Robert A. Askins—
The window at my desk looks down a snowy slope through gray tree trunks to a heavily forested ridge on the far side of the valley. This is the view in winter, when nearly all of the trees and shrubs are leafless. By mid May, after leaf buds have opened and leaves have unfurled, I’ll only be able to see a short distance into the forest understory. This dramatic seasonal change is characteristic of deciduous forests, the northern forests where most woody plants lose their leaves in the autumn and grow new leaves in the spring.
The view from this ridge would have been very different in 1780 or 1880. As in much of New England, the landscape was almost completely open then, and I would have looked across a wide expanse of pastures and orchards to plowed fields on the flat-topped ridge across the valley. Today the only signs of most of these farms are stone walls and the foundations of houses and barns deep inside the woods. As people moved to factory towns or headed west to cultivate flatter, more fertile land in the Midwest, the forest grew back. In most of New England the landscape went from more than 80% cleared, with forest restricted to small woodlots or steep slopes, to more than 70% forested. In some respects this new forest is different from the forests encountered by English colonists in the 1600s—the trees are typically younger, and a few species such as the American chestnut, gray wolf and passenger pigeon are now missing —but it still supports a remarkable diversity of forest-dwelling plants and animals. This recovery is still in progress, with beavers, fishers, turkeys, and black bears recently returning to areas where they had not been seen for more than 150 years. How could an ecosystem that had been so completely disrupted over an entire region recover so fully?
The reassembly of a complex ecosystem following widespread destruction is nothing new for the eastern deciduous forests. Along the Atlantic Coast the forest is mowed down by a strong hurricane every few decades, but it quickly grows back after each storm. When forests are logged, new trees rapidly emerge from stump sprouts and after only 10 or 15 years a new tree canopy begins to form and woodland birds return. More severe disturbances such as agricultural clearing or the flooding of a stream valley by beavers kill the root systems of trees, but when trees again have a chance to grow the forest returns within a few decades. Forest now covers abandoned farmland not only in New England, but also along the entire Appalachian Mountain range.
Over a longer time scale, the eastern deciduous forest was not just disrupted, but completely destroyed over immense areas by continental glaciers. The glaciers bulldozed the forest soil, along with seeds and root systems, leaving no vestige of the original forest. Tree species survived far to the south, in relatively warm refuges in river valleys along the Gulf Coast. The pollen record in the sediments of lakes and bogs shows that these refuges were surprisingly small, and that different refuges supported different species of deciduous forest plants. When the glaciers receded, tree species expanded their ranges northward at different rates. Deciduous forests began to replace coniferous forests in southern New England about 8,000 years ago, but the composition of the forest steadily changed as one tree species after another spread northward. Thus, the forest cleared by Native American farmers or English settlers was not an ancient and stable ecological community that had survived intact for millennia. Instead, it had slowly assembled over thousands of years, resulting in a continually shifting set of tree species. And the process of disassembly and reassembly of forest ecosystems had occurred repeatedly as massive glaciers expanded southward and then retreated 18 or 20 times during the past 1.6 million years.
Deciduous forests in eastern North America have shown remarkably resilience in both recent history and deep geological time. Will they be as resilient to the types of changes predicted by climate models that project the effects of greenhouse warming? One possibility is that tree species will expand their geographical ranges northward or, in the mountains, upward to regions where they can still thrive in climates with a cold winter and warm summer. Many bird species have already responded to warmer winters by spreading northward; some common garden birds such as red-bellied woodpecker, tufted titmouse and cardinal moved northward into New England during the past hundred years. Birds, of course, are more mobile than trees and potentially can respond more quickly to climate change. Trees spread northward as seeds over many generations. Wind-dispersed seeds may spread relatively quickly, while seeds dispersed by squirrels and other animals move northward at a slow rate. The fear is that the rate of climate change will be so rapid that many tree species will not shift their distributions fast enough to survive. Another problem is that people have created new barriers such as expressways and densely built-up metropolitan areas that will slow the rate of tree dispersal.
One obvious solution is to preserve green corridors so that both plants and animals have a path for shifting their distributions in response to climate change. Protecting continuous woodlands along rivers, ridges and mountain ranges is part of this effort. A more controversial approach called assisted migration might be especially appropriate for plants such as nut-bearing trees and ant-dispersed wildflowers that tend to shift their ranges slowly. These species could be transplanted north of the northern border of their current geographical ranges. This approach is anathema to many conservationists, however, who are concerned about the spread of invasive species that were introduced from other parts of the world. The problem species were introduced from Europe or East Asia, however, not from nearby areas in North America. The pollen record shows that plant species have been reshuffled into different sets of species repeatedly because of glacial cycles, so assisted migration mimics a natural process. By transplanting native forest species progressively northward, we could move them across artificial barriers and ensure that their distributions adjust quickly to climate change.
The remarkable resilience of deciduous forests in the recent past and over millions of years of climate cycles suggests that they should survive greenhouse warming. The local composition of forests might change markedly, however. Sugar maples and paper birches might disappear from New England as sweetgum, tulip trees and magnolias spread northward. But climate change is a more serious and immediate threat to some other ecosystems. Sea level rise could flood and destroy tidal marshes, particularly if development next to the marsh prevents the marsh from moving inland. Species that depend on mountaintop habitats such as alpine meadows and krummholz are also threatened because they may have no place to move as the climate changes; eventually lower elevation plants may spread upward to occupy the entire mountain range. Far northern ecosystems such as tundra and sea ice are at risk for similar reasons. In contrast, the deciduous forests of eastern North America should continue to be resilient to climate change, particularly if we enhance their resilience with forest corridors and careful assistance with northward migration.
Robert A. Askins is Katherine Blunt Professor of Biology, Connecticut College. He is the author of Saving the World’s Deciduous Forests.