By Whit Bronaugh
From out of the north, it came. Inanimate, inexorable and indifferent: Nothing could withstand its cancerous growth. With the patience of inevitability, it slowly consumed North America. It chewed up the terrain, pulverized granite and left nothing alive in its wake. When it had grown to maximum size, some seven million square miles, it broke the back of the continent. Nothing can withstand…The Ice Age!
No, this is not a preview for another post-apocalyptic Hollywood survival movie. This catastrophe is a major part of Earth’s recent history and is best known for cavemen, mammoths and, well, ice. Lots of ice. It is less well-known as the engine of change in North American forests for the last two and a half million years: Forests were consumed, driven south, rearranged and reordered. As unfortunate as this sounds, without the ice age, the forests of today would not be as they are.
If you draw a line from Washington, D.C., to Washington state on one of those multi-colored vegetation or ecoregion maps, you will see that it crosses a dozen or so different kinds of forest from the broadleaf deciduous forests of the East to the temperate rainforests of the Pacific Northwest. Unfortunately, reality does not exactly match the map. More than 99 percent of old-growth, eastern forests have been cleared for agricultural and urban environments or have been replaced by second-growth forests devoid of the once-dominant American chestnuts. The oak savannas of the Midwest have been reduced to tiny remnants crowded by corn fields. Diverse, ancient forests of the Pacific Northwest have been clear-cut and replaced with a monoculture of skinny, young trees in a checkerboard pattern right up to national park boundaries.
Each of these views is but a snapshot in time, and none is sufficient for a proper understanding of our forests. Our present time is marked by rapid and profound change. But change from what? How can we know the degree to which our activities are changing the long-term health, composition and extent of North American forests if we don’t understand what they once were and how they became what they are today? We can’t answer these questions with snapshots. What we need is, well, something more like a Hollywood movie. In this first of a two-part series, we’ll see that the natural baseline is a moving target.
Trees on the Move
Woody forests have been on Earth since 385 million years ago, when the palm-like, sporeproducing Wattieza dominated the world. The more familiar, seed-bearing conifers came on the scene around 300 million years ago, and the flowering or broadleaf trees arrived 140 million years ago. Since then, forests have dominated the world since they did not have to compete with grasslands, deserts or shrub lands until about 15 million years ago and tundra ecosystems about seven million years ago. But forests have always shifted their borders and their composition in concert with the prevailing climate.
The land that a given tree species can inhabit fluctuates as the climate gets drier or wetter, warmer or colder, more or less seasonal, or more or less stable over the long term. In turn, climate depends on the position of the continents, their effect on ocean currents and the amount of carbon dioxide and other greenhouse gases in the atmosphere.
Around 50 million years ago in the Eocene Epoch, back when horses were the size of foxes, the levels of carbon dioxide and methane in the atmosphere rose dramatically. Released by volcanoes and ocean rifts as the continents pulled apart, these greenhouse gases heated an already warm world to one of its all-time highs. Except for some mountaintops, there was no ice on the planet. In some places, tropical forests pushed north of the Arctic Circle. This extreme global warming forced many deciduous trees, like maples and birches, so far north that they nearly ran out of habitable land and barely escaped extinction. Meanwhile, warm-loving deciduous trees like magnolias, basswoods, elms and sweetgums were able to expand their ranges.
But on a dynamic Earth, both good times and bad times never last. As the Eocene greenhouse gases were gradually sequestered by forests and geologic activities, the world slowly cooled. At the end of the Eocene, 34 million years ago, South America and Australia separated from Antarctica enough to create the Antarctic Circumpolar Current, which prevented the mixing of Antarctic and tropical waters. This spawned the Antarctic Ice Cap, causing world temperatures to plunge and allowing cool, temperate coniferous and deciduous trees to make a comeback.
It’s All About Glaciers
Around three million years ago, South America ended its long isolation and joined North America at Panama. This blocked ocean currents between the Atlantic and Pacific and, with the Arctic Ocean nearly walled off by North America and Eurasia, helped usher in the Pleistocene ice age which has been going on for the last two and a half million years. At the moment, we are enjoying a warm period between glaciers — just the latest of many periodic lulls. The Pleistocene ice age is really a succession of 17 or more glacial periods separated by warmer times, called interglacial periods, when the ice retreats. These episodes of glaciation are thought to be caused by cyclical changes in the Earth’s orbit, tilt and orientation, collectively known as Milankovich cycles.
At the peak of glacial periods, ice can cover 75 percent of North America, nearly two miles thick in places and weighing so much that it can depress the continent by as much as 1,000 feet. On the front lines, trees withdraw behind the advancing strip of tundra or are buried and pulverized beneath surging lobes of the glacier. Of course, the individual trees don’t move. They die on the front line, but the species’ geographic range shifts southward. Because North America extends to the lower latitudes, there has always been room for most tree species to retreat. Throughout the Pleistocene, the forests advanced and retreated with the glaciers.
About 18,000 years ago, when the ice was last at its peak, it extended to the Ohio River in the east and just south of Canada in the west. Most of the ice front was bordered by a strip of tundra a few to 100 miles wide. South of the tundra, a broad band of boreal forest, dominated by white spruce and jack pine, stretched from Pennsylvania to Tennessee and westward to cover nearly all the Great Plains. The Gulf Coastal Plain, which was up to 200 miles wider because of lowered sea levels, was mostly oak-hickory and southern pine forest. Florida, then twice its current width, was mostly sand dunes and shrub lands.
Our knowledge of when and where different trees once grew comes mostly from taking core samples of bogs, lakes and other wetlands, and identifying fossil pollen grains at different levels. Such sites only cover a tiny percentage of the continent, and it took some trial and error to find the right ones. Radiocarbon dating provides the timeline, and fortunately, pollen can usually be identified to specific species, so we get a good picture of the forests that once covered our continent.
The Rocky Mountain forests of pine, fir and spruce were pushed 2,000 feet downslope and squeezed against the edge of the dry valleys. The lower-elevation ponderosa pines were practically eliminated from the Rockies altogether, and sought refuge in the mountains of the Southwest and Mexico. The Coast Range in the Pacific Northwest was subject to colder, dryer weather that banished the range’s now iconic Douglas-fir and red alder to the Puget lowlands. The southwestern deserts escaped to Mexico and were replaced by juniper, pinyon pine and oak forests.
Today, the vast Great Plains are dominated by grasslands and the eastern half of the U.S. is mostly mixed deciduous forest. But the climate at glacial maximum reduced the prairies and deciduous forests to such small refuges that scientists have only recently found them. They discovered that the grasslands shrank back to the panhandle and Edwards Plateau of Texas, and that remnants of mixed deciduous forests, now so common, were found to have survived only in small pockets along the bluffs of rivers emptying into the Gulf of Mexico.
While glaciers and climate were the driving force behind these changes, our picture of these forests is incomplete without the large mammals that evolved with them for millions of years and helped to shape them in many ways. Mastodons, giant ground-sloths and other ice-age mammals browsed heavily on tree foliage, nuts, fruits, bark and buds. Like African elephants today, they broke large limbs or even pushed over trees to get what they could not reach. They kept the canopy of many forests open, allowing grasses and herbs to intermingle with the trees. For some tree species — such as coffeetree, honeylocust and Osage orange — these animals were the primary means of dispersing seeds. And who knows what effects the horses, camels, mammoths, giant beavers, peccaries, stag-moose and woodland muskox had on forest ecology.
Obviously, the distribution of North American forests is quite different today; the ranges of different species are constantly changing, even if too slowly for us to see. When the ice made its last retreat, trees generally chased it north and climbed lost mountain heights — but they did not move in concert with each other. It was every tree species for itself. Over the proper time frame, the species goes where its preferred climate goes.
Take, for example, today’s forest on the south side of Lake Superior. Ten thousand years ago, as the climate warmed, the spruces came first, quickly followed by aspen, birch and tamarack because they are all pioneer species able to colonize open ground. Hardwoods like American elm, black ash, oaks and maples had to wait for the conditions created by the pioneer trees. Several thousand years later, eastern white pine arrived, having traveled north from the exposed continental shelf near Virginia, over the Appalachians and around the Great Lakes. American beech arrived still later, around 6,000 years ago, having followed a circuitous route from southern refuges around the east side of the Appalachians. It was only able to spread across the Great Lakes when water levels were low enough for blue jays and passenger pigeons to island hop and plant its seeds.
Meanwhile, white spruce forests evacuated the Great Plains, leaving a few outposts in Montana and the Black Hills of South Dakota. Throughout the West, trees climbed the mountains at different rates, creating strange mixtures before settling out in the familiar elevation bands we see today. Ponderosa pine, banished to the southwest corner of the continent for so long, came out of exile to dominate the lower slopes of western mountains, but the conquest took 11,000 years to advance from southern New Mexico to Wyoming.
Depending on the species, trees can live for hundreds, even thousands of years, but outside of the fantasy of Tolkien’s Middle Earth, they are firmly rooted in the ground. As species, they only migrate via dispersal of their seeds, but it’s a slow process. All this puts them at risk if climate change is relatively rapid or if they are forced into barriers they cannot surmount or habitats they cannot endure. Tuliptrees and hickories, for instance, once spanned the temperate zones of North America and Eurasia, but became extinct in Europe when the first glacial period forced them south against the uninhabitable Alps, Pyrenees and Mediterranean Basin. Fortunately for North American trees, all our major mountain ranges run more or less north-south and allow easier passage. If the Appalachians had run along a latitudinal line or the Gulf Coast had been a few hundred miles further north, many of our eastern deciduous trees would now be extinct.
The North American trees of today have endured many glacial advances and retreats and countless fires, storms, droughts and floods. They have moved across the continent like dancers with the ice and climate, but at the whim of seed-dispersing wind, water and animals. In the process, they have created and abandoned multiple alliances with other tree species.
The current interglacial period has seen the most drastic changes in our forests, but not because the climate shifts were so different from previous episodes. At the last glacial maximum, the lower sea levels once again connected Asia and North America, and this time a new character was added to the cast. About 15,000 years ago, a powerful agent of change crossed the Bering Land Bridge on two legs and left strange tracks never before seen in North America.
Tune into the next issue for the sequel: From out of Siberia it came … The Ice Age 2: Human Invasion!
Whit Bronaugh writes from Eugene, Oregon.