Why the tree of heaven spreads so devilishly and harms Pa. forests

The Ailanthus altissima, or tree of heaven, is an invasive species that’s taken root in Pennsylvania forests, crowding out native trees, damaging urban infrastructure, and hosting a crop-afflicting insect.

And it has bedeviled foresters trying to get rid of the tree, which first was imported to the Philadelphia area in the 18th century.

Now, thanks to a former Pennsylvania State University student, we know why it’s been so tough to control: The tree of heaven has one heck of a sex drive.  Unlike many other species, it can drop millions of seeds over a lifetime and keep doing it well into old age.

“We were able to figure out that a fecund female could produce up to 52 million offspring, or seeds, in its lifetime,” says Matthew Kasson, coauthor of a study on the plant’s astounding reproductive power. “We were floored by that. We said, ‘Wow, that’s really impressive. There are other trees that can produce a million seeds a year, but they don’t have that kind of lifespan.”

Not only that, but those seeds have a much higher potential to germinate when compared with many other plants.

Kasson, who earned his Ph.D. at Penn State, is now a professor at West Virginia University.  He coauthored an article in the current issue of  the journal Forests about the tree of heaven, along with West Virginia  Ph.D. student Kristen Wickert; his Penn State mentor, professor Donald Davis; and Eric O’Neal from the Davey Tree Expert Co. in Horsham.

The news gets even worse for those trying to control the spread, he said.

“Not only can it produce buckets of seeds, but it can spread through root and stump sprouts,” Kasson said. “So even if you cut it down, you get a thousand more popping up through the root system. Cutting it down just makes it even worse.”

The tree also plays host to the spotted lanternfly, first detected in the United States in Berks County in 2014, a significant threat to grapes, apples, stone fruits, and hardwoods.  The spotted lanternfly, also native to China, lays egg masses in the trees, helping the insect spread.

“And it all started in Philadelphia,” notes Kasson, who has also researched the history of the tree.

The tree was introduced to the U.S. in 1785 by William Hamilton from England.  Hamilton got that first Ailanthus tree from England, but it was originally from China.  Hamilton planted it at his Philadelphia estate, known as the Woodlands, which is now a cemetery.  Williams even gave a tree to a member of the Bartram family as a wedding gift.

Camera icon Matthew Kasson
Woodlands Cemetery, Philadelphia, where “Ailanthus altissima,” or the tree of heaven, was introduced into the U.S. by William Hamilton. Two large Ailanthus, 91 and 107 years old, still stand on the grounds of the cemetery. Hamilton’s Federalist-style mansion also still stands.

The earliest known seed production was found at the Philadelphia Herbarium and the Academy of Natural Sciences in the early 1800s — likely with seeds from Bartram’s Garden, still the oldest known botanic garden in the nation.  From there, the tree spread to more than 40 states.

Camera icon Matthew Kasson
An Ailanthus altissima, or tree of heaven, in Philadelphia.

The tree was largely contained along urban areas and roadsides.  Indeed, passers-by might assume the saplings are sumac or weeds.

But it really began to spread during the 1980s, when a massive gypsy moth infestation threatened large sections of forests.  The moths defoliated hardwoods, which were then logged by salvaging operations, Kasson said. Whole swaths of forest lands were left open to plant invasion.

That gave Ailanthus a clear path into eastern forests where it thrived, putting down rhizomes which spread horizontally underground, crowding out native species.  The trees can reach 80 feet, and live a long time. A tree at the Lemon Hill mansion in Fairmount Park is almost 110 years old and still producing seeds, he said.

Ryan Reed, an environmental education specialist with the Pennsylvania Department of Conservation and Natural Resources, calls the tree’s spread, coupled with spotted lanternfly, an “emerging threat.”

“One of the things that should be noted is just how costly it is,” Reed said. “When you think about lost productivity of the forest, how many years will it be set back by invasives? Think of the man-hours, the time, the money, and all the resources dedicated to getting rid of this stuff.”

Reed said he has seen the trees take root and spread quickly, covering acres in just a few years.  It took him five years to eliminate a two-acre stand, “and I still find survivors.” The trees form a monoculture, crowding out desirable native hardwoods such as cherry, maple, oak, and walnut, he said.

Reed calls the study on the seed viability of the trees “very eye-opening,” considering it is now found in just about every county in the state.

The tree can also be destructive in urban and suburban environments.  In fact, the tree was at the center of a recent legal battle between the parents of Marisa Tomei and Sean Lennon in New York.  Gary and Addie Tomei sued Lennon two years ago, claiming the roots of his Ailanthus tree at his Greenwich Village home were destroying their home next door.

O’Neal, of Davey, said the tree is a double-edged sword in urban environments. He said many find it a pretty tree, especially in bloom, and it grows where others won’t, giving greenery to what otherwise might be barren lots.

“It has an increased ability to colonize just about anywhere — sidewalk cracks, foundation cracks — anywhere it’s a harsh environment,” O’Neal said.  “It has the ability to quickly grow.  But that also means it can damage foundations and get into sewer lines. … The tree sap can also cause a painful rash through contact dermatitis.”

But there is some local hope in controlling the tree, according to Penn State’s Davis, a professor in plant pathology.  He has shown that a native fungus, Verticillium nonalfalfae, can kill the tree without attacking other native species. He discovered the fungus’ impact after examining a stand of dead Ailanthus.

“We inoculated the trees and seedlings and 100 percent of the Ailanthus died,” Davis said. “Every tree that we inoculated died. And, in addition, the fungus spread out.  To make a long story short, we inoculated less than 100 trees in south-central Pennsylvania, but that resulted in about 20,000 other trees dying.

“We looked at maples, oaks, and ash and we saw no other injuries to those trees,” Davis said, calling the fungus a safe bio-control.

He noted there are millions of trees of heaven still out there, and there is no centralized program to use the fungus.

His former student, Kasson, plans to keep studying the tree in order to help foresters.  He said the fungus is promising, and so are efforts by foresters to cut down female trees before they can produce seed.

There’s plenty yet to learn, he said. “I started my Ph.D. on trying to control this tree, and I’m still researching it.”