How Invasive Pests and Plants Are a Threat to Fall Foliage
The introduction of invasive species can be devastating to the local landscape. Learn more about New England invasive species and how you can help keep them from spreading.
Note: In 2021, the name gypsy moth was replaced withLDD moth (for Lymantria dispar dispar) in most entomology circles, as well as in the press. The LDD moth has long been a dreaded sight during foliage season in New England. Photo Credit : Image by Helmut Kloubert from PixabayWhen we look at the prospect for fall foliage every year, there are many things to consider. The biggest drivers of the autumn season are temperature and daylight: As days get shorter and the weather turns cooler, trees reach the point where it takes more energy to keep their leaves than to shed them. This happens at roughly the same time every year. Other factors across the year determine the timing, intensity, predominant color palette, and duration of the autumn leaves. For instance, the weather during the preceding winter, spring, and summer is incredibly impactful, as is the general health of the forest in terms of rainfall and drought. Regional outbreaks of bugs and fungus also play a role. Even the amount of energy that trees put into producing nuts and seeds has an effect on autumn color. Beautiful fall color in New England is a result of a balance between climate, forest health and native trees. Long-range, though, the biggest risks to fall foliage in New England are invasive species and climate change. These will ultimately determine whether the autumn show in New England continues to look as glorious as it does now. And history offers a few thoughtful lessons.
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The year was 1869 and the plan was simple. Etienne Trouvelot was a French immigrant who settled in Medford, Massachusetts, with the goal of expanding the lagging U.S. silk production. He imported a number of silk-making species, most notably Lymantria dispar dispar, or the LDD moth. Inevitably, these insects escaped, and they immediately made an impact on the local forest. From there, the LLD moth slowly spread across New England, erupting in annual outbreaks. In the 20th century, DDT was used to fight back the moth’s advance, but when that chemical was rightly banned in 1972, outbreaks intensified and the moths spread across the Mid-Atlantic region as well. A fungus native to the moths’ home range was eventually introduced as a biological control, and a reasonable population balance was created. However, the run of dry spring weather over the past five years or so has taken a toll on the fungus, and New England has once again seen major LDD moth outbreaks. In northern New England, 2021 was the worst in more than 30 years, with residents in the outbreak areas reporting that the moths’ falling frass (excrement) sounded like constant rain through the forest. After a LDD moth outbreak, forests canopies can look very thin mid-summer. Photo Credit : Jim SalgeLDD moths rarely kill the trees that they defoliate, but the lack of leaves allows more light to shine on the forest floor, and the energy required to put out new leaves weakens the forest as a whole. It also has a more obvious and immediate impact on leaf-peeping.
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There are many stories in our past like that of the LDD moth, a good intention gone horribly awry. The intentional introduction of the European starling comes to mind, too, along with all the impacts that that species has had on native birds.Other invasive species have arrived by complete accident, though, and some of these have caused the most significant damage to forests. A century ago, the chestnut blight completely eliminated that keystone species, along with all the food for forest dwellers that they provided. Dutch elm disease did nearly the same: We see an Elm Street in practically every New England city but few elm trees.
These terrible past impacts come to mind as we see relatively new invasives such as the emerald ash borer, the Asian longhorn beetle, and the hemlock woolly adelgid becoming more established. They threaten to eliminate more key species from the forest and further limit native biodiversity.Invasive insects, like the hemlock wooly adelgid, threaten to again reshape New England Forests. Photo Credit : Jim SalgeAnd, though less harmful long-term, generalist invasive caterpillars such as the winter moth and the browntail moth do pose a significant risk for the annual display of colors.
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Bugs aren’t the only threats to the New England forest. Invasive plants, too, take a toll, including bittersweet, which is entangling and bringing down mature trees on forest edges at alarming rates, and black locust, which is prolific in many eastern forests and is now crowding out the maples. But a bigger risk to long-term forest health has to do with early successional species — that is, the plants that jump-start the regeneration of disturbed and logged forests. Native plants that are first to fill meadows, such as poplar and birch, usually last for only a few decades before mature maple and beech forests grow around them. Native trees are being outcompeted for early succession by aggressive invasive shrubs like Autumn Olive. Photo Credit : Jim SalgeThese early natives are now being replaced by species that have never grown here before, including multiflora rose, autumn olive, and glossy buckthorn, which all grow much faster than the native trees. When these plants move in first, are our whole native forests in jeopardy? New England’s most iconic foliage tree is unquestionably the sugar maple, which is native throughout the region and even dominant over oaks in the northern states. While some have expressed concern that this beautiful tree will find New England less habitable as annual temperatures warm, it’s more likely the sugar maple will simply become less competitive with all these other species and represent a smaller portion of the forest canopy. Oaks are shifting northward, and a forest more like the ones found in the Mid-Atlantic states will probably be the eventual outcome. With the threat of climate change, will oak trees displace sugar maples in much of their native range? Photo Credit : Jim Salge
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While the forests of New England are always changing and always beautiful, maintaining native diversity should be the goal — especially as we see the power of the tourism economy (think: leaf peeping) to promote education and conservation. The good news is that there are many things being done locally and regionally to combat invasive species. At the most basic level are the public campaigns reminding people not to move transport firewood when vacationing or camping, and to buy wood locally; this severely limits the movement of many invasives. And biological controls — such as the fungus that combats the LDD moth — continue to be a source of optimism. When purple loosestrife began taking over meadows across North America, for instance, a beetle from its home range was brought over and is now helping to manage the population. Similar measures will likely be tried with many other species in the years ahead. In the meantime, landowners and conservation groups are seeking help in eradicating invasive species from private properties, community forests, and protected lands. And citizen-scientist programs are engaging and empowering many to assist in monitoring forest health. Contacting local and regional land trusts can open the door to many volunteer opportunities. Until then, we can all enjoy the beauty of our forests as they are, with an awareness that we can all affect what they become!
Jim Salge
As a former meteorologist at the Mount Washington Observatory, foliage reporter Jim Salge is a keen observer of the progression of the seasons in New England. He uses his knowledge of weather, geography and climate to pinpoint the best time to visit various New England locations to find the best light, atmosphere, and most importantly, color.
