Forest Pest and Invasive Plant Update

By Brett McLeod

This area (former pasture, hayfield and cornfield) in Brookfield, Vt., is now overgrown with wild chervil. The left side of the photo shows where mowing was done in an attempt to cut plants before flowers developed seeds.
Photos by Courtney Haynes unless otherwise noted.

Nationally, invasive species are estimated to cause nearly $120 billion in environmental damages per year. While this estimate by researchers at Cornell University includes both plant and animal species, nonnative invasive plants pose what is arguably the greatest threat to the future of forests in the Northeast.

The enormity of this problem is largely due to effective seed dispersal mechanisms and rapid colonization of native habitats by invasive plants. This conversion often results in native species being displaced by more aggressive nonnative species. Loss of biodiversity can, in turn, lead to a condition called "ecosystem simplification," where a forest lacks pathways of resiliency, thereby compromising forest health.

Most of the nonnative invasive plants in the Northeast were introduced either as ornamental species or for food and fiber purposes. However, issues of forest health are compounded by a variety of nonnative forest insects. In the Northeast, three forest pests are of particular concern: the emerald ash borer (EAB), Asian long-horned beetle (ALB) and hemlock woolly adelgid (HWA). Collectively, these three forest pests pose a significant threat to the region, since nearly half of the trees in the Northeast are host species for one of these three insects. Foresters and invasive species experts often refer to the dominance of nonnative plants and the threat of nonnative insects as a "perfect storm," capable of changing forests in unimaginable ways.

To better understand the risks landowners are likely to face, I recently sat down with Courtney Haynes, a forester and invasive species specialist with Redstart Forestry, based in Corinth, Vt.

BM: In the context of forests, why are invasive plants a problem in the Northeast?

CH: As consulting foresters, we quickly realized that nonnative species were preventing native species from regenerating in the forest. These native species occupy an important ecological niche in the northern forest, but also contribute in economic and social ways. It's easy to see, for example, how maple sugaring is both an economic and cultural activity that has thrived because of healthy forest communities.

Without a doubt, invasive species pose a significant threat not just at the stand and forest level, but also at the landscape level. Invasive plants and forest pests threaten the function and structure of a highly complex, interdependent community. For example, deer, mice and tick imbalances have developed due to Japanese knotweed. Honeysuckle and autumn olive are believed to be poor nutrition for birds, and black swallow-wort is cited as poisonous to butterflies.

Asian long-horned beetle.
Daniel Herms, The Ohio State University,

BM: So it sounds like this is just as much about human values as ecological values.

CH: Absolutely, managing invasive species is still anthropocentric. As a culture, we tend to value things that are native to this region, and because they provide either intrinsic value, monetary value or some other value. For most people, it's rooted in preserving a set of values that we've become accustomed to and have evolved with.

For me, as a forester, I continually come back to the idea of regeneration when we talk about invasive plants and forest pests. Without regeneration of native species, forests as we know them will not be the same in 50 to 100 years.

A former pasture, hayfield and cornfield in Brookfield, Vt., prior to mowing. The area is overgrown with wild chervil.

BM: What are the invasive species that you view as most threatening to the health of farm and forest communities?

CH: The threat of invasive plants really depends on what ecosystem and region you are in. The woody-stemmed invasive plants, such as common and glossy buckthorn, honeysuckle, and Japanese and common barberry, are relatively easy to manage compared to others and comprise a large portion of our work here in central Vermont. The vine species (Asiatic bittersweet, black swallow-wort) are more difficult and will likely pose a threat in the future. Multiflora rose and autumn olive have thorns; thus, treatment from the ground can be difficult.

Japanese barberry invading a forest understory in Woodstock, Vt.

In wetland environments, Japanese knotweed and phragmites are the most common threats. Almost every watershed, at least in Vermont, is affected by Japanese knotweed infestations. These traditionally wetland species are of concern because we're beginning to see them move into other ecological communities as well.

In field settings and pasture settings, we regularly encounter autumn olive and multiflora rose, which were traditionally used as living fences. In Vermont, we encounter large infestations of wild chervil, which is appearing in greater abundance, particularly in abandoned farm fields. Other states are concerned about spotted knapweed.

BM: What management options exist for these invasive plants?

CH: It's about long-term diligence. Rarely is it as simple as a single treatment - monitoring and educating landowners is just as important as the treatment tools that we use.

At Redstart Forestry, we practice integrated pest management (IPM). This strategy includes mechanical approaches (hand-pulling, cutting, repetitive mowing, grazing) as well as chemical and biological controls. In forest settings, we have found good ways to integrate mechanical and chemical options. Biological control is still in its infancy and has to be managed at a higher degree, given the risks of introducing new biotic agents into the ecosystem.

We're also looking at ways to make invasive species treatment more cost-effective for landowners. This may include integrating invasive species control either before or during a timber harvest. At the end of the day, I view my job as helping landowners to bring infestation down to manageable levels. Complete eradication is rare because of seed banks that can exist in the soil for centuries, and because of continual introduction of seed sources from other neighboring properties.

Poison parsnip invading a formerly hayed farm field in Thetford, Vt.

BM: What are the greatest challenges you face in managing invasive species?

CH: There are two real challenges to practicing invasive species management. The first is changing the way people think about invasive species. For example, the control of many forest pests is best achieved by removing the threatened species as a way to prevent the spread of a particular pest. Many people can't conceive of cutting perfectly good trees simply as a preventive measure. On the other hand, the current threat of the ALB, HWA and EAB might have been better controlled with more aggressive quarantine tactics.

The other challenge we face is that landscape-level management is really tough to implement. It involves a long process of educating landowners and then convincing them to work with their neighbors, not just for the good of their own land, but for the good of all. My long-term vision is to work toward creating invasive-free pockets that can be connected to form invasive-free landscapes.

After one problem (honeysuckle) was dealt with in Charlotte, Vt., another marched in (garlic mustard). Garlic mustard is difficult to manage, in the same vein as bittersweet and black swallow-wort.

BM: Is there funding to help control invasive plants on private land?

CH: Yes. The U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS) offers federal invasive treatment funding that is administered at a state level. The NRCS has been working hard with private landowners to help treat invasive plants in forested and agriculture settings. We have worked with other state, private and nonprofit organizations that have dedicated funds for invasive species treatment. Sometimes invasive plant treatment can be a component in a larger project, such as a riparian tree planting or the creation of early successional habitat for wildlife.

BM: Given the risks we've discussed, is there any such thing as preventive maintenance? If so, what advice can you offer?

CH: Absolutely! It will take preventative measures to help combat this problem. I believe that education is our first line of defense. Education must target a wide spectrum of the population, from convincing campers to not move firewood, to loggers washing their skidders to prevent the spread of invasive seed stock from one forest to the next. As a private landowner, learn which plants are posing a threat to your forest and fields. Even simple measures like maintaining old pastures through mowing can be an effective tool to prevent the establishment of invasive species.

A classic infestation in a forest understory in Shelburne, Vt., featuring common and glossy buckthorn, along with honeysuckle.

BM: In closing, what final bit of advice would you offer landowners?

CH: Landowners must remember that managing invasive species is a cyclical process that requires monitoring, and in many cases repeat treatments, either through mechanical, chemical or biological controls. If you do find invasive species on your property, it's always easier to address these issues sooner rather than later. You can start by contacting your local USDA office or a consulting forester.

Brett McLeod is an associate professor of forestry and natural resources at Paul Smith's College, located in the Adirondacks of northern New York.