Eastern Parks Reduce Herds to Benefit Forests, Deer, and People

National parks protect a small but critical part of eastern U.S. forests. They’re now responding to an accelerating crisis as overabundant white-tailed deer coincide with a profusion of invasive plants.

By Megan Nortrup

About this article

This article was originally published in the "Features" section of Park Science magazine, Volume 37, Number 2, Winter 2023 (December 29, 2023).

Lush, native, understory vegetation fills an area fenced off from deer in Valley Forge National Historical Park. Outside the fenced area, the forest floor only has invasive Japanese stiltgrass.

Image credit: NPS

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It was summer 2023, and we stood neck high in a shaded clump of sweet-scented spicebush talking to a group of reporters about forest regeneration.

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Despite the sweltering weather, we’d climbed a steep slope to get to this spot in Washington, DC’s Rock Creek Park. Park botanist Ana Chuquin and I were leading the reporters through the woods to show them the damage white-tailed deer had done to the forest. We had spent the few hours before in a different part of the park. There, shrubby vegetation and plants were sparse from the ground up to about seven feet, where deer could no longer reach. In contrast, small square plots of forest fenced off from deer stood out like lush green cubes on an open plain.

Under ideal circumstances, eastern forests can sustain themselves without intensive human intervention. But we are currently teetering on the brink of a catastrophic loss of eastern U.S. forests. And forests in national parks, whose ecological value is even greater than those in the surrounding landscape, are not exempt. A study by National Park Service and other scientists, described in a paper published in March 2023, demonstrates that the main culprits for this impending disaster are overabundant populations of white-tailed deer and invasive plants.

The deer eat certain native plants and tree seedlings, often leaving behind just a few unpalatable natives and many non-native invasives. These stressors, along with increasingly frequent disturbances brought on by climate change, like droughts, extreme weather events, and pest outbreaks, reduce overall forest resilience. They are pushing forests towards becoming invasive shrub thickets and better habitat for disease-carrying ticks.

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From where we were standing in Rock Creek Park on that hot summer day, we could see that work to control deer populations was paying off.

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But from where we were standing in Rock Creek Park on that hot summer day, we could see that work to control deer populations was paying off. There, amid the spicebush shrubs, tree seedlings and saplings poked up from the forest floor. It was a classic example of how eastern forests have regenerated themselves for centuries. The young tree saplings and seedlings bide their time until a large, neighboring tree topples or dies. They then compete to grow and fill in the resulting canopy gap. That spot in Rock Creek is one of the places where those important forest processes still play out.

Left image
A healthy understory in a closed-canopy hardwood forest at Black River National Wild and Scenic River. Move the slider to the left or right to see the entire image.
Credit: NPS

Right image
A severely deer-browsed forest at Roosevelt-Vanderbilt National Historical Site
Credit: NPS

A Planetary Powerhouse

Globally, forests are critical to the health of the planet. They store carbon (important for reducing greenhouse gases), mitigate urban heat island effects, and protect drinking water. They’re also fun to recreate in and good for your health. They provide economic benefits to gateway communities, and they’re crucial for maintaining plant and animal biodiversity.

The United States has about eight percent of the world’s forests, covering about one-third of the country. Most U.S. forests are in the country’s eastern half. Based on 2010 U.S. Census data, the authors of the March 2023 paper calculated that nearly a quarter of the U.S. population lives within 100 miles of a forested, eastern national park, and 16 percent lives within 30 miles.

And although they occupy less than one percent of national park lands, eastern national park forests that are part of the agency’s Resilient Forest Initiative have outsized importance. The authors looked at National Park Service visitor statistics through 2021 and found that these forested eastern parks receive nearly 20 percent of the agency’s annual visitors. If those forests are lost, one in four Americans will lose the benefits of them and face the risk of increased exposure to disease-carrying ticks.

Scattered Results and a Strategic Decision

The study described in the 2023 paper showed that parks that started controlling deer populations decades ago are seeing higher rates of forest regeneration than others. Rock Creek Park, which began reducing deer populations in 2013, has seen tree seedling density more than double. And Catoctin Mountain Park, which began reducing deer populations in 2010, has seen a 21-fold increase in seedling density. This huge increase comes in part because forest regeneration was in such an advanced state of decline when the park began reducing deer numbers.

Data from Catoctin Mountain Park (left) and Rock Creek Park (right) show an increase in seedlings (blue lines) in response to a decline in deer population (red lines). Data were collected starting in 2010 for Catoctin and 2013 for Rock Creek. One hectare (ha) is about two and a half acres. One square kilometer (km²) is a little under half a square mile.

Image credit: NPS

Narrative description of graphs

On the graph, years are measured on the x axis. Seedlings per hectare are measured on the left y axis, and deer per square kilometer are measured on the right y axis.

Catoctin Mountain Park: A vertical dashed black line shows the beginning of deer management in 2010. To the left of that line, there’s a blue dot showing that deer were over 40 individuals per km² and seedlings were fewer than 1,000 per ha. On the line for 2010, a red dot indicates deer numbers decreased significantly and a blue dot shows that seedlings had increased slightly.

After 2010, the lines show that seedling numbers increased as deer numbers declined. There’s an inflection point at 2012, where the blue line for seedling numbers surpasses the red line for deer. After that, the seedling line increases significantly, and the deer line fluctuates but remains well below the seedling line. In 2016, deer were just above 10 per km², and seedlings were above 7,000 per ha. In 2020, deer were still just above 10 per km² and seedlings were more than 8,000 per ha.

Rock Creek Park: A vertical dashed black line shows the beginning of deer management in 2013. To the left of that line, deer numbers fluctuated but were more than or just below 30 per km², and seedlings were slightly above 2,000 per ha. After deer management began, deer numbers increased slightly to 30 per km², and seedlings increased to just below 3,000 per ha. In 2016, deer were fewer than 10 per km², and seedlings were more than 4,000 per ha. Between 2016 and 2018 is an inflection point, where the line for deer slightly surpasses the line for seedlings. After that, deer numbers trend downward steeply, while seedling numbers climb sharply. In 2020, deer were fewer than 5 per km², and seedlings were more than 6,000 per ha.

But in other national parks, forests aren’t doing as well. Many parks lack canopy-forming tree seedlings and saplings, a condition known as regeneration debt. Some park forests are so degraded, they could easily be lost altogether and convert to impenetrable shrub thickets. All it would take is a nudge from a new forest pest or a particularly destructive weather event. A forest monitoring plot in Morristown National Historical Park, included in the 2023 study, demonstrated this downward spiral from 2009 to 2022 as it lost canopy trees and was overtaken by invasive shrubs.

A forest plot in Morristown National Historical Park shows the progression of invasive understory plants: in 2009, no seedlings present and invasive Japanese barberry waist high; in 2013, storm damage knocked over canopy trees; in 2017, no canopy-forming tree seedlings, impenetrable thicket of invasive plants; in 2022, native forest lost, invasive shrub thicket over 6 ft high.

Image credit: NPS

Although Rock Creek Park’s seedling recovery has been good, retired park biologist Ken Ferebee wishes they had started deer population management sooner. “If we’d started a few years earlier, we would be a little better off,” said Ferebee. “The forest wouldn’t have been so far gone. We lost a lot of the seed bank. Everything that might’ve been there was eaten by animals.”

This potent mix of deer and invasive plants had been slowly pushing self-sustaining eastern forests out of balance for decades, and national parks noticed. In the 1970s and 80s, forests were starting to feel emptier and less familiar. Native orchids and other wildflowers were disappearing, and so were tree seedlings. Even as Gettysburg National Military Park began to reduce deer populations in the 1990s, other parks like Catoctin Mountain Park and Rock Creek Park began studying and tracking deer damage to plants and young trees.

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As division scientists analyzed forest health data from eastern national parks, they found the problem to be more pervasive than previously thought.

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When the National Park Service established its Inventory and Monitoring Division in 2000, the agency gained a means to understand the issue more comprehensively. As division scientists analyzed forest health data from eastern national parks, they found the problem to be more pervasive than previously thought. According to the 2023 paper, 12 years of forest data from 39 eastern parks showed that 27 of them were in imminent or probable danger of “regeneration failure,” which is a technical measure of how close forests are to being lost. Evidence of deer eating seedlings was the strongest predictor of severe regeneration debt.

“In a healthy forest, when a big tree falls or dies, seedlings and saplings in the forest understory grow to fill the gap in the canopy,” said Kate Miller, a quantitative ecologist and lead author of the 2023 study. “This regeneration is how forests continue to be forests. If there’s a lack of seedlings and saplings, the forest can’t maintain itself. That regeneration debt is what we found in the majority of eastern national park forests, and it’s very concerning.”

Map showing regeneration status of forests in eastern national parks from 2007-2009. Secure: ACAD; Insecure: SAGA, ROVA, GARI, NERI, FRSP, RICH, APCO, PETE, BOWA, WOTR, NACE; Probable Failure: MABI, MIMA, WEFA, DEWA, ALPO, JOFL, FONE, BLUE, HOFU, GETT, ANTI, MONO, ROCR, PRWI, COLO; Imminent Failure: SARA, MORR, FRHI, VAFO, CATO, CHOH, HAFE, GWMP, MANA, SAHI, THST, GEWA. From Figure 1 in the March 2023 paper.

Image credit: NPS

Networks, park codes, and corresponding park names

Network Code	Park Name	Park Code
NETN	Acadia National Park	ACAD
ERMN	Allegheny Portage Railroad National Historic Site	ALPO
ERMN	Bluestone National Scenic River	BLUE
ERMN	Delaware Water Gap National Recreation Area	DEWA
ERMN	Fort Necessity National Battlefield	FONE
ERMN	Friendship Hill National Historic Site	FRHI
ERMN	Gauley River National Recreation Area	GARI
ERMN	Johnstown Flood National Memorial	JOFL
ERMN	New River Gorge National Park and Preserve	NERI
MIDN	Appomattox Court House National Historical Park	APCO
MIDN	Booker T. Washington National Monument	BOWA
MIDN	Fredericksburg & Spotsylvania National Military Park	FRSP
MIDN	Gettysburg National Military Park	GETT
MIDN	Hopewell Furnace National Historic Site	HOFU
MIDN	Petersburg National Battlefield	PETE
MIDN	Richmond National Battlefield Park	RICH
MIDN	Valley Forge National Historical Park	VAFO
NCBN	Colonial National Historical Park	COLO
NCBN	George Washington Birthplace National Monument	GEWA
NCBN	Sagamore Hill National Historic Site	SAHI
NCBN	Thomas Stone National Historic Site	THST
NCRN	Antietam National Battlefield	ANTI
NCRN	Catoctin Mountain Park	CATO
NCRN	Chesapeake and Ohio Canal National Historical Park	CHOH
NCRN	George Washington Memorial Parkway	GWMP
NCRN	Harpers Ferry National Historical Park	HAFE
NCRN	Manassas National Battlefield Park	MANA
NCRN	Monocacy National Battlefield	MONO
NCRN	National Capital Parks East	NACE
NCRN	Prince William Forest Park	PRWI
NCRN	Rock Creek Park	ROCR
NCRN	Wolf Trap Park for the Performing Arts	WOTR
NETN	Marsh-Billings-Rockefeller National Historical Park	MABI
NETN	Minute Man National Historical Park	MIMA
NETN	Morristown National Historical Park	MORR
NETN	Roosevelt-Vanderbilt National Historic Sites	ROVA
NETN	Saint-Gaudens National Historical Park	SAGA
NETN	Saratoga National Historical Park	SARA
NETN	Weir Farm National Historical Park	WEFA

NETN = Northeast Temperate Network
ERMN = Eastern Rivers and Mountains Network
MIDN = Mid-Atlantic Network
NCBN = Northeast Coastal and Barrier Network
NCRN = National Capital Region Network


 

Seedling and Sapling Numbers Aren’t the Whole Story

Regeneration debt can also occur from regeneration mismatch, when seedlings are of different species than mature canopy trees. This means the canopy trees will not be replaced in kind when they die. You can see this in places where the seedlings and saplings of smaller, non-canopy trees that deer find less desirable as food outnumber those of the larger canopy trees. Less palatable species like pawpaw, American holly, and American hornbeam are small-stature trees, unable to grow to canopy height, and they block the light that young canopy trees need to survive.

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These stressors underscore the importance of having a diverse mix of young seedlings and saplings in forests.

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According to the 2023 study, the devastating impact of pests and pathogens like the emerald ash borer are also driving regeneration mismatch. The emerald ash borer eliminated ash trees in many eastern parks. And the emergence of beech leaf disease now threatens to do the same to American beech trees. Beech leaf disease is newly widespread in national parks throughout the east, with a few exceptions (Vermont, New York, and the District of Columbia). Other tree pest threats include hemlock woolly adelgid and spongy moth. These stressors underscore the importance of having a diverse mix of young seedlings and saplings in forests, which helps forests withstand the impact of pests and pathogens.

Gettysburg National Military Park, once on the verge of complete forest failure, has controlled its deer populations since the early 1990s. The park’s forest regeneration (as measured by the number of seedlings and saplings) is now among the highest anywhere in the east. But most of the seedlings and saplings are ash. The dominance of one species, likely doomed by emerald ash borer, is why the study authors classified this park as a “probable regeneration failure” despite years of success with reducing deer impacts.

Gettysburg National Military Park has seen an increase in understory tree seedlings and saplings since managing deer populations in the 1990s. But the forest's relative lack of tree diversity puts it at risk from insect pests and plant diseases.

Image credit: NPS

In the past two decades, Gettysburg also created forest canopy gaps in some areas through forest thinning to let in sunlight, in hopes of promoting native seedling and sapling growth. But these gaps were instead exploited by invasive plants. Gettysburg now has one of the highest levels of invasives of any park in the mid-Atlantic. Follow-up actions like strategically managing invasive plants and planting native canopy trees will help get Gettysburg’s forests back on track.

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“If the species composition of the forest changes...it has trickle-down effects in every way. Even from a cultural landscape viewpoint.”

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Zach Bolitho is the division lead for Resource Stewardship and Planning at Gettysburg. “If the species composition of the forest changes, the animals, the birds, the insects, they change too,” he said. “And it has trickle-down effects in every way. Even from a cultural landscape viewpoint, it matters. People used certain trees [here] for certain reasons.”

Long Road to Recovery

The experiences of parks that have controlled deer impacts for as long as three decades show that it’s possible to start reviving a forest through dedicated and adaptive long-term management. As the study described in the 2023 paper showed, Valley Forge National Historical Park has seen significant improvements in its forest condition since the park began addressing deer impacts in 2010. The number of canopy tree seedlings has increased by nearly 26 times. Like Catoctin, these huge increases come in part from starting with regeneration nearly in the red. Although these seedlings are only the start of a long recovery process, this positive trajectory should eventually lead to healthy forests in Valley Forge.

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Funding through the Bipartisan Infrastructure Law and the Inflation Reduction Act is helping parks find cost-effective ways to improve forest health.

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Restoring eastern forests requires a significant and sustained investment. Different parks hold different pieces of the puzzle. Some have had success in managing invasive plant species and others in bringing deer populations into balance long enough to allow plants to begin recovering. Still others are reintroducing fire to the forest landscape. Collaboration among parks to find solutions is key, and the National Park Service’s Resilient Forest Initiative can help parks with that.

But controlling non-native invasive species, rebalancing deer populations, and replanting trees are also costly, and it may take decades to see meaningful results. That’s why funding through the recently enacted Bipartisan Infrastructure Law and the Inflation Reduction Act is so important. Several projects funded by these laws are helping parks find cost-effective ways to improve forest health. By pulling all the levers available to us, we’ll give eastern forests a fighting chance.

Acknowledgements

The authors would like to thank the park staff of all the eastern parks in the study who do the incredible work of caring for and managing these forests every day. Thanks also to Inventory & Monitoring staff Kate Miller, Stephanie Perles, Elizabeth Matthews, John Paul Schmit, Brian Mitchell, Jim Comiskey, Ed Sharron, and many others for their dedicated work monitoring vegetation, analyzing data, sharing their findings far and wide in support of eastern forests, and for working with parks to identify management actions that promote forest resilience. 

About the author
_{Megan Nortrup is an information-sharing specialist with the National Park Service, National Capital Region Resource Stewardship and Science team. Image credit: NPS}