Invaders coming soon to a forest near you
November in a forest of invasive plants in Pennsylvania. New Hampshire’s climate is predicted to be equal to this within the next 50 years. (Courtesy)
Forests define New Hampshire’s landscape. At more than 80 percent forested, our state is the second most forest covered state in the union, just after Maine.
It hasn’t always been that way, as historians will remind us. When European settlers arrived, they found a forest that they assumed was primeval, or untouched. It was in some places, but in others it was a managed forest, groomed by Indigenous people to favor the wildlife and plants that provided sustenance for their cultures. They most often managed the forest using fire, and for centuries lit periodic fires in order to clear the understory of shrubs and plants that were less favorable for wildlife like deer, moose, and woodland caribou. The resprouting woody and herbaceous plants provided more browse for these grazing animals and thus greater populations of potential food.
The arrival of European settlers changed all that. They wanted land for agriculture. They were highly industrious and managed to clear most of the state of trees and replace them with pasture for domestic animals and fields for crops. Of course, the legacy of all that hard work was a landscape lined by thousands of miles of stone walls, created as the settlers discovered that under all those eons of fallen forest leaves the soil was covering a huge crop of boulders left behind 12,000 years earlier by the retreating glaciers. Stone walls were built to define property boundaries, but often were primarily a place to deposit stones, both large and small that were an impediment to farming.
Following the Civil War, a great many farm families decided that subsistence or commercial farming in our rocky landscape was simply too challenging, and tales of the deep and rock free soils of the Midwest lured them away from the hills of New Hampshire. Thus began the slow but inevitable return of most of the New Hampshire landscape to forests.
Trees in our climate are the primary occupiers of abandoned land. When the hill farms were left to themselves, first came the white and gray birches, oaks, and pines. As these trees matured, species that favored sprouting in the shade, the maples, beech, yellow birch, and others would become established again. These forests, what ecologists call “second growth” dominate our landscape today. Some are very old, perhaps more than 200 years, but very rare is the plot of forest that never witnessed sheep and later cows.
But these second growth forests are extraordinary places. They host an enormous diversity of wildlife, plants, and fungi. They are the principal filter of rain water that runs into streams, lakes, and the ground, and which sustains our human presence on the land and in our cities.
With some exceptions, they have contained a host of species that have been familiar to us for as long as people have lived here. The maples, oaks, pines, beech, birch, and other native species along with a host of native shrubs still thrive. Lost are the American Chestnut and American Elm that were decimated by diseases inadvertently brought here by trade from other continents. Today we are losing our ash trees to another invading insect from Asia.
Will we continue to recognize the forests of our future? Today’s forests developed in a relatively stable climate over many centuries since the last ice age. But the climate is changing faster than any time during human history and our forests cannot help but notice.
How will they respond? Species that require colder winter and cool summers (fir and spruce) will likely die off at lower elevations, while expanding their habitat north and to higher elevations. Oaks that like our central New Hampshire climate will appear more frequently farther north. Some have worried that sugar maples will decline, but it’s too early to know for sure, and scientists are working to figure that out.
But one thing is almost certain. A host of exotic invasive species from other continents that have become well established in forests to our south are rapidly moving our way.
I was just in central Pennsylvania, where the native shrubs and nonwoody plants of the forest understory are rapidly disappearing. They are being replaced by nonnative multi-flora rose, autumn olive, buckthorn, burning bush, Japanese honeysuckle, oriental bittersweet, and others that are aggressive and most often unpalatable to the native wildlife. In the forests I visited, “bush whacking” through the thicket of thorny plants has become a hazardous activity. Deer seek out suburban yards to find landscape plants that are edible, when the food in the forest is not.
These invasive plants are already here in New Hampshire in some places, but not most. That is changing rapidly as birds, who often find the seeds of these invaders a good food source, spread them far and wide.
My own woodlot in Weare is changing. The autumn olive and Japanese knotweed have been here for years, but are spreading more rapidly. Oriental bittersweet would be choking the native trees if I didn’t go out every summer and cut it back. In my maple sugar bush, where the tree seedlings were once all sugar maples, there are ever increasing numbers of shagbark hickory, a species once rare this far north.
There’s not much we can do about this invasion, except be prepared. Tending our forests will be more challenging if we want to retain our native species. Rooting out exotic invasive plants is no easy task, and no doubt we will sometimes have to resort to using chemical herbicides. Deer will likely become more of a nuisance as their favorite foods are replaced by invasives that they find inedible and they seek out our gardens and home landscapes.
If you own a woodlot or just have a wooded house lot, it’s time to educate yourself about the invaders that are coming or recently arrived. Consult the UNH Cooperative Extension for lists of exotic invasive plants. Get an app for your phone to identify them. Be diligent at pulling or killing them when you find them and you may be able to keep them at bay. Ignore them and they will take over before you know it.
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