mountains and Cascade and Coast Ranges, broke the forests into a patchwork of fragments containing different mixtures of species. Each species grew where the soil, temperature, and moisture conditions were favorable. Windstorms, floods, and fires also broke up the forests into even smaller patches containing various sizes and kinds of trees intermixed with grasses, sedges and other herbs, and shrubs.


One tree more than any other defined forests near the ice sheets when the glaciers were at their maximum about 18,000 years ago -- white spruce. It is a short tree with a thin trunk and low-hanging branches that form a narrow cone of pointed blue-green needles. It grows on relatively dry soils, while black spruce, an even shorter tree, grows in swamps and bogs within the same forest. Spruce forests grew in a band hundreds of miles wide from the Rocky Mountains to the East Coast. They hugged the southern edge of the tundra and cold steppes at the foot of the glaciers. Occasionally fingers and patches of spruce also protruded into the tundra and worked their way up to the edge of the ice.

Finding out where forests grew and how they looked thousands of years ago requires detective work. Most evidence comes from cores taken from deep within sediments that build up at the bottom of very old ponds. Sand and silt settles in layers as rainwater washes soil into the ponds each year. Pollen from trees and other plants blows into the ponds and mixes with the sediments as they fall to the bottom. Sometimes cones and needles, charcoal, and even insect parts stay preserved in these sediments. These pieces of plants, grains of pollen, and insect parts gradually become fossilized. This creates a nearly permanent record of the surrounding vegetation.

Fossil pollen is especially useful in determining the number of different kinds of trees, shrubs, and other plants that grew around a pond when each layer formed. The core samples that scientists take from the bottom of these ponds capture this record in long cylinders of mud. Then scientists date each layer using radiocarbon methods and identify and count the fossilized pollen grains. Thus the ooze at the bottom of a pond can provide the key to unlocking the ancient history of a forest.

Scientists reconstructed the vegetation that grew on the Great Plains and elsewhere during the Ice Age using fossil pollen and macrofossils, or fossilized plant parts such as seeds, needles, cones, wood, and twigs. Much of this evidence comes from cores, but plant parts also accumulate in pack rat dens, glued together with their droppings into a mass called a midden. These middens provide valuable evidence of past vegetation because they last for thousands of years. Additional evidence comes from logs and land snails that became buried beneath the thick layers of soil that glacial winds spread across the land. Scientists know from this kind of evidence that the enormous expanse of grasslands so familiar to us did not exist on the Great Plains during the Ice Age. White spruce forests covered the plains, although grasses still grew in openings among the trees.

A white spruce forest blanketed the Great Plains 18,000 years ago. It extended from the base of the Rocky Mountains through eastern Kansas and up on to the Ozark Plateaus. Colorado blue spruce and stubby limber pine seeking refuge in the lowlands from Rocky Mountain glaciers also spread over the plains to add diversity to parts of the spruce forest. The white spruce forest also extended east of the Mississippi River and into the Southwest, mixing with an unknown species of pine in the Texas panhandle and eastern New Mexico. Scientists found enough herb pollen to show that the forest was relatively open because these plants cannot grow well in shade. Thus the Ice Age white spruce forest might have looked similar to the open spruce forests or parklands that occupy south-central Alaska today.

The Ice Age white spruce forest differed somewhat in species from east to west and from north to south, but overall it consisted of groups of short spindly trees scattered over prairies and swamps. The red-brown bark of juniper added streaks of color here and there among the dark green trees. Interspersed with the groups of spruce were groves of aspen and birch that brightened the forest with their white trunks and light green leaves, which turned vivid yellow in the fall. Clumps of eastern larch or tamarack also flashed golden yellow in the fall before their needles dropped, like bursts of light in the shadowy swamps and bogs. Sagebrush with its silvery leaves and bright pink patches of fireweed dotted the clearings between trees providing accents and highlights to an already picturesque landscape.

The immense tracts of open spruce forest were not just lifeless art hanging in our imagination. They were as real as today's forests, full of smells, sounds, and motion. Bitter winds poured from the glaciers, roared through openings between the clumps of little spruce trees, and whipped dry grasses and sedges back and forth into swirls. Trees bent to absorb the impact as cold winds shrieked through their branches carrying along bits of bark and needles, and dusting the ground with cottony willow seeds in late spring. Shaking limbs filled the air with clicking and scratching as they dragged over one another. Trees fell here and there, hitting the dry musty smelling ground with muffled crashes because their shallow roots could no longer resist the onslaught. During lulls, other sounds would fill the air, such as the snapping of twigs, chomping and grinding, swishing branches, and the dull rhythmic thud of mammoth and mastodon feet (Fig. 2.2).

These primeval forests were rich in species beyond anything known today. The open patchy character of spruce forests and the mosaic of different species and sizes of trees and shrubs, intermixed with prairies and swamps, provided an incredible diversity of habitats. Mammoths coexisted with horses, camels, and mastodons, which looked similar but they lacked the mammoth's enormous curved tusks and dome-shaped head. Their ranges overlapped, but mammoths lived mainly in the West where grass was more plentiful than in the East. They could live together because mastodons browsed on shrubs and tree needles and mammoths ate mostly grass that grew in the clearings. Neither species could live in a closed forest. Grass cannot grow in the shade of trees, and dense shade causes lower limbs to die, leaving green foliage too high on the trees for the mastodons to reach. Smoky shrews, thirteen-lined ground squirrels, heather voles, and Ungava lemmings scurried beneath the feet of these massive beasts, only to go their separate ways when the ice melted and the forest changed. Thus the diversity of the Ice Age spruce forest made it possible for mammoth and mastodon to live with each other and with many other kinds of extinct and living animals that no longer coexist.

In the East, the band of spruce forest extended down to northern North Carolina. It also spread onto the broad continental shelf beyond where the waves of the Atlantic Ocean now pound the shore. It pushed eastern white pine and hemlock trees even farther out on the exposed continental shelf, and down into some unknown location in the Appalachian foothills. West of the Appalachian Mountains spruce grew as far south as central Missouri, Arkansas, and southeastern Oklahoma. Black ash, a small hardwood with a crooked trunk and an open crown, probably grew among spruce as scattered trees in wet areas just as it does today. Other hardwoods, such as hornbeam or ironwood and northern red oak, probably lived with spruce in some areas as well. A narrow stringer of white spruce forest also followed the cold air draining from the glaciers down the Mississippi Valley, reaching into Louisiana. A thin ring of mixed-hardwood forest, including oak, hickory, beech, and walnut, bordered the spruce forest on the warmer south-facing bluffs above the lower reaches of the Mississippi River. Hardwoods also grew along the bluffs of other rivers in the South.


Valley glaciers pouring from mountain summits in the West pushed high-elevation or subalpine trees such as Englemann spruce, subalpine fir, and lodgepole pine to the lower slopes surrounding the valleys. These subalpine species probably mingled with white spruce in the northern Rocky Mountains of Montana and Wyoming where some white spruce trees still grow. Treeline, or the highest place trees can survive, also moved more than 2000 feet lower because of the terrible cold. This compressed subalpine forests into narrow belts. Englemann spruce and subalpine fir moved down the mountain sides and squeezed lodgepole pine forests against the edge of the dry windswept valleys below. Warmer weather forests of ponderosa pine that originally grew around the valleys had to migrate out of the northern Rocky Mountains entirely. Similarly, western larch probably found refuge somewhere in the Pacific Northwest, but no one knows for certain. Thus an unusual mosaic of open subalpine forests intermixed with tundra and cold steppes wound through the western mountains and valleys below the glaciers.

In the Pacific Northwest, the mild and moist climate that seems normal today was far different when glaciers were at their maximum during the late Ice Age. The shoreline moved westward as ocean levels fell and Puget Sound became a dry basin filled with glacial ice at its upper end. Thus the climate in the Puget Trough became continental rather than maritime. Ocean temperatures also dropped, and the tall glaciers split the jet stream winds causing one branch to follow the distant coastline toward the south. A southern shift in the jet stream also robbed the region of its moisture. As a result, average temperatures fell 13 degrees F. in the Pacific Northwest and rainfall only reached 50% of current levels.

The dry cold air pushed temperate or mild climate trees such as Douglas-fir and red alder out of the Coast Range and into small protected areas at the southern end of the Puget Trough. The area of Douglas-fir forests even shrank in northern California. However, Douglas-fir also probably took refuge on the exposed coastal shelf to as far south as southern California. Even today, 17,000-year-old Douglas-fir logs are still eroding out of gullies on the Channel Islands. Thus the vast forests of huge Douglas-fir trees seen by early European explorers did not exist in the Pacific Northwest during the Ice Age.

The cool moist climate along the Oregon coast sheltered Sitka spruce and western hemlock trees in some of the same places they still grow. However, these temperate trees only survived in protected areas and possibly on the now submerged coastal shelf near the sea. Subalpine forests that normally live near the upper limit of tree growth on mountains pushed them aside as they occupied the lowlands. As a result, mountain hemlock, lodgepole pine, and Englemann spruce formed a parkland that covered most of the Washington and Oregon coast. However, mountain hemlock only remained within the forest during moderately cold periods when conditions were moist and foggy. It moved south during the dry cold periods that only Englemann spruce and lodgepole pine could tolerate. Tundra and steppe vegetation temporarily pushed even these cold-weather trees farther south during the coldest periods.

Jet stream winds split by the continental ice sheets carried moisture from the Pacific Ocean into the Southwest. As a result, winter rainfall increased, allowing pygmy conifer-oak woodlands to invade the deserts of West Texas and parts of the Southwest. They even moved into the Chihuahuan Desert and the Sonoran Desert where cactuses grow today. The giant saguaro cactus and the palo verde, in turn, retreated far into Mexico. Woodlands also expanded into the southern Mojave Desert in California and Nevada and northward into the lower reaches of the Great Basin. However, pi&#241on pine and evergreen oaks could not grow with juniper on the dry valley floor of the Mojave Desert or around the edge of the Grand Canyon in Arizona. Mainly sagebrush grew among junipers in these areas, which formed a juniper-sage woodland. This woodland may have extended along the eastern edge of California's Sierra Nevada to as far north as the shores of Ice Age Lake Lahontan. On the other hand, pi&#241on pine did grow among junipers on the lower slopes of the mountains near Flagstaff, Arizona. They also grew together in the more humid climate around the edge of Owens Lake on the east side of the Sierra Nevada.

Pygmy pine woodlands grew on the mountainsides directly above the juniper-sage woodlands that sat on the valley floors of the Mojave and southern Great Basin during the late Wisconsin. They consisted of limber pine and bristlecone pine, the world's longest living tree. Both conifers have thick drooping branches, with little plumes of needles, connected to a short, thick trunk that tapers toward the top. They prefer to grow on dry, rocky slopes and ridges. Englemann spruce grew on the moister slopes above them until it reached treeline, which was much lower than today. A closed Englemann spruce forest also probably grew on the Mogollon Rim in northern Arizona where ponderosa pine forests now grow. Fir, spruce, sagebrush, and limber pine coexisted above the Grand Canyon and on parts of the Colorado Plateau in Arizona, New Mexico, and Utah. Mixed-conifer forests also grew within canyon bottoms such as Chaco Canyon in northwestern New Mexico, but cold steppes dominated the surrounding mesa tops.

Today ponderosa pine forests dominate many parts of the Colorado Plateau where fir, spruce, sagebrush, and limber pine grew during the Ice Age. Ponderosa pine is a majestic tree with a tall barrel-shaped trunk covered by plates of yellow-brown bark. It now grows on lower mountain slopes in all states west of the Great Plains. During maximum glaciation, however, the cold climate forced ponderosa pine out of the Rocky Mountains. Some may have survived in a few favorable and yet unknown areas in the southern part of the mountains. However, most of the ponderosa pine trees retreated south to the Santa Catalina Mountains in southern Arizona, the San Andres Mountains of southern New Mexico, and places farther south.

California's weather became cooler, drier, and more continental during the late Wisconsin. Santa Ana winds also increased, blowing great clouds of sand from the Mojave Desert westward over southern California and piling it up in dunes near the coast. Even so, California's Mediterranean climate buffered forests from the extreme cold that lay over most of North America during the Ice Age. Thus, forests of giant redwoods probably grew along the northern California coast at this time just as they had done for at least the past 5 million years. However, the coast was farther west during the Ice Age than it is today. In addition, Douglas fir and other trees that now grow among the redwoods most likely grew with them back then as well. More frequent coastal fogs may also have increased moisture in southern California. Here forests of pine and Douglas-fir extended onto the exposed coastal shelf and over southern California's Channel Islands. Today only a few small relicts of the Ice Age Douglas-fir forests still exist in southern California's coastal mountains. Likewise, ponderosa pine retreated into a few isolated areas high in these mountains.

The mild, but still cold and dry, Ice Age climate in California filled the upper valleys of the Sierra Nevada with glaciers. So forests shifted as much as 2000 feet down the mountains to escape the cold. As a result, trees such as ponderosa pine, juniper, sugar pine, incense-cedar, and fir moved to the lower slopes to form diverse forests that also included subalpine species. These forests also filled canyon bottoms in the Sierra Nevada, and giant sequoia found refuge in shady ravines around the canyons. Sagebrush-grass communities mixed with pine and juniper occupied mid-elevation slopes on the west side of the southern Sierras. A mixed conifer-oak forest also fanned out on the foothills farther down the Sierras and over much of the California Coast Range. Fingers of juniper-sage woodland from the Great Basin extended beyond the foothills along streams and rivers and into the San Joaquin valley. These migrating forests and woodlands probably pushed grasslands and brushlands out of the foothills and valleys and onto the exposed coastal shelf. A pine-cedar forest that consisted of a mosaic of open and closed patches of pine, cedar, juniper, fir, and sagebrush also spread over inland areas in northwest California during the coldest periods.


Jack and red pine grew at the southern edge of the white spruce forest. Jack pine is a small-diameter tree that looks scraggly because of a shaggy crown and a trunk that usually bristles with dead branches. Red pine is tall and thick with a clear trunk covered by flaky orange-red bark topped by an oval crown of dark-green needles. Red pine looks majestic by comparison to jack pine, but jack pine dominated the forest. Fingers of spruce also penetrated the jack pine forest in a few places, reaching down to the Gulf of Mexico.

Only small numbers of temperate hardwoods such as ash and maple grew in favorable spots throughout the jack pine forest. They probably settled on warmer south-facing slopes because the climate then was like Maine or Minnesota is today. The open and patchy character of the forest allowed grasses, sedges, sagebrush, daisies, and many other plants to grow among the pines. Scattered prairies also broke up the forest and increased its diversity. This Ice Age jack pine forest formed a short third band of vegetation below the glaciers. It extended eastward onto the now submerged continental shelf of the Atlantic Ocean. It reached its southern limit in central Georgia, but it only grew westward into Tennessee, where it mixed with spruce. The jack pine forest stopped at the edge of a mixed-hardwood forest toward the west, near Memphis. Here a pocket of oak, hornbeam or ironwood, ash, hickory, maple, beech, and chestnut found refuge in the warmth of the lower Mississippi Valley.

An open southern pine-oak forest grew in a band on the lower edge of the jack pine forest. The widely spaced and tall pine trees stood like columns supporting a thin green canopy over a carpet of grass. A scattering of oak and hickory also grew among the pines. This final strip of forest below the glaciers extended downward into north-central Florida. The pine forest in Florida probably consisted of sand pine, which is a short two-needle species of southern pine that looks like jack pine. The southern pine-oak forest did not penetrate farther south in Florida because conditions were too dry to support trees. However, thin lines of small trees and shrubs, or scrub, probably snaked through the parched landscape along streams. Even so, active sand dunes and prairies covered most of southern Florida during the coldest part of the Ice Age.

The southern pine-oak forest continued toward the west into Texas. In Texas, however, a semidesert pygmy conifer-oak woodland composed of pi&#214on pine, juniper, and shrubby evergreen oaks replaced the southern pine-oak forest. This pygmy conifer-oak woodland continued through Texas and into parts of New Mexico and Arizona. In Texas, however, it grew around the edges of two small areas of grassland that later expanded to cover the Great Plains.


Until recently, no one could find evidence that prairies existed in the center of North America during the Ice Age. Many scientists thought that the Great Plains shrank and eventually disappeared in the glacial cold. Then the Ice Age prairies reappeared as if rising from the dead. Scientists found fossil pollen that had blown into a cave, and settled to the bottom of an old lake, from Ice Age prairies in Texas. These pollen grains showed that one small piece of the Great Plains grassland took refuge from the cold in the Llano Estacado, or High Plains of the Texas panhandle and eastern New Mexico. Another piece of the Great Plains grassland found safety just below the High Plains on the Edwards Plateau in central Texas. Even so, these Ice Age prairies did not look like the modern shortgrass and tall-grass prairies that we know. Both of them contained more herbs and fewer trees than today. In addition, sagebrush grew among the grasses and herbs in the Ice Age High Plains prairie, but they did not grow in the Edwards Plateau grassland.

These Ice Age prairies sat within the bands of forest like lakes of grass. The High Plains sagebrush-grassland rested within the southwestern part of the white spruce forest that covered the Great Plains, but the Edwards Plateau grassland cut through the band of pygmy conifer-oak woodland farther south. This woodland wound around the edge of the plateau within deeply eroded stream channels. A few hardwood trees such as hickory, ash, elm, walnut, alder, and sweetgum also survived the cold in these moist valleys within the pygmy conifer-oak woodland. Other areas of prairie may also have existed during the Ice Age, perhaps in the northwestern Great Plains and even in the central plains.


Small mammals, such as the arctic ground squirrel, arctic shrew, yellow-cheeked vole, lemmings, least weasel, and ermine, followed the tundra and cold steppes southward through the ice-free corridor between the glaciers. There they temporarily mixed with nonarctic species such as pocket gophers, eastern chipmunks, and sage voles that also took advantage of the new habitat. Joining them were musk ox, reindeer, and caribou. The caribou even found its way from the far north down to northern Alabama and Georgia. Only a few mammals migrated northward through the cold treeless plain within the corridor, including the giant short-faced bear, camel, badger, and black-footed ferret.

The long-horned bison that came across the Bering land bridge into Alaska from Asia also pushed its way down the passage between the ice sheets into the United States. This southern population evolved into the giant bison after the glaciers closed the passage and separated it from the ancestral long-horned bison to the north. The range of the bison became smaller after the glaciers finally melted, leaving only three isolated populations. Horns grew shorter and bodies smaller as the bison's habitat shrank. Ultimately, these isolated populations evolved into the short-horned bison of Europe, the wood bison of eastern Siberia, Alaska, and Canada, and the plains bison of the United States. Similarly, the American bighorn sheep evolved from an Asian ancestor that traveled through the passage and then found itself trapped below the glaciers.

No single plant or animal symbolizes the Ice Age better than the woolly mammoth. It evolved in Eurasia and migrated across the Bering land bridge into North America about one-half million years ago. The smaller and more primitive mastodon, a forest dweller that lived on shrubs and trees, joins the mammoth as a symbol of the Ice Age. Coarse golden-brown hair covered the animal, but it was not as shaggy as the hair of the woolly mammoth. It roamed North America and parts of South America for several million years before the woolly mammoth arrived. Woolly mammoths plodded through cold steppes and tundra in the far north and at the southern edge of the glaciers. They also flourished within the open spruce and pine forests, and the Columbian and Jefferson mammoths lived to as far south as northern Florida, southern California, and Central America.

Mammoths looked something like a modern elephant. However, instead of rough gray skin, thick brown hairs covered their entire body, even the trunk. A 6-inch woolly undercoat provided additional protection from the cold. Their eyes were like protruding saucers, and their dull white tusks stuck out 12 feet and curled sharply upward. They swung their tusks back and forth to scrape away snow so that they could eat the underlying grass much like bison use their noses for the same purpose. Their ears were small and round, and long hair draped over their dome-shaped heads like a bad toupee. Wide padded feet designed for snow or marshy ground supported the 8-ton weight of the shaggy beasts as they lumbered along in search of food.

An abundance of other extinct herbivores lived among these giants, such as shrub-ox, stag-moose with antlers that combined the shapes of modern moose and elk, 7-foot-long armadillos, and a beaver the size of a black bear. Giant ground sloths weighing several tons ranged as far north as Alaska, and Harrington's mountain goat lived with them in the juniper woodlands of the Southwest. These primeval communities also included the Western camel, horse, and long-legged and short-legged llamas. Enormous herds of saiga antelope and reindeer also immigrated across the Bering land bridge into western Alaska to take advantage of the expansion of their cold steppe habitat. However, they became locally extinct when they withdrew into Eurasia at the end of the Ice Age. A giant condorlike vulture with a 15- to 17-foot wingspan called Merriam's Teratorn, and a tortoise, also became extinct. Many carnivores also disappeared, including the dire wolf, saber-toothed cats, the American lion, and the American cheetah. The most terrifying extinct carnivore of all is the giant short-faced bear. It stood as high as a moose and it used its long legs to run swiftly after prey across the tundra and cold steppes.

Modern animals also lived among these extinct beasts. Ice Age animals common in North America today include elk (wapiti), white-tailed deer, mule deer, musk ox, caribou, wolves, cougars, black bears, and such small animals as red squirrels, the western jumping mouse, and voles. Moose and grizzly bear lived in Alaska and the Yukon, but they did not move south until the glaciers retreated.

North America's Ice Age forests started to disintegrate roughly 17,000 years ago when the climate warmed and the massive ice sheets began to melt. Occasional sharp increases and decreases in temperature did not reverse the warming trend. The glaciers reluctantly backed away, lurching forward here and there, and then retreating like a dying beast. Each time the glaciers lurched forward, trees moved back as if cautiously avoiding the beast in its last moments of life. Then the trees advanced again, reassembling into the ancient forests that European explorers found as the glaciers continued their inevitable decline into oblivion.

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