North American Forests in the Age of Man
By Whit Bronaugh
Imagine you are an alien from outer space, intelligent of course, who came to observe Earth and its less intelligent life forms around 200,000 years BP (before present). Your lifespan is in the millions of years, so you are quite patient and park yourself in a stationary orbit above the equator a few hundred miles west of the Galapagos Islands. At a glance, you can see all of North and South America, half the Pacific Ocean, much of the western Atlantic and a sliver of Antarctica. You have a partner in stationary orbit on the opposite side of the world, who soon discovers two-legged animals that make stone tools and control fire. After meticulous observations, you must report, “No sign of intelligent life,” from your side of the world.
While lots of interesting things are recorded by your antipodal partner, you are forced to repeat the same observation for the next 185,000 years. You amuse yourself by watching the forests of North America migrate and change composition in a dance with the advancing and retreating glaciers, but you curse the luck that gave you the uninhabited side of Earth to observe.
Then, around 15,000 years ago, up in the far northwestern corner of North America, small bands of humans dressed in the furs of their prey slowly make their way across Beringia and into Alaska before the rising seas drown the ice age land bridge behind them. Intelligent life forms have finally arrived on your side of the world, and your future suddenly looks a lot brighter.
North and South America are the New World not only because Columbus blundered into them, making them new for western civilization in the 1400s, but because people from eastern Siberia wandered there about 14,500 years earlier and made them new for all of humankind. A thousand or so years later, nearly one third of the habitable world in the Americas had been discovered and populated by these first Americans. Previously, the flora and fauna of North America had an evolutionary and ecological history completely apart from the influence of humans. But that was all about to change.
Not long after humans made it to North America, they developed the Clovis culture, known for its distinctive stone spear points up to eight inches long with fluted bases. These people hunted mammoths, mastodons and other large mammals — likely causing their extinction. Within a couple thousand years, North America had lost all its mammoths, mastodons, giant ground-sloths, tapirs, camels, llamas, glyptodonts, giant beavers and other large species. Gone were 19 herbivorous mammals bigger than elk, with seven of them ranging from the size of bison to elephants.
All the tree species we know today were present before the first Americans arrived, but their distribution across forest habitats was likely altered by the loss of so many large herbivores and seed dispersers. These effects are not easy to detect in the fossil record, but are strongly suggested by known ecology. Just one modern elephant, for example, can consume nearly 1,000 pounds of vegetation in a single day, topple or break sizeable trees and disperse tree seeds up to 35 miles from the parent tree. Imagine how millions of mammoths and mastodons would have affected the trees and forests of North America.
Recently, scientists have discovered that even much smaller herbivores, like elk, can greatly affect the distribution of trees. After wolves were eliminated from Yellowstone National Park in 1926, cottonwood tree regeneration dropped precipitously, as elk could then munch on saplings without risk of being hunted. By 1995, when wolves were reintroduced to the park, the park’s cottonwood trees were either seedlings or mature trees more than 70 years old thanks to the elk population. Since then, the wolves have reduced the elk population and forced them to avoid open riparian areas, where cottonwood saplings have now made a dramatic comeback. Who knows how, and in what ways, now-extinct ice-age mammals influenced forests and other habitats across North America.
After the big mammals became extinct, early Americans did not just melt into ecological insignificance. Nor did they, despite popular belief, live in perfect balance with nature. If you think all North American forests were natural, old-growth icons at the time of Columbus, you are in the vast but mistaken majority. By that time, American Indians had already been modifying forest habitats for millennia.
Paleoindians used fire to open forests for more grass, herbs, shrubs and tree saplings favored by elk, deer, bison and other animals they hunted. Burning along trails made travel easier and eliminated hiding places for predators and human enemies. As long as these people were strict hunter-gatherers, the economy of that lifestyle forced most groups — and their ecological impacts — to remain small and dispersed. When they began to practice agriculture around 4,500 BP, things changed.
Farming allowed human population densities to rise and cluster into villages and chiefdoms. By about 1,000 BP, significant areas of forest were converted to intensive agriculture use for maize, squash and beans. Based on archaeological excavations of their village as described by ecologists Paul and Hazel Delcourt, the Cahokia people in Illinois (800-700 BP) cut one million trees to house 25,000 people. They also surrounded the village with a two-mile-long stockade composed of 15,000 oak and hickory logs 21 feet tall. Add all the trees they cut for fuel, and it wasn’t long before the Cahokia had leveled the forest within nine miles of their village.
Forests were also changed or eliminated by many other activities. Fires were still set to augment hunting opportunities, but the woods were also burned to make way for specific tree species. Oaks and hickories, which produce abundant, nutritious fruits in certain years, were spared or actively propagated. Northern beech and maple forests were converted into oak and pine woods interspersed with fields. The result of these management activities was a mosaic of agriculture, grassland, savanna, woodlands and forests, rather than an unbroken expanse of natural forest.
If this sounds at odds with the accounts of early explorers and settlers of pristine forests, there is a tragic reason: American Indians lacked immunity to European diseases, like smallpox and influenza, which wiped out 25 to 50 percent — sometimes 90 percent — of tribal populations. These diseases spread so quickly that, by the time a majority of European explorers arrived in an area, Indian populations had long been decimated. This allowed trees to invade the old fields, savannas and other open areas the Indians had long maintained, giving the advancing colonists a false impression of a continent-wide, untrammeled wilderness.
After Columbus, Europeans and their descendants also modified forests for their benefit, but with major differences. Paramount was the near-universal perspective that forests were either a threat that hid enemies, an obstacle to settlement, a resource to be converted to profit or all the above. In 1807, Irish author Isaac Weld wrote that Americans had an “unconquerable aversion to trees.” This attitude was backed up by steel axes, saws and plows that greatly increased the efficiency of converting forests to pastureland and crop fields. Selling fuel wood, bark for tanning and other wood products helped cover the cost of clearing the forest or provided extra income for established farmers. Initially, the pace of clearance was relatively slow, but the impacts accelerated with the ever-expanding population, which, by the 1800s, was doubling every 20 to 30 years. By 1850, the lowland forests of the Atlantic seaboard, New England and much of the Midwest had largely been cleared.
In mid-1800s, the Great Lakes, Appalachia, Southeast and West were still relatively unscathed. Then, perhaps the most sudden and drastic change to American forests ensued with the Industrial Revolution. Railroads and steam-generated power helped make lumber a large-scale industrial commodity. This vastly increased the demand for lumber far beyond local needs. For the first time in history, large-scale deforestation took place for no local reason, and mountain slopes were no longer a safe haven for forests.
Timber production soared from one billion board feet in 1840 to 46 billion board feet in 1904, enough to fill more than 10,000,000 modern logging trucks. By 1880, lumber had overtaken agriculture as the most important driver of deforestation. By 1920, more than two-thirds of American forests had been leveled at least once, including the vast majority of eastern forests. Timber companies simply harvested the forest and moved on, from the Great Lakes to the South and across the West, leaving behind stumps, fire prone slash and dead or dying lumber towns. Finally, they were stopped by the Pacific Ocean and forced to begin replanting practices.
Although deforestation continued apace, the overall decline in forest cover finally plateaued around 1920, as trees reclaimed a portion of abandoned farms and clearcuts. Now, in the 48 contiguous states, logging mainly takes place on lands that have been previously harvested, and agricultural land use has generally stabilized. Although a relatively small fraction of forest is currently being lost to urbanization, the amount is increasing, and none of it will be returned to forest anytime soon. Today, the total amount of forested lands is about 70 percent of the original cover, but that’s just the quantitative story.
NOT ALL FORESTS ARE CREATED EQUAL
Human impacts, from colonial times to the present, have drastically changed not just the size, but the nature of American forests, whether you consider the baseline for what is natural to be 1492 CE or 15,000 BP.
The trees in mature forests are adapted to soil characteristics, light intensities and moisture levels created by the forest’s species themselves. Remove these species, and all those factors change. The resulting forest is now composed of pioneer species — those first to grow in a tree-less location, like aspen, birch and alder. The old-growth forest species must wait until the pioneer species recreate their required soil, light and moisture conditions to reemerge. Similar changes in forest composition are created by natural events such as fires and wind storms, and the mature forest regenerates naturally. The difference is that most managed forests today are harvested so frequently that they never reach the optimal conditions for the species that prefer mature conditions. Instead of a complex, old-growth structure of multi-layered canopies with a spectrum of young to ancient trees and tree fall gaps, decaying down wood, standing dead trees and high species diversity, forests today have relatively young, dense, even-aged and even-canopied stands of fewer species.
Simply replanting trees does not always mean the forest has returned. In places where timber companies have replanted with native trees — whether in rows on a plantation or less orderly in wilder areas — the new forest is a monoculture of commercial species that lacks most of the biodiversity associated with the original forest. Smaller patches of forest, or forest fragmentation, has also reduced forest biodiversity because the smaller fragments cannot support wide-ranging wildlife species. In addition, the small, isolated populations of other species, including some trees, are more susceptible to local extinction.
Other post-Columbian human activities have caused changes rarely or never before seen in our forests. Commercial interests and overseas trade introduced many pathogens and pests that have significantly reduced the health and abundance of various trees. The chestnut blight fungus — accidentally imported from Europe around 1900 — took only 40 years to reduce mighty groves of American chestnuts, which had comprised up to 24 percent of many eastern forests, to scattered saplings that never mature. Other species affected by non-native disease or insects include the eastern white pine (pine blister rust), American elm (Dutch elm disease), oaks (sudden oak death syndrome), ash (emerald ash borer) and hemlocks (woolly adelgid). The introduction of foreign trees also displaced native trees, as imported trees became invasive, like the tamarisk and Russian olive in the West and the cajeput tree in the Everglades.
Sometimes even the best of intentions have resulted in wholescale transformations of our forests. In the Smoky Bear era, fire suppression allowed the buildup of dead woody debris. Now, instead of frequent small fires that rarely reach the crowns of trees, we often have large conflagrations that destroy hundreds of thousands of acres.
A CHANGING FUTURE
The forests of North America have endured many changes wrought by glaciers, climate fluctuations, sea levels, the rise and fall of mountains and the wholesale movement of the continent. In response, some tree species and forest types disappeared, while others flourished in their wake or simply bided their time in a protected refuge. Usually, the changes were slow enough that most species and habitats survived by migrating.
In recent decades, however, human impacts on climate change have eclipsed the natural variation caused by the slow changes in Earth’s orbit, solar output, ocean currents and natural emissions of greenhouse gases from volcanoes and other sources. In the last two decades, the death rate of trees in western old-growth forests has doubled. The northward and higher-elevation migrations of many tree species has already begun, as the trees in the hottest and driest parts of their geographic ranges are increasingly susceptible to fire, disease and pests. Climate has always pushed trees around the continent, but now, many of their escape routes have been cut off by roads, cities and farms.
As the old comic strip character Pogo famously proclaimed, “We have met the enemy and he is us.” It doesn’t matter if the “us” is a spear-wielding Clovis hunter, a Cahokia Indian farmer, an 18th-century settler, a modern logger, a low-MPG driver, an office worker who only prints on one side of the paper or even the most dedicated conservationist. We all use and impact forests to some degree. Their future and ours will depend largely on how well we understand how different tree species and forest types across North America change in response to the forces of both nature and humanity. We have learned to value trees not just for their products as wood, paper and fuel, but for their aesthetic and ecosystem-service values as living and life-giving forests. Now, unlike most of our ancestors, we know that there are no more frontiers. There is only a future with the forests we have and the consequences of the decisions we make.
Whit Bronaugh writes from Eugene, Oregon.