Too Many Deer: How Eastern National Parks Are Saving Their Forests

A deer-proof fence at Valley Forge National Historical Park: native plants thrive inside; outside, only invasive Japanese stiltgrass covers the ground. NPS.

A forest the deer can't reach

In summer 2023, writer Megan Nortrup and park botanist Ana Chuquin led reporters up a steep slope in Washington, D.C.'s Rock Creek Park to a shady thicket of spicebush. Elsewhere in the park, the forest was bare from the ground to about seven feet — the height deer can reach — while small plots fenced against deer stood out like green cubes on an open plain. Here, though, tree seedlings and saplings were coming up: deer control was working. It was the classic way eastern forests renew themselves — young trees wait until a big neighbor falls, then race to fill the gap.

The danger

Left alone, eastern forests can sustain themselves. But they now face a catastrophic loss, and national park forests — even more valuable ecologically than those around them — are no exception. A study by Park Service and other scientists, published in March 2023, names the main causes: overabundant white-tailed deer and invasive plants.

• Deer eat certain native plants and tree seedlings, leaving a few unpalatable natives and many invasives.
• Climate change adds more frequent droughts, extreme weather and pest outbreaks, which erode resilience.
• The forests drift toward invasive shrub thickets — better habitat for disease-carrying ticks.

A healthy understory under a closed canopy at Black River National Wild and Scenic River. NPS.

A forest severely browsed by deer at Roosevelt-Vanderbilt National Historic Site. NPS.

Why these forests matter

Forests store carbon, cool urban heat islands, protect drinking water, support biodiversity, bring money to gateway towns, and are good for our health. The United States holds about eight percent of the world's forests, covering about one-third of the country, mostly in the east.

• Nearly a quarter of Americans live within 100 miles of a forested eastern national park, and 16 percent within 30 miles.
• These parks, part of the Park Service's Resilient Forest Initiative, cover less than one percent of park land but get nearly 20 percent of its visitors.
• Lose them, and one in four Americans loses their benefits and faces more exposure to ticks.

Where deer control worked

Parks that began controlling deer years ago are seeing more regeneration:

Catoctin's and Valley Forge's huge gains partly reflect how far gone their forests were when they started.

Seedlings (blue) rose as deer (red) fell at Catoctin Mountain Park, from 2010, and Rock Creek Park, from 2013. NPS.

Retired Rock Creek biologist Ken Ferebee still wishes they had started sooner: "If we'd started a few years earlier, we would be a little better off," he said. "The forest wouldn't have been so far gone. We lost a lot of the seed bank."

Where it's failing

Many parks now carry regeneration debt: too few seedlings and saplings of canopy trees to replace the old ones. Some forests are one new pest or bad storm away from becoming impassable shrub thickets. A monitoring plot at Morristown National Historical Park shows the slide from 2009 to 2022.

One Morristown plot: in 2009, no seedlings and waist-high invasive Japanese barberry; in 2013, storm-felled canopy trees; in 2017, an impenetrable invasive thicket; in 2022, the native forest gone under invasive shrubs over 6 feet tall. NPS.

Parks noticed the imbalance decades ago. By the 1970s and 80s, native orchids, wildflowers and tree seedlings were vanishing. Gettysburg began reducing deer in the 1990s, while Catoctin and Rock Creek began tracking deer damage.

The Park Service's Inventory and Monitoring Division, created in 2000, revealed how widespread the problem is. In 12 years of data from 39 eastern parks, 27 were in imminent or probable danger of "regeneration failure" — and signs of deer eating seedlings were 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 quantitative ecologist Kate Miller, the study's lead author. "If there's a lack of seedlings and saplings, the forest can't maintain itself."

Forest regeneration status in eastern national parks. Only Acadia was secure; Saratoga, Morristown, Valley Forge, Catoctin and others faced imminent failure. NPS, from the March 2023 paper.

The wrong seedlings

Numbers aren't everything. In regeneration mismatch, seedlings belong to different species than the canopy. Small trees deer dislike — pawpaw, American holly, American hornbeam — can't reach the canopy and shade out the young canopy trees that could.

Pests make it worse. The emerald ash borer has wiped out ash in many eastern parks; beech leaf disease, now widespread in eastern parks except in Vermont, New York and D.C., threatens American beech; hemlock woolly adelgid and spongy moth are other threats. A diverse mix of young trees helps forests ride out pests and disease.

Gettysburg shows the trap. Once near complete forest failure, it has controlled deer since the early 1990s, and its regeneration is now among the highest in the East — but most of its young trees are ash, likely doomed, so the study still rated it a probable failure. Gaps it thinned to let in light were taken over by invasive plants; Gettysburg now has one of the highest invasive levels in the mid-Atlantic, and targeted invasive control and planting native canopy trees should help. "If the species composition of the forest changes, the animals, the birds, the insects, they change too," said Zach Bolitho, Gettysburg's lead for resource stewardship and planning. "Even from a cultural landscape viewpoint, it matters."

Gettysburg's understory has filled in since deer management began in the 1990s, but its lack of tree diversity leaves it exposed to pests and disease. NPS.

The long road back

Three decades of deer control in some parks show forests can be revived through long-term, adaptive management. Different parks hold different pieces of the answer: some have beaten back invasive plants, others have kept deer in balance long enough for plants to return, and others are bringing back fire. Controlling invasives, balancing deer and replanting are costly, and results may take decades — which is why funding from the Bipartisan Infrastructure Law and the Inflation Reduction Act matters: several projects are finding cost-effective ways to improve forest health.

Sources

Based on "Eastern Parks Reduce Herds to Benefit Forests, Deer, and People," by Megan Nortrup, information-sharing specialist with the National Park Service's National Capital Region, Park Science magazine, Volume 37, Number 2, Winter 2023 (December 29, 2023), National Park Service; a work of the United States government in the public domain.