Forest Regeneration 2024

By Emma Brentjens, NCRN I&M Science Communications Intern

Seedling growth at a forest plot in Catoctin Mountain Park

Left image

2007

Credit: NPS

Right image

2024

Credit: NPS

Forest Regeneration: How Slow and Steady Wins the Race

Forest regeneration continues its slow recovery in National Capital Region (NCR) parks. While year-to-year changes in the growth of young trees can be small, conditions have improved over time at most parks since routine forest monitoring began in 2006. Seedling density in several parks continues to inch up.

Forests are an important habitat that accounts for three fourths of the landcover in national parks of the greater Washington, DC, region (including Maryland, Virginia, and a small part of West Virginia). Regeneration, the process of seedlings growing up to replace older trees when they die, sustains forests. Without it, gaps in the canopy are never filled, and forests decline. Forest vegetation data collected by National Capital Region Inventory and Monitoring Network (NRCN I&M) staff starting in 2006 showed that regeneration levels in all NCR parks was very low. In the years since, most NCR parks have ramped up invasive plant management, begun deer population management, and steady regeneration declines have begun to reverse.

The capital region has increasingly urbanized since at least the 1990s, and a lack of significant natural predators and favorable landscape changes have helped deer populations swell. A 2023 National Park Service study using 2008-2019 monitoring data from northeast and mid-Atlantic national parks identified white-tailed deer overpopulation and the proliferation of invasive plants as the biggest threats to forest regeneration.

Assessing forest regeneration: Stocking Index

To determine the current state of regeneration in park forests, NRCN I&M staff analyzed forest vegetation data using the Stocking Index (a calculation of regeneration potential on a plot-by-plot basis) and rates of seedling density per hectare.

The Stocking Index is calculated based on the number and size of seedlings and small saplings in each monitored plot. It includes seedlings over 15 cm tall and gives higher value to larger plants. As a result, it is less sensitive to changes in the density of small seedlings. For forest regeneration to be considered acceptable, 67% of plots must be adequately stocked.

The change in Stocking Index between 2006-2009 to 2020-2024 for all parks is shown below in Figure 1. Since 2006-09, most parks have seen an increase in the Stocking Index, including most sites managing deer populations. No parks have more than 30% of plots adequately stocked.

Figure 1. Stocking Index (left: 2006-2009, right: 2020-2024). A park or smaller distinct park area is considered healthy if the Stocking Index shows that 67% of plots are adequately stocked with trees (dashed line). Asterisks show parks managing deer. The number of plots monitored is listed in parenthesis after each park name. T-shaped error bars indicate 95% confidence intervals. Stocking index includes canopy and non-canopy, and native and non-native tree species.

Narrative figure description

These horizontal bar graphs show the percent of plots adequately stocked in each park. The data in these graphs were collected from one visit to each plot over four to five years.The x-axis shows the percentage of plots adequately stocked from zero to 100 percent. On the y-axis, 18 parks and park subsites in NCR are listed alphabetically and correspond to one horizontal data bar. Parks managing deer and marked with an asterisk are: Antietam National Battlefield, Catoctin Mountain Park, Chesapeake & Ohio Canal National Historical Park Potomac Gorge (from Chain Bridge flats to Bealls Island), Greenbelt Park, Harpers Ferry National Historical Park, Manassas National Battlefield Park, Monocacy National Battlefield, and Rock Creek Park. Stocking Index from 2020-2024 was generally higher at most parks than in 2006-2009. The parks with the greatest percentage of adequately stocked plots in the 2020-2024 monitoring cycle were Baltimore Washington Parkway, George Washington Memorial Parkway Below Gorge, and Harpers Ferry National Historical Park. The parks that had no adequately stocked plots in 2024 are Chesapeake & Ohio Canal National Historical Park Far West, Greenbelt Park, National Capital Parks-East, Piscataway Park, and Wolf Trap National Park for the Performing Arts.

Seedling density: Steady increase at many sites

Most parks that are managing their deer populations show growth in seedling density (Figure 2, graphs with blue dashed lines). This figure shows changes in average seedling density by park. Each point represents the cumulative average of a four- to five-year monitoring cycle. Seedling density has steadily increased at Catoctin Mountain Park, Harpers Ferry National Historical Park, and Monocacy National Battlefield. Rock Creek Park, which also manages deer, has seen a decline in seedlings over the past three years, however, this is because many of their seedlings have grown into saplings which in turn pushed up the park Stocking Index score.

Some parks not managing deer have had small increases in seedling densities, namely Baltimore-Washington Parkway and Chesapeake & Ohio Canal Central (from Violettes Lock to Harpers Ferry). However, most of the sites with declines in seedling density have not implemented deer management: Chesapeake & Ohio Canal National Historical Park West (from Harpers Ferry to Four Locks) and Far West (from Four Locks to Cumberland), George Washington Memorial Parkway’s Potomac Gorge area, National Capital Parks - East in DC, Piscataway (includes Fort Washington Park), Prince William Forest Park, and Wolf Trap National Park for the Performing Arts.

While increasing seedling density in many parks is promising, seedling levels are highly variable and do not guarantee forest regeneration. Continued management is necessary to sustain improvements.

Figure 2. Change in seedlings per hectare from 2009-2024 in parks and smaller distinct park areas. Each point represents a running average of seedling density across all plots at a park (measured over a four- to five-year monitoring cycle). Vertical lines are 95% confidence intervals. Dashed blue vertical lines indicate the year a park began deer population management. Seedling density includes all canopy and non-canopy, and native and non-native species. NPS

Narrative figure description

This figure shows seedling density per hectare in 18 parks and park subunits from 2009 to 2024. Eighteen small graphs are arranged alphabetically by name in six rows of three. Each has time in years on the x-axis, and seedlings per hectare on the y-axis. The y-axis varies from park to park due to a large variance in number of seedlings and the size of confidence intervals. The parks with the greatest current seedling densities are Wolf Trap National Park for the Performing Arts and Manassas National Battlefield Park. The parks showing increases in seedling density from 2023 are Antietam National Battlefield, Baltimore Washington Parkway, Catoctin Mountain Park, Chesapeake & Ohio Canal National Historical Park Potomac Gorge (from Chain Bridge flats to Bealls Island) and Central (from Violettes Lock to Harpers Ferry), Harpers Ferry National Historic Park, Manassas National Battlefield Park, and Monocacy National Battlefield.

Conclusion: Park Management and Monitoring

Forest health and regeneration condition (measured by the Stocking Index and seedling density rates) have improved at several NCR parks since 2006. To expand and build on these improvements, continued forest management including control of deer and invasive plant populations is crucial. Parks across the NCR continue to implement integrated forest management tactics, including Catoctin Mountain Park where park managers are removing invasive shrubs and planting trees, and at Rock Creek Park where staff continue to manage deer and invasive plants with support from the Rock Creek Conservancy and their Forest Resilience Framework plan, forthcoming in 2025.

As part of deer management, parks are also monitoring the spread of chronic wasting disease (CWD) among white-tailed deer. This neurological disease spreads more easily in areas of high deer density. And although it is fatal, studies show that CWD does not reduce deer populations until infection rates reach about 29% of the population. Current CWD rates are very low (less than one percent) and rates approaching 29% are not likely to occur in the NCR for at least 20 years (Scott Bates, email message to authors, February 2025). Since 2024, four NCR parks—Antietam National Battlefield, Harpers Ferry National Historical Park, Manassas National Battlefield Park, and Monocacy National Battlefield—have confirmed cases of CWD.

Read more about ongoing efforts across northeastern national parks through the Resilient Forest Initiative: https://www.nps.gov/im/resilient-forests-initiative.htm.

Learn More

About the National Park Service's Inventory & Monitoring Efforts

To help protect natural resources ranging from bird populations to forest health and water quality, National Park Service scientists perform ecological Inventory & Monitoring (I&M) work in parks across the country. The National Capital Region, Inventory & Monitoring Network (NCRN I&M) serves national parks in the greater Washington, DC area (DC, MD, VA, and a bit of WV). To learn more about NCRN I&M forest monitoring, you can visit the NCRN forest monitoring webpage.

Further Reading: Forest Regeneration in the Press

Since the 2023 publication assessing regeneration in eastern national parks, forest management efforts in the NPS have gained media attention.

Park Service works to change the future of failing forests, Chesapeake Bay Journal, 2025. NRCN Quantitative Ecologist John Paul Schmit was interviewed about the state of forest regeneration in Mid-Atlantic and Northeast national parks.

Study finds most forests in eastern national parks at risk, WAMC Northeast Public Radio Midday Magazine, 2024. Kate Miller, Quantitative Ecologist with the Northeast Temperate I&M Network, spoke with WAMC about threats to forest regeneration and management efforts in northeast parks.

Could DC Lose Its Forests?, City Cast DC, 2024. In this podcast episode, NCRN Quantitative Ecologist John Paul Schmit discussed forest regeneration challenges in DC parks and how managing deer and invasive plants can improve forest regeneration.

Local Forests Are Failing To Produce The Next Generation Of Trees, DCist, 2023. Former NCRN Program Manager Liz Matthews discussed NPS monitoring and analysis and the importance of sustained management to prevent regeneration failure.

References

Horsley, S.B., S.L. Stout, and D.S. DeCalesta. 2003. White-tailed deer impact on the vegetation dynamics of a northern hardwood forest. Ecological Applications 13:98-118.

Miller K., Perles S., Schmit J.P., Matthews E., Weed A., Comiskey J., Marshall M., Nelson P., and Fisichelli N. 2023. Overabundant deer and invasive plants drive widespread regeneration debt in eastern national parks. Ecological Applications.

NPS. 2024. Catoctin Mountain Park Pursues Forest Re-growth and Restoration through Bold Efforts and Infusion of Funding. Available at: https://www.nps.gov/cato/learn/nature/forests.htm.

Schmit, J.P., E.R. Matthews and A. Brolis. 2000. Effects of culling white-tailed deer on tree regeneration and Microstegium vimineum an invasive grass. Forest Ecology and Management 463:118015.

Wisconsin Department of Natural Resources. “How does CWD affect Wisconsin’s deer population,” Field Notes, January 2025, https://dnr.wisconsin.gov/topic/Research/articles/January2025#articleThree.

Where this belongs

• Anacostia Park
• Antietam National Battlefield
• Baltimore-Washington Parkway
• Catoctin Mountain Park
• Chesapeake & Ohio Canal National Historical Park
• Fort Dupont Park

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