By Morgan Spehar
The roots of the tree-of-heaven, or Ailanthus altissima, spread out like feelers beneath the soil, creeping in all directions. Occasionally, a sprout breaks off from the roots and shoots up, bursting through the membrane of the soil. This new sprout can grow three to four feet in a year, and will quickly grow into a full-size tree. Someone walking in what feels like a natural forest may be unaware that they are surrounded, not by individual trees, but by a single organism, whose veins twist and turn beneath their very feet.
In the U.S., the Fish and Wildlife Service estimates that there are over 4,300 invasive plant species, which are classified as invasive if they are non-native to the ecosystem and cause significant environmental or economic harm. These species damage the environment by out-competing and replacing native species that are important to the food web or the economic concerns of the region. Unfortunately for the environments across the United States where it can now be found, Ailanthus altissima is incredibly invasive.
Joanne Rebbeck, a U.S. Forest Service plant physiologist who was stationed at the Northern Research Center in Delaware, Ohio, spent the latter part of her career searching for a solution to the problem of just how invasive Ailanthus is. She began looking in the early 2000s, after another project made her notice how widespread the tree-of-heaven had become.
“I was studying the effects of prescribed fire in oak forests,” she said, “we were doing a big experiment, and I started seeing Ailanthus around. It appeared that the fire stimulated the Ailanthus, and I started seeing more and more Ailanthus in our study areas.”
The areas where Rebbeck was studying were located in the Midwest, southern Ohio in particular, where the invasive Ailanthus is mainly a threat to the native hardwood trees that populate the forests of the region. Hardwood trees aren’t actually identified by their use, thickness, or appearance, but rather by the fact that they reproduce using fruits or nuts. They tend to grow slowly, and because the tree-of-heaven is a voracious consumer of space in both the soil and the canopy, it often easily outcompetes native trees. Ailanthus also releases phytotoxic, or poisonous, compounds into the soil, ensuring that other plants can’t compete for the space that it wants to inhabit.
Oak and hickory, for example, lose out on nutrients when competing with Ailanthus, which is problematic for a variety of reasons. Oak trees produce acorns, which feed everything from chipmunks to turkeys to deer, none of which can feed on Ailanthus seeds. In fact, Forest Service Ecologist Bill Borovicka said small animals such as rabbits may feed on the tree-of-heaven’s seeds, but only in years where other food resources are extremely scarce.
A native species, however, may prove to be effective in weeding out Ailanthus. In 2002, researchers at Penn State University began noticing that a natural fungus called Verticillium nonalfalfae was killing off thousands of Ailanthus trees. Rebbeck was immediately interested, but she couldn’t transport the fungus across state lines for risk of spreading the Verticillium where it didn’t naturally grow, so she had to find the fungus in Ohio before she could test anything.
“Basically,” she said, “I went in and talked to all sorts of different groups and private landowners and said, ‘Here’s the symptoms [sic] you need to look for.’ So with the help of a private forester, who came up to me after a talk, we found the fungus in 2012.”
The fungus’ discovery in Pike County, Ohio, meant that Rebbeck could start her research. She partnered with the Ohio Division of Forestry, the Ohio Division of Wildlife and The Wilds, a safari park and conservation center in Cumberland, Ohio, to find the best plots of forest land to test it.
To the untrained eye, it can be difficult to identify the tree-of-heaven because it blends so well into the hardwoods that surround it. When the tree is still young, the bark looks as though someone stenciled diamonds onto it, and as it gets older, it starts to look not unlike the bark of an oak tree. The large leaves are palm- or fern-like, with a single stem holding several smaller leaves, like the leaves of a sumac or walnut tree. Once the tree is a few years old, the leaves are further up, leaving more of the thin trunk exposed.
Once the tree is infected with Verticillium, its appearance quickly changes. Rebbeck explained that the fungus infects the vascular tissue, which means it targets the systems that spread nutrients and water throughout the body of Ailanthus. At first, those big fern-like leaves just start to droop, but as the wilt takes hold, they fully shrivel up and turn a darker green. Eventually, all of the leaves and seeds fall off, leaving the grayish-brown trunk bare; if the bark were to be peeled away, the wood inside would appear a sickly yellow. Once inoculated, the tree-of-heaven succumbs to the Verticillium and wilts in about two weeks, but the fungus doesn’t stop there.
Ailanthus is like a many-headed hydra; cut down a few of its stems without killing the root system, and more young sprouts will quickly shoot up to take their place. This clonal method of expansion is part of what helped the tree-of-heaven spread so quickly; it was brought to the United States from China in 1784 and has since spread so aggressively that it can be found in the majority of the lower 48 states. The extensive root systems make Ailanthus difficult to kill with traditional methods like herbicides or prescribed fire.
But when Verticillium is introduced to the environment, suddenly Ailanthus’ method of spread isn’t so good for its survival. Because the trees have those complex root systems that connect each trunk to its neighbor, treating a few Ailanthus trees with fungus can kill off an entire stand. The Forest Service estimates that the fungus can spread 200-400 feet per year via root transport, which is an integral part of what makes Verticillium treatment so effective.
“The only way to control some of these invasive species is by some sort of biological control that can spread on its own,” research ecologist Todd Hutchinson said. Hutchinson, along with other Forest Service researchers Leila Pinchot and Lauren Pile, picked up where Rebbeck left off when she retired in February 2019. The team has been trying to figure out how the fungus could be used to manage Ailanthus on a large scale, when it has historically been such a difficult invasive species to control.
Because invasive species like the tree-of-heaven aren’t native to an ecosystem, the environment often doesn’t include any natural predators or resource competition that would control the invasive population. There is no plant in the southern United States that can compete with kudzu, and European Starlings across America are rarely threatened by another animal once they reach adulthood. Bringing in the species’ natural predator from its place of origin is sometimes an option, but these new species can cause more problems than they solve.
Hutchinson worked on some of the early projects with Rebbeck, and saw firsthand how typical methods like prescribed fire were ineffective against the tree-of-heaven.
“Herbicide is pretty expensive and won’t spread on its own. The [prescribed] fire will kill the stems, but then they’ll just resprout,” he said.
Once the stems sprout, they grow much faster than hardwood trees. The tree-of-heaven’s name actually comes from the fact that it grows incredibly quickly; “ailanthus” literally means “tree reaching for the sky.” The tree can grow to be 80 feet tall in its average lifespan of 50-75 years, which makes it an attractive choice for urban garden cover. Gardeners in the Northeast were historically fond of the tree-of-heaven because it could be grown in areas with poor soil and air quality, and could provide shade in areas where other trees wouldn’t grow.
But the traits that make Ailanthus a good tree for urban environments, like rapid growth rate and resilience, are exactly what make it such a threat to the health of the United States’ midwestern hardwood forest ecosystems.
For example, hardwood trees have deep root systems that protect the ecosystem from soil erosion and excessive runoff. Ailanthus roots spread far rather than deep, and when the tree-of-heaven replaces oak or hickory trees, the environment doesn’t get the same benefits. Pile pointed out that hardwoods are also more economically valuable because their strong wood can be used for building materials and products such as oak barrels. Because they grow slowly, hardwoods tend to have wood that is much denser, and therefore more durable, than fast-growing trees like the tree-of-heaven. With Ailanthus taking up valuable space, the delicate balance of hardwood ecosystems and economic systems can be thrown off.
Another problem with the tree-of-heaven is that it reproduces in extremely large numbers. Ailanthus trees are either male or female and depend on pollination for reproduction. In the spring, male trees produce foul-smelling flowers and huge amounts of pollen, which contribute to making allergy season more severe. The pollen is carried by the wind and pollinators to female trees. Once pollinated, the thin, papery seeds hold onto the branches of the tree until after the leaves fall off each year; the seeds’ design means that they can easily be carried long distances by the wind before remaining in the soil for years, waiting for the perfect opportunity to sprout. Given that female Ailanthus can produce over 350,000 seeds annually, when the researchers started looking for test plots to inoculate with Verticillium, it wasn’t a difficult tree to find.
In the Marietta Unit of the Wayne National Forest in southern Ohio alone, the Forest Service found almost 3,000 seed-bearing Ailanthus trees in a 124,000 acre area. The test plots for using the fungus treatment can be found in the Wayne National Forest, the Wilds, Pike State Forest, Tar Hollow State Forest and Blue Rock State Forest.
“These [sites] were chosen because they had a lot of tree-of-heaven and we wanted to spread across a lot of different counties and different areas,” Hutchinson said.
Researchers inoculated the trees using a method called the Hack-N-Squirt: first, three quick chops of a hatchet expose the soft wood under the bark on the base of the Ailanthus, then, a very precise number of fungal spores mixed in a water solution are inserted into the exposed wood with a syringe. Borovicka, who monitors the Ailanthus test plots, said that the amount of the fungus in each syringe was calibrated down to the exact number of spores, and that the researchers were trying to determine the lowest number of spores needed to kill a healthy Ailanthus tree.
Treating the Ailanthus with the fungus is also much less labor-intensive and much less expensive than treating the trees with herbicides.
“One of the big advantages is it’s fairly inexpensive to culture,” Rebbeck said. “If you culture it in a laboratory and inoculate the Ailanthus, you don’t have to treat every single stem in that area, it spreads through the roots. We’re studying if you can get 100% kill by having it travel through the root system. Herbicides are never 100% effective.”
Herbicides also don’t discriminate when it comes to killing plants, so introducing Verticillium, a native organism, seems to have a better effect on the environment than chemicals in the long run. The Forest Service inoculated many of Ohio’s native plant species with the fungus in the lab before commencing tests in actual forest environments and found that only three–striped maple, devil’s walkingstick and sumac–were in any way affected by the wilt, but the effects of the fungus on these plants wasn’t nearly as extreme as its effect on Ailanthus.
In all of the test plots, Borovicka has witnessed the wilt spread outside of the plots’ original boundaries, although he said that the kill rate wasn’t 100%. Some of the Ailanthus came close to dying before regrowing their crown and seeming to recover. He’s not worried about the trees forming a tolerance, though–the Ailanthus wilted a second time and it seems like the Verticillium is successful for now.
The study that Rebbeck started in summer 2015 ended with the final observations being taken in the 2019 summer field season. Hutchinson said they’re hoping to analyze the data and publish a paper by the end of the year about Verticillium’s effects and potential future uses.
Now the focus of the study is shifting toward how the areas that were once filled by Ailanthus are recovering. Once the tree-of-heaven is gone, there is a sizable gap left in the ecosystem, but that gap isn’t necessarily being filled with desirable native species.
“In a forest where there are other invasive species, the mortality caused by Verticillium wilt creates a response from other invasive species,” said Pinchot, who specializes in native species reintroduction and forest ecosystem restoration. “There may not be an overall reduction in invasive species in the long run.”
But that’s without intervention. The next step for the project may be introducing native trees like white oak, chestnut and elm back into the places where Ailanthus once dominated so that other invasive species like honeysuckle don’t move in to replace it.
“The issue is that the sites are in really degraded environments,” Borovicka said, “so there’s not really any natural species growing back right now.”
The end goal for the researchers is to create future forests that are diverse, healthy and able to support all sorts of native wildlife. Other than reintroducing native plant species, the researchers are still working on methods of achieving that goal–the final solution may include a combination of tools, so Verticillium could be working right alongside herbicides to keep invasive species from spreading further.
Right now, Verticillium can only be used for research and development purposes, but a private company is looking to change that. It is working with public agencies, like the Forest Service, to get a strain of Verticillium approved by the Environmental Protection Agency to be used as a minor-use herbicide. The process for approval can take five years, and Rebbeck said the company was only in year one.
“The EPA has a lot of regulations, which is really good, because you don’t want to introduce anything bad into the environment, it just takes time and money,” she said.
Eventually Verticillium could be used by private landowners, as well as public agencies, all over Ohio to keep Ailanthus from spreading beyond its current borders and further threatening forests’ biodiversity. But it’s unlikely that the tree-of-heaven will ever completely disappear from Ohio’s ecosystem.
“With something as broad as Ailanthus, we’re never going to be able to completely eradicate it,” said Pile, who is studying the Ailanthus wilt from a research station in Missouri, “but using different tools, we can use different methods to control the plant and keep it from spreading even more.”
The Ailanthus project is ongoing, and brings up the possibility that using biological controls like Verticillium could be the answer to the question of how to save natural biodiversity. Each lost hardwood tree that Ailanthus edges out is a threat to the health of these ecosystems, so controlling the tree-of-heaven and other invasive species is important to their survival. For now, the Forest Service will continue to monitor the test plots and conduct research, as the roots of the tree of heaven continue to spread.
This story was originally produced for the Fall 2019 Environmental and Science Journalism class taught by Dr. Bernhard Debatin. All photos are credited to the author unless otherwise noted.