Agricultural fields. Credit: Zen Sutherland
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.
This article concludes a two-part series on the history of North American forests. Read part one, “North American Forests in the Age of Nature,” in our Spring 2012 Issue.