Charting the Course for Recovery: Villanova Geography and the Environment Professor Turns

November 17, 2025

Invasive pests and pathogens are attacking native trees and threatening the biodiversity of our historic Eastern forests. For Jen Santoro, PhD, the key to recovering these important species—like the American chestnut—begins with the coordinates.

Dr. Santoro poses with a young American chestnut sprout that has yet to be infected with chestnut blight.

What remains of a population of 4 billion American chestnut trees in eastern North America a few centuries ago are now trapped in a cruel cycle of existence.

In the early 1900s, a fungal pathogen known as chestnut blight was accidentally transported to the United States on imported Japanese chestnuts and quickly attacked the susceptible native variety. The blight kills the tree’s living cells to feed itself, yet it usually cannot survive in the soil. That means the fungus kills the above-ground growth, but then the root system pushes out another sprout—a sprout which ultimately succumbs to the same fate, never reaching maturity or producing fruit. Eventually, the cycle ends with the death of the entire tree.

The once-massive population of mature American chestnuts has been reduced to only a few million of these sprouts, and by some estimates, only hundreds of mature trees. The species is functionally extinct in the wild, meaning reproduction has nearly ceased.

Chestnut blight is just one of many deadly invasive threats that have taken a stranglehold on native American trees at various points over the last 125 years, all of which have been introduced to the continent via human movement, according to Jen Santoro, PhD, assistant teaching professor in the Department of Geography and the Environment at Villanova University.

Chestnut blight attacks the living tissue of the tree in order to feed itself. Image courtesy of The American Chestnut Foundation.

Fungal pathogens such as Dutch elm disease have ravaged large populations of elm trees since the 1930s, while the more recent threat of beech leaf disease—discovered to be transmitted by small worms called nematodes in the early 2010s—is causing swift dieback in American beech trees across 13 states. Invasive insects like the emerald ash borer have killed tens of millions of ash trees in North America in the short 20 years since it has been detected. The list goes on.

“We might not have the biggest trees, but on the East Coast, from Maine to Florida and out to the Mississippi River, we have the highest tree diversity on the North American continent,” said Dr. Santoro, who uses geographic information systems (GIS) and statistical models to aid in the battle to recover populations of trees like the American chestnut. “That diversity is incredibly important, and losing trees left and right has cascading impacts on the ecosystem.”

A Gut Punch to Ecology and Economy

The value of the American chestnut trees that lined the forests of Eastern North America was incalculable both ecologically and industrially. Growing up to 100 feet tall and producing large, edible and nutritious nuts, these trees served as both a habitat and reliable primary food source for numerous species of wildlife. For humans, their strong, straight grained, rot-resistant wood was used in everything from dwellings to furniture to railroad ties. Both the timber and the nuts—one of the most well-known symbols of the holiday season—were of incredible economic importance.

According to The American Chestnut Foundation, within 50 years of the introduction of the blight, paired with another introduced fungal pathogen called Phytophthora root rot, the industry built around the timber and nuts of the American variety had collapsed. Now, the organization says, no new chestnut lumber has been sold in the US for decades, and all of the country’s nuts are either harvested from Asian varieties of the tree or imported.

The consequences, Dr. Santoro says, were felt strongly throughout the country, but especially in Appalachia, whose inhabitants relied heavily on the timber and even the nuts; farmers used them to bulk up their hogs before sale, but lost that free source of animal feed once the species population declined.

Chestnuts are packed with nutrients, and the decline of the American species removed an important food source for wild and domestic animals. Image courtesy of The American Chestnut Foundation.

“There is a link between the loss of the American chestnut and the decline of the Appalachian economy that persists to this day,” she said.

The dramatic loss of the American chestnut underscores the pervasive issues facing many species in Eastern US forests—and in turn, the people and animals who depend either on them directly or the habitats they help sustain.

Emerging threats to the versatile American beech tree have contributed to a diminished food source for wildlife, like the black bear, and have increased soil erosion due to a lack of its slow-decomposing leaf litter. The long-living eastern hemlock—the state tree of Pennsylvania—shades streams and cools water for trout in addition to stabilizing stream banks, but the invasive hemlock wooly adelgid insect has caused widespread damage and death to the species—especially in the South—threatening trout populations and disrupting stream ecology.

Elms, aside from being forest anchors, “were once a staple shade tree of neighborhoods because they are hardy, tolerant of pollution and had an appealing vase-shaped canopy,” Dr. Santoro said. “But once Dutch elm disease came, it wiped out tons of street trees and also cost cities a lot of money.”

Invasive pests and pathogens affecting old growth forests have threatened the habitats of native animals and altered forest ecology. Image courtesy of The American Chestnut Foundation.

And of course there is the ash. The white ash is a large canopy tree whose leaf litter creates nutrient dense soils, while the black ash thrives in wetlands and stabilizes the water table. The rapid population decline of these trees in recent years has led to wide-ranging issues, from the loss of access to natural resources, to near elimination of the wood historically used for common items like baseball bats.

When these trees are lost, they are not necessarily replaced by native trees or shrubs. Ever notice the prevalence of thick, thorny underbrush in Eastern forests? Diminished native canopies give plants such as the tick-friendly multiflora rose and Japanese barberry—along with other invasive species—a chance to take over.

“It’s a compounding effect,” Dr. Santoro said. “The individual threats certain species are facing, paired with prevailing climate threats to all trees, like hotter summers, warmer winters and precipitation changes, really has a profound collective impact on both the ecosystem and on human lives.”

Destined for the Forest

The forests have always called to Dr. Santoro. Her father, a forest ecologist himself, sparked her interest in trees from a young age, and while pursuing her degree in environmental science, she began to grasp their all-encompassing importance.

“I was particularly moved by the story of the loss of the American chestnut and how that not only altered the ecology of the entire Appalachian ecoregion from Maine to Georgia, but also the livelihood of people who depended on the forest,” she said.

“Extractive industries like industrial logging recovered, finding both other species and other markets for wood. But local people, who often used chestnut most sustainably, were hurt the most, and are in some cases still hurting, from the loss of the trees which contributed so greatly to their livelihoods.”

Dr. Santoro, seen here examining an afflicted beech tree, became interested in trees and forest ecology at a young age.

It is what drew her to an internship, and later partnership, with The American Chestnut Foundation, whose mission is simple and clear: to return the iconic American chestnut to its native range. Doing so, however, is a little more complicated.

Biologically, geneticists are working on potential solutions to repopulate forests with trees resistant to the blight. By taking surviving mature specimens and selectively breeding hybrids, there is hope that ultimately one of these genetically-resistant American chestnut populations can take hold.

“If we can get enough herd immunity by having a lot of healthy trees, then the blight impacts will be less in the population,” Dr. Santoro said. “We can get that nice kind of genetic, natural crossing and evolution back.”

But where would they be planted?

“It’s not like birds are going to carry the nuts and drop them in the perfect spots,” Dr. Santoro said. “That’s where our work comes in.”

Because of the widespread chestnut blight, traditional forestry maps, which show changes to tree populations over time, are far less useful in studying that species. However, the urgent need to study healthy chestnut genetics has led to members of the public reporting surviving specimens they find in the wild to The American Chestnut Foundation. Those trees are verified, and coordinates are easily plugged into a database.

Dr. Santoro began to look at recorded specimens of healthy trees in Virginia’s Shenandoah National Park while working on her master’s thesis. Years later, when she arrived at Villanova, an undergraduate named Alec Henderson ’22 CLAS took up an interest in Dr. Santoro’s previous work with the iconic tree after attending a seminar she gave on the subject.

“Her energy and enthusiasm for this work were so apparent, it was impossible not to,” Henderson said, further noting the influence Dr. Santoro’s mentorship and classes had on both his work mapping chestnuts and wildlife habitat distribution—the subject of his current graduate studies at the University of Connecticut.

A broad view of Henderson’s Pennsylvania American chestnut suitability map, originally published in the journal Forest and Ecology Management.

Henderson joined her on an independent study project, and the two of them began to plot the reported specimens in Pennsylvania—one of the most important parts of the historic range. They analyzed dozens of potential environmental factors that could lead to each single tree’s success throughout those regions and incorporated them into a model.

“We’d look at soil profiles—maybe it’s sandy, or contains a little silt and clay,” Dr. Santoro said. “Then, things like average monthly precipitation over several decades and average monthly temperatures. What is the land use around it? Are there potential contaminants? Sun exposure, soil acidity, all of it.

“We had about 20 different factors with the goal of finding the ones that create the most suitable environment, so that we can ultimately plant trees in those locations.”

Henderson, who ran the model, determined that it was indeed possible to calculate. His project with Dr. Santoro and Associate Professor of Geography and Environment Peleg Kremer, PhD, was a probable suitable habitat range for the entire Commonwealth of Pennsylvania, at a very fine scale. So fine, The American Chestnut Foundation expressed interest in a suitable habitat map of all areas east of the Mississippi River down to a 30-by-30-meter scale. That means for every piece of land in that range—more than half of the contiguous US—every 900 square meters, or about the size of two NBA basketball courts, would have its own suitability score based on those aforementioned factors and more.

“This fine-scale work is what leads to the most actionable results, particularly in looking at soil chemistry, which is so important in considering plausible habitats for these trees,” Henderson said. “And it’s being done over such a large area so we can better understand chestnut habitats in broader strokes.”

That project is what Dr. Santoro, Henderson and other collaborators are currently working on together. In doing so, they aim to not only uncover suitable habitats within the historical native range, but also new habitats outside of that range, where these trees could now thrive where they historically could not.

With dedicated research and planning, scientists hope to restore the American chestnut to its historic native range and beyond.

Additionally, they are developing public-facing materials to help community members engage with the American chestnut’s recovery story. Given the scarcity of the species, there is a lot of need for help and little room for error.

“We have to be pretty intentional about this,” Dr. Santoro said. “I’d give someone a heart attack if I took an expensive, specially-bred Chestnut seed, tossed it out of the car window and hope it grows. Work like this helps target the optimal places where humans can plant these seeds to help start to regrow these forests. And all of this could, in theory, be applied to other types of trees being critically threatened as well.”

But for now, her focus is on the American chestnut.

“I’m so passionate about its restoration efforts from an ecological perspective—to help our eastern forests remain resilient in the face of climate change—but also to bring back a tree that was so culturally intertwined in East Coast history and helped so many people make a living off the land,” Dr. Santoro said.

That recovery may happen one at a time for now, but every one is a victory. One multiplies to two, then four, then eight. Double it 29 more times, and you’re over 4 billion.

“We have to think in terms of tree lifetimes,” Dr. Santoro said. “You hope that people will continue to build on all that’s being done to recover this species and keep carrying it on.”