Terrestrial invasive plants can wreak havoc with forests
By Joe Rankin
Forests for Maine’s Future writer
Licensed forester Jeff Williams does the usual things foresters do: writes management plans, runs boundary lines, oversees harvests, lays out logging roads, marks trees. But more and more these days he’s having to deal with invasive forest plants.
Williams, who owns Maine Forest Management in Hollis, said 30 to 40 percent of his time is spent helping his clients cope with the likes of glossy buckthorn and Japanese barberry, and that percentage goes up every year.
In fact, it’s very seldom that he gets a job these days that doesn’t involve invasive plant issues. Sometimes he even does herbicide applications (he has a master applicator’s license) if a client can’t find a contractor to do the work at an affordable price.
“As bad as it is now it’s inevitable that it’ll get worse,” Williams said. “As it is now there are pockets where it’s a real problem. In 50 years it’ll be a huge problem for southern Maine and forests in Maine period.”
Woodland invasive plants are also known as terrestrial or upland invasives. They haven’t gotten the publicity that exotic insect pests or aquatic invasive plants have. Non-woodsy types who have heard of the emerald ash borer or recognize Eurasian milfoil on sight might give you a blank look at a mention of glossy buckthorn, black swallowwort or Asiatic bittersweet.
That’s not unexpected, said Tom Doak, the executive director of the Small Woodland Owners Association of Maine.
Exotic insect pests prompt more attention because they kill trees outright and the damage is highly visible; the uncontrolled spread of invasive aquatic plant species threatens to imperil the state’s thousands of lakes and ponds, said Doak.
In contrast, woodland invasives are more insidious: they’re green in a landscape of green and growing things. “They don’t generally kill the trees, but they occupy the land and prevent forest trees from growing,” said Doak.
An invasive plant is generally defined as one that spreads quickly and crowds out other plants and trees. Most are exotics, immigrants from Asia, Africa or Europe. Many are sunlight lovers. They invade marshes (common reed), wetlands (purple loosestrife), grassy areas and roadsides (Japanese knotweed), and field-woodland edges (honeysuckle and autumn olive.)
Generally speaking, it’s harder for an invasive plant to make it in a healthy forest with a full canopy. But some have no problem. There are vines, trees and shrubs already invading forests in Maine and others that will likely be here soon.
So, what species would a list of Maine’s “most unwanted” woodland invasives include? The experts we talked to reeled these off:
Glossy buckthorn (Frangula alnus) and common buckthorn (Rhamnus cathartica) are fast-growing shrubs that form dense thickets in wetlands and woodlands. Their habit of leafing out before other plants and retaining their leaves late into the fall gives them an advantage and helps them shade out native species.
Japanese barberry (Berberis japonicus) is, as its name suggests, a spiny shrub originally from Asia. It grows three to six feet high. It forms dense thickets that can be impossible to bull your way through. The red berries are highly attractive to birds, which help spread it.
Asiatic bittersweet (Celastrus orbiculata) is the Boston strangler of invasives. This relative of American bittersweet grows fast and twines around trees as it reaches for sunlight, eventually smothering the host plant. It not only reproduces using attractive red berries, but also root suckers.
Norway maple (Acer platanoides) is an escapee from the nursery trade. This sugar maple look-alike, still planted as a shade and street tree, grows fast, even in shade, and forms dense colonies that can elbow aside native trees and shrubs.
Garlic mustard (Alliaria petiiolata) is a biennial herb that tolerates a wide spectrum of soils and growing conditions, and is especially aggressive in rich, moist upland forest soils, where it forms dense colonies of three-foot tall plants, shouldering aside all manner of native wildflowers and herbs.
Black swallowwort (Cynanchum louiseae) is a viney native of southwestern Europe that likes moist soils. It engulfs native plants and creates thickets.
Morrow honeysuckle (Lonicera morrowii) is a native of Asia that grows as high as 16 feet, forming dense thickets and shading out native plants. While it likes sunlit forest edges it will also invade mature forests.
You’ll note some common themes in the descriptions. These invasive plants tend to be fast growing; they form dense thickets, a sort of non-compete strategy; they adapt to a wide range of habitats and they have shiny,brightly-colored fruit, a sure-fire reproductive strategy since birds ingest the fruit then excrete the seeds far and wide along with a little fertilizer.
Some of these plants were introduced accidentally, some on purpose, either as ornamentals or for erosion control. It wasn’t until later, sometimes decades later, that the threat they posed was realized. However, even today, some, such as Norway maple and Japanese barberry, are staples of the nursery trade.
These forest invasives currently pose the greatest threat in southern Maine, say, south and west of Augusta. Not coincidentally that’s where the most people live. In fact, these weedy plants tend to be closely associated with another weedy species: humans.
“The behavior of invasive plants follows human activity very closely. Humans are an edge species. Our yards are open. When we look to the woods we see the edge. Invasive plants thrive on the disturbance we humans create. We are bombarded by invasive plants that are basically just trying to heal the wounds that we inflict on the land,” said Tom Rawinski, a botanist with the U.S. Forest Service’s Northeastern Area State and Private Forestry program. He works on invasive plant issues throughout New England and New York.
Many invasive plants get their start at the edge where grasslands (read, lawns and fields) meet the woods and sunlight is ample. For some it’s just a beachhead to invade the forest. Sometimes we help them do it, by creating disturbance. A timber harvest is just such a disturbance.
Harvesting opens the canopy, flooding the forest floor with sunlight. If invasives like barberry or buckthorn are already present it’s like throwing gasoline on a fire: they enjoy an explosion of growth.
“Many landowners come to us and ask for harvest oversight and don’t realize they have invasives,” said Williams. “We’ll recommend that they treat the invasives first. And in some cases we won’t administer the harvest if they’re unwilling to so. In other situations, if they’re young plants we’ll do mechanical treatment in the spring when the soil is soft and you can pull them. In the worst situations we do recommend herbicide control.”
Sometimes, humans create disturbances in other, more subtle, ways, by encouraging or favoring one species over another.
In southern New England burgeoning white-tailed deer populations have in some places wiped out native wildflowers and understory plants, essentially clearing the field for invaders like garlic mustard or Japanese stiltgrass, said Rawinski.
“The millions of acres of stiltgrass in the eastern U.S. is not the problem,” he said. “The deer are the problem, because they’ve eaten all the natural competitors.”
At Laudholm Reserve in Wells deer pressure resulted in an understory “where there is nothing but Japanese barberry. It’s actually impenetrable. It’s quite spectacular,” said Ann Gibbs, the state horticulturist with the Maine Department of Agriculture, Conservation and Forestry and an expert on invasive plants.
One of the things that makes invasive plants so successful is that they’re unpalatable to wildlife. Deer won’t touch barberry. Ditto for black swallowwort.
While invasive plants can crowd out native species and alter an ecosystem beyond recognition in a few years, some pose an even more pernicious threat, to the very genome of related plants.
Asiatic bittersweet, for instance, hybridizes readily with American bittersweet. In parts of Massachusetts, said Rawinski, it’s hard to find a pure American bittersweet anymore. It’s a victim of “genetic swamping” by its Asian relative and saving it could require eliminating Asiatic bittersweet “for perhaps a mile, which is almost impossible,” Rawinski said.
Invasive plants can be controlled, if detected early enough. If you suddenly wake up to discover you’ve got acres and acres of glossy buckthorn in your forest, it’s almost impossible to deal with, even with herbicides.
“Early detection and rapid response are the answers,” said Gibbs. “Once you get something established in an area it’s a major undertaking to control and very expensive. The best thing is to keep things out.”
Think of your garden: if you wait until the weeds get established, reclaiming your cabbages and cucumbers may be just too much trouble, then you swap the tiller for the lawnmower.
Rawinski remembers a sugarbush in Putney, Vermont. It had “majestic old sugar maples, but essentially the whole understory was glossy buckthorn. The task of running the lines from tree to tree or just negotiating that sugarbush would have been a nightmare. That’s a situation where, with early detection, you could have contained it.”
The Maine Forest Service, SWOAM, and conservation organizations such as Maine Audubon, Maine Coast Heritage Trust and The Nature Conservancy have been working to educate people about invasive plants by offering workshops and field tours. A couple of good places to start your own education — the National Invasive Species Information Center website and the Maine Natural Areas Program invasive plants website.
The control issue is complicated by the fact that there is such a broad spectrum of invasive species and those invasives don’t recognize the property lines we think are so important.
“It’s a landscape problem, not an individual property problem,” said Williams, the forester from Hollis. “The biggest hurdle is educating landowners, even the person who has a one-acre lot adjoining a property that’s managed, and trying to get everyone to work together.
“We’ve had some luck going to neighbors and saying, ‘are you willing to work with us and control these in a responsible manner?’ And I’ve had landowners who are willing to pay for control of invasives on an abutting landowner’s property just because they’ll reap the benefit in the long term. And sometimes landowners will share the costs.”
And when it comes to most of these species that’s what we’re talking about — control. Nobody is talking eradication. And, in fact not every plant can be controlled in every area. The scale of the problem is just too immense. On the one side you have plants that seed and sucker with abandon to spread their genes and on the other you have humans with limited financial resources to fight them. In other words, we need to pick our battles, knowing we won’t ever be able to declare “mission accomplished.”
“The greatest challenge is to make sure our limited resources and limited energy are directed appropriately and strategically,” said Rawinski. To protect a beautiful hardwood forest, perhaps, or a marsh or a wetland with endangered native plant species, a community forest, park or a wildlife refuge.
It’s not a battle for the faint of heart. As Gibbs points out, “These problems didn’t happen overnight and you can’t take care of them overnight. If you want to be successful and control an invasive plant population you have to be persistent and in it for the long haul.”
And it’s not as though the invaders we’re fighting now are going to be the only ones. There are others on our doorstep or headed our way.
Among them is mile-a-minute vine (Persicaria perfoliata) — the common name gives you an idea of how fast it grows. It has barbs on the stem, is self-pollinating and a prolific seeder. It likes edges, but will grow in woods as well, where it climbs trees to get the sun it needs. Deer won’t touch it.
Then there is Japanese stiltgrass (Microstegium vimineum), which now covers millions of acres in more than two dozen states. Stiltgrass is a prolific seeder and very tolerant of low light levels. Deer won’t eat it, either. Stiltgrass is one of those stealth invasives. It looks like other native grasses. Even experts might not pick it out.
It’s not the only one. Rawinski said Linden arrowwood (Viburnum dilitatum) an understory shrub that resembles our native arrowwood, is a common ornamental that can live in deep forest. “It’s not on anyone’s invasive plant list, but it probably should be,” he said.
He feels the same about rusty willow (Salix atrocinerea), also known as large gray willow, which he’s found in York and Cumberland counties. He calls it a “sneaky invader.” First identified on Cape Cod a century ago, it’s managed to spread widely in the northeast, choking the banks of ponds and lakes, because it so closely resembles some native willows that even botanists don’t give it a second glance.
Which brings us back to educating yourself about the trees, shrubs, wildflowers and vines in your woodlot or your community forest or the local park. Many people keep a list of birds they see on their property. Fewer have a plant list. But it’s a good first step.
“You ought to learn to recognize the most common invasive plants, and then spend some time on your property,” said Doak. “Don’t always look up at the trees, look down as well. Get a sense of whether you have any of them and then learn whether you have a problem.”
Joe Rankin writes forestry articles and keeps honeybees at his home in New Sharon.