Forest Regeneration 2023

By Hannah Harkness, NCRN I&M Science Communications Intern

Growth of Box Elder Seedlings into Saplings at a Forest Plot at Rock Creek Park between 2018 and 2023

Left image
2018
Credit: NPS

Right image
2023
Credit: NPS

Overview

Forest regeneration in national parks of the National Capital Region (NCR) continues to show slow and steady improvement. While the stocking index remains relatively unchanged for most parks, seedling densities are increasing. Continued forest management efforts are important for preserving this progress and fully restoring regeneration in these forests for the future.

The Significance of Regeneration

Forests make up around three quarters of all landcover in national parks of the NCR. These protected forests (part of the Eastern Deciduous Forest ecosystem), have an ecological value even greater than forests in the surrounding landscape due to their large area and high biodiversity. For these forests to regenerate, new tree seedlings and saplings must grow and fill out canopy gaps left when larger trees die. This process is crucial for sustaining forests as well as the wildlife that lives within them.

Threats to Regeneration

Due to high populations of white-tailed deer, combined with increases in invasive plants, forests in most eastern national parks have regeneration levels too low to maintain forests in the long term. Forest pests and pathogens and climate change pose additional threats. The sustained impacts of these stressors are significantly changed forest structure and conversion of forests to invasive shrub thickets.

The National Capital Region, Inventory & Monitoring Network (NCRN I&M) has been collecting forest data since 2006 to better assess these threats and forest conditions.

Gradual Improvements to the Stocking Index

Forest regeneration is slowly improving across the region, with the most recent improvements at Monocacy National Battlefield, in the western portion of the Chesapeake & Ohio Canal National Historical Park, at Manassas National Battlefield Park, and at Rock Creek Park when compared to 2022 forest regeneration data (Figure 1).

The Stocking Index is an important metric for measuring forest regeneration. The advantage of the stocking index is that it uses data across the entire park and takes into account all sizes of regenerating trees.

It calculates regeneration potential on a plot-by-plot basis based on the abundance and size of saplings and seedlings. Because they are more likely to survive and grow into mature trees, larger seedlings and saplings have more influence on the stocking index score.

A park is considered to have sufficient regeneration if the stocking index shows that 67% of its forest plots are adequately stocked with seedlings and small saplings. Since forest monitoring began in 2006, no NCR park has had more than 30% of their plots adequately stocked.

Figure 1. Stocking Index 2019-2023. For a park or smaller distinct park area to have healthy regenerative capacity, 67% of plots must be adequately stocked (dashed line). The number of NCRN forest monitoring plots is listed in parenthesis after each park name. Asterisks show parks managing deer. T-shaped error bars indicate 95% confidence intervals.

NPS

Narrative Description of Figure 1

This bar graph shows the percent of plots adequately stocked in each park. The x-axis shows the percentage of plots adequately stocked from zero to 100. On the y-axis, the 17 parks of the NCR are listed alphabetically.  There is one horizontal bar per park. The parks with the greatest number of adequately stocked plots are B-W Parkway, GW Parkway Below Gorge, and Harpers Ferry. The parks that have no adequately stocked plots are C&O Far West, National Capital Parks East, and Piscataway. No parks have more than 30% of plots adequately stocked.

Positive Change in Seedling Density

There was a general increase in seedling density in most parks (Figure 2). This trend is consistent with previous years. But while seedling density is improving in the region, it does not guarantee future regeneration in these forests. It takes many years for seedlings to grow into saplings and resilient mature trees, and they are vulnerable to many threats along the way.

Figure 2. Change in seedlings per hectare 2009-2023 in parks and smaller distinct park areas. Each circle represents the average seedling data of the last visit to each plot (a complete sample of all plots at the park). Vertical lines are 95% confidence intervals. Dashed blue vertical lines indicate the year a park began deer population management. The y-axis varies from park to park due to a large variance in number of seedlings and the size of confidence intervals.

NPS

Narrative Description of Figure 2

This figure shows seedling density per hectare in 18 parks from 2009 to 2023. Eighteen small graphs are arranged alphabetically by park in six rows of three. Each has time in years on the x-axis, and seedlings per hectare on the y-axis. The parks with the greatest current seedling densities are Greenbelt and Wolf Trap. The parks showing increases in seedling density are Antietam, B-W Parkway, Catoctin, C&O Potomac Gorge, C&O Central, C&O West, C&O Far West, Harpers Ferry, Manassas, Monocacy, Piscataway, Prince William, and Wolf Trap. 

Implications of Stocking Index and Seedling Density

Together, the stocking index and seedling measures help to give a comprehensive view of the current state of regeneration. Most parks managing deer populations show improvements in seedling densities, but the stocking index remains largely unchanged.

At Rock Creek Park in 2023, the stocking index did increase while seedling density decreased. This is due to one plot where many box elders shot up from seedlings into saplings, disappearing from the seedling ranks and bumping up the Stocking Index.

Monocacy National Battlefield, and the western reaches of C&O Canal also saw increases to the Stocking Index. This is attributed to an increase in large Pawpaw at one plot in each park.

Meanwhile, many of the new seedlings at Manassas National Battlefield Park are ashes, likely doomed by the now naturalized emerald ash borer. Some other emerging seedlings are non-canopy species (e.g., pawpaw) that will be unable to fill canopy gaps.

It is important to note that neither the stocking index nor measures of seedling density consider tree species composition. Native canopy species and non-natives or non-canopy species are equally considered in both measures. Other ways to measure forest regeneration that take into consideration factors like native/non-native status were used in a 2023 analysis of forest data from 39 northeastern national parks published in Ecological Applications.

Parks Taking Action

Some parks are planning and implementing integrated forest management tactics that may help to mitigate the significant threats to regeneration in the region. Catoctin Mountain Park, which has been removing invasive plants and reducing deer populations for over a decade, plans to expand their work into an area they’ve cleared of Japanese barberry and other brambles and plant more than 5,000 native trees.

George Washington Memorial Parkway is developing an Urban Forest Management Plan for their park with researchers at Virginia Tech University, and in 2023 Rock Creek Park worked with the Rock Creek Conservancy to design a Forest Resilience Framework to help manage forest resources and fundraise for management support.

Overall, low regeneration levels remain a cause for concern in the region. Significant and sustained effort is needed to help restore NCR forests for the long term.

Further Reading

• 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., Fisichelli N. 2023. Overabundant deer and invasive plants drive widespread regeneration debt in eastern national parks. Ecological Applications.
• Schmit JP, Matthews ER, Brolis A. 2020. Effects of culling white-tailed deer on tree regeneration and Microstegium vimineum, an invasive grass. Forest Ecology and Management. 463(118015).
• Schmit, JP. and M. Nortrup. 2013. NCRN Resource Brief: Forest Regeneration 2013.
• Schmit, JP and M. Nortrup. 2020. Stiltgrass and Tree Seedling Recovery
• Schmit JP, Matthews E, Brolis A. 2023. Trends in woody forest vegetation in Prince William Forest Park, 2006–2017. Natural Resource Report. NPS/NCRN/NRR—2023/2495. National Park Service. Fort Collins, Colorado.

Learn More about the National Park Service's Inventory & Monitoring Efforts

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