There’s an invasive tree that is becoming a threat to Pennsylvania’s forests. And it’s one that you see all the time. Ailanthus altissima, better known as tree of heaven, is a tough urban tree that sprouts out of sidewalks. It also happens to be the tree from the book A Tree Grows in Brooklyn.  The tree came to the U.S. from China by way of England in the 1700s. And since then, it has thrived. The tree of heaven is found in 40 states, including most of Pennsylvania.

Matt Kasson is an assistant professor of forest pathology at West Virginia University. He’s been studying the tree for more than a decade, and his latest research was recently published in the journal Forests. The study was co-authored by Kristen Wickert, Eric O’Neal and Dr. Don Davis.

Kara Holsopple spoke to him about what he and his team have learned about the tree’s spread.

Commonly thought of as an urban tree, the tree of heaven has now invaded forests across Pennsylvania. (Photo: Matt Kasson)

Kara Holsopple: Your newest research clues us into why this non-native tree has done really well. What did you find?

Matt Kasson: We’ve known for a while now that Ailanthus is pretty aggressive in colonizing the landscape. But what was unclear to us, and really the motivation behind the study, was to figure out its reproductive potential. If you drive down the highways right now you could see these brilliant red to orange colored seed clusters, prolific seed production on individual trees. But we wondered how much seed could these trees actually produce in a single year and over a lifetime.

KH: And what did you find?

MK: We found that there are some exceptionally reproductively capable individuals across the landscape. We found one tree in particular on the campus of Penn State University that routinely puts out about 700,000 seeds a year, and that’s been doing so for almost a century. So the cumulative seed production of that tree is somewhere north of 50 million seeds over its lifetime. Now that’s an exceptional tree. But we found that on average a tree that lived 40 years and was reproductively capable during that window could produce upwards of 10 million seeds.

LISTEN: “Tree of Heaven Creates Hell for Native Forests”

KH: Your study mentions viability. That’s the success of the seed once it gets into the ground if it’s going to become a tree or or not?

MK: I think viability is a really important aspect because if a tree can produce a million seeds per year, and viability is 3 percent, that’s not a lot of reproductive capacity.  But if viability is north of 70 percent like we found on a few of our individuals including this 104 year old individual in Pennsylvania, the potential for producing a lot of progeny over a lifetime is exceptional.

KH: So why is this tree’s kind of super reproductive power and it’s spread a problem?

MK: It wasn’t always a problem. And I think the ways we manage land and landscapes have changed over time. Following another invasive pest, the gypsy moth, we saw widespread land clearing which really set up these forests for Ailanthus. Gypsy moth has been here since the 1800s and it started to devastate forest in Pennsylvania and throughout the Appalachian region in the late 70s to early 80s. We saw widespread salvage harvesting in the aftermath of this gypsy moth defoliation because the defoliation was so severe that it actually killed a lot of the oak trees that were occupying these ridge tops in South Central Pennsylvania and elsewhere throughout the mid-Atlantic. So one reproductively capable female tree at the edge of this clear cut seeded in the hole clear cut. And now you’re left, not with native trees regenerating, but a whole entire stand of invasive trees.

KH: Is this something that’s worrying for foresters?

MK: Absolutely. Tree of heaven, not only can outcompete native species that might seed into those same clearings, but it also produces compounds that inhibit the germination of native plants. It’s called allelopathic and what it does is it produces these compounds that make the site more suitable for itself to regenerate and less suitable for native plants. If we think about the spotted lanternfly which is a new pest that’s just been found in Pennsylvania, it actually requires Tree of Heaven to fulfill its lifecycle: It feeds on the foliage, it lays eggs on the stems, and it’s a real threat to the grape industry. Tree of heaven is pretty well established throughout most of the counties in Pennsylvania with the exception of a few of the northern tier counties. So, in that sense, it could follow tree of heaven up towards Erie and up to New York into the Finger Lakes region where it could directly threaten and impact the grape and wine industries in those regions.

KH: How can you get rid of them? Can you cut them down? What are some of the ways that you can remove them?

MK: I’ve been working on biological control using native fungi to kill Tree of Heaven in these forest settings for about a decade now. But there’s still plenty of stands of Atlantis that haven’t been controlled. And until they’re removed, whether it be through biological control or chemical applications or mechanical removal, they’re going to continue to be under-productive forests that supplant native species and prevent future generations of native forested stands from establishing.

Seed clusters on the Ailanthus. Photo: Matt Kesson

KH: Tree of Heaven is commonly thought of as an urban tree. How did they move into the forest?

MK:  That’s a good question. For a long time it just kind of hung out in Philadelphia and New York is kind of a botanical oddity passed around among early botanists and then seed producing trees were established and soon seedlings were available through a lot of the nursery trade and followed the railroad corridors east and west. We noticed a huge spike in the spread of tree of heaven in Pennsylvania following the completion of the Horseshoe Curve which connected the east and western part of the state. Tree of heaven moved along those transportation corridors much like they move along our interstate highways now. But as to how they got in the forest, with building of roads and things like that, we see movement of invasive species whether it be through gravel or just that there’s a lot of exposed soil which allows prolific seed producing species to establish. There was a recent study done out of Penn State that showed that these Marcellus shale well sites are now being taken over by a number of invasive plant species. We predict that tree of heaven will follow suit because it does occur up in those northern tier counties but they’re not really widespread as of yet. But with all this new road building and road construction to support the gas industry, I think we’ll see a lot more spread of these invasive species that are common throughout the the southern part of the state.


Matt Kasson is an assistant professor of forest pathology at West Virginia University.