Why EarthWorks Is Removing Trees
by Joel Gerwein

"You're asking me to cut down a tree to help the environment?! No way!" These were the words of a teenager volunteering with EarthWorks in McLaughlin Orchard last summer. EarthWorkers were removing non-native, weedy trees such as Ailanthus (Tree of Heaven) from the park's slopes and replacing them with native red oaks and white pines. Why were we interfering with plants that had grown in the park naturally? Why do we think that oaks and pines are better than Ailanthus? And why will we be doing similar projects at a dozen urban wilds in Boston during the next fifteen months?

The Ailanthus trees EarthWorks removed from McLaughlin are a non-native, invasive species. This term means just what you might think it does- Ailanthus trees did not grow in North America until they were brought here by people. Since their arrival, they have spread rapidly, invading natural habitats and displacing the native trees that grew here before them. In North America, Ailanthus trees are free of many of the constraints they face in their native China. The insects that specialize in eating Ailanthus leaves and the fungi that infect the tree are largely absent from this continent. Our native animals did not evolve to eat Ailanthus seeds, and our native plants did not evolve to tolerate the toxins Ailanthus trees release into the soil to discourage competition.

Ailanthus trees may sound like they are the natural winners in the evolutionary contest. However, there are ecological as well as economic reasons that we should intervene to control the spread of Ailanthus and 400 other invasive plant species in North America. While species have moved across oceans and mountain ranges throughout evolutionary history, the rate of introductions never approached that reached in the last 200 years. Before human commerce, plants could only cross the ocean when their seeds stuck on the feet of the occasional misdirected bird or floating raft of seaweed. Now they are transported vast distances everyday, either intentionally for human use, or accidentally with shipments of food, timber, or other products. This flood of species can disrupt the web of interactions that keep our environment in a dynamic equilibrium.

Not all non-native species are problems. About 4,000 of the 22,000 plant species in North America have been introduced, but only about 400 of those are serious pests. These problem species, however, have had serious environmental and economic impacts. Their principal ecological impact is to reduce biodiversity by crowding out native plants and the animals that depend on native plants. Plants like Ailanthus, Glossy Buckthorn, Japanese Knotweed, Multiflora Rose and Garlic Mustard can all form dense stands in which native species are unable to get a foothold. These stands often spread vigorously by shoots and runners, so that a few knotweed plants can eventually fill a large area, as they have done in the Franklin Park Wilderness. Invasive species are weedy species, producing many seeds to establish new populations quickly.

People assist invasives in their spread. In addition to the help we have already given by carrying these species to their new homes, we often further help them by disturbing the habitats they invade through construction, off-trail damage from Off-Road-Vehicles and mountain bikes, and through dumping fill and yard waste. All of these activities disrupt native plant populations and provide opportunities for problem plants to invade. Worst of all, people plant invasive species as ornamentals in their gardens, providing additional seed sources and further enlarging their range. Purple loosestrife, a deceptively beautiful plant that is choking our country's already stressed wetlands and riverbanks, is a popular landscaping plant.

In addition to directly displacing other plants, invasives displace the animals that depend on native plants. For example, the West Virginia White Butterfly is on the decline in Massachusetts, partly because of the invasive Garlic Mustard. The West Virginia White prefers to lay its eggs on toothwort plants, but these have been largely displaced by Garlic Mustard. To add insult to injury, when the butterfly lays its eggs on Garlic Mustard leaves, the eggs fail to develop at all. The West Virginia White is not alone in its suffering at the leaves of invasive plants. Invasive species are regarded as the second biggest threat to biodiversity in the United States. Only habitat destruction is more of a danger. 40% of endangered species in the U.S. face primary threats from competition with or predation by non-native invasive plants and animals.

Invasive species cause significant economic damage, to the tune of $137 billion per year in agricultural, forestry and public health losses. Whether the losses are due to Ailanthus trees spreading across farm pastures, Eurasian watermilfoil choking waterways, or the Hemlock Wooly Adelgid beetle eating all of our Canada Hemlock trees, they are clearly a cause for concern. Conservation agencies have responded with different control strategies, ranging from hand-pulling for small infestations to herbicide sprays to biological controls. All of these strategies have their problems. Herbicides can damage native plants and pollute our soil and water. Biological controls, such as the weevils introduced to control purple loosestrife, involve introducing another non-native species. While scientists now test non-native biological controls to see if they will pose a threat to native plants, new introductions can always bring surprises. Once these genies are out of their bottles, they are virtually impossible to get back inside. EarthWorks will attempt to control stands of Ailanthus, Glossy Buckthorn, Garlic Mustard and Japanese Knotweed primarily through repeated hand-pulling. Eventually this will exhaust the plants' root systems.

What can you do about invasive species? One great way to help is to lend us a hand on an EarthWorks workday or at a workday at a park in your neighborhood. Another key step is to make sure that your garden is not harboring invasive species, such as Japanese knotweed or Purple loosestrife. Educate your friends about the problem, too, so they will plant native species in their garden. Learn more about the problem from the resources listed below. Next time you hear that EarthWorks is chopping down trees, you will understand that it is an act of restoration, not of destruction.

Resources for more information:
Precious Heritage: The Status of Biodiversity in the United States. Edited by B.A. Stein, L.S. Kutner and J.S. Adams. 2000. NY, NY: Oxford Univ. Press. This book gives an excellent overview of biodiversity in the US and the threats facing it, including invasive species.
Websites:
www.nps.gov/plants/alien/index.htm
This National Park Service site gives excellent information on recognizing and controlling invasives.
www.cyberonic.com/~gwlt/invasive.html
The Greater Worcester Land Trust's website includes a list of invasive species that are problems in our area, and suggestions for action.
Nursery and living museum:
www.newfs.org/garden.htm
The New England Wildflower Society's Garden in the Woods in Framingham features classes, plantings, and a nursery where you can buy native plants and seeds for your own garden.