Forests and deer in the National Capital Area: what 2020 monitoring shows

Tree regeneration is how a forest sustains itself: seedlings and saplings grow and survive to replace large trees as they die. A National Park Service brief looks at how it fared in 2020 in national parks of the National Capital Area, where large white-tailed deer populations can hold it back.

A dense patch of young white ash seedlings and saplings at Catoctin Mountain Park. NPS/Loncosky.

Why deer matter

Deer densities above 8 per square kilometer, as in these parks, can cut regeneration significantly by browsing seedlings so they never grow into saplings. Over time that can also reduce the diversity and density of tree species, so many parks are working to reduce their deer. The Park Service's National Capital Region Network Inventory and Monitoring program began long-term forest monitoring in 2006, before any deer management, and tracks seedlings, saplings and mature trees in over 400 plots across the parks.

Regeneration in 2020

Because of reduced operations during the COVID-19 pandemic, the field crew measured only three parks in 2020: Catoctin Mountain Park, Monocacy National Battlefield and Rock Creek Park, all with deer management under way.

Seedling counts are the most sensitive measure, but they swing with seed production, browsing, weather and disease, and more seedlings do not guarantee future regeneration, since small seedlings face years of threats. In 2020 seedling numbers kept improving in all three parks. At Rock Creek tree seedlings have almost tripled since deer management began, and at Catoctin they have risen 13-fold.

Change in seedling numbers at each park. NPS figure.

A better measure is the Stocking Index, which looks at plot-by-plot regeneration potential, counting seedlings and small saplings, their size and how they are spread across the park. A park is considered to be regenerating healthily if 67 percent of its forest plots are adequately stocked. Since monitoring began in 2006 no park in the area has had more than 30 percent. In 2020 the seedling recovery that Catoctin has seen for years finally registered: one of its 49 forest plots is now adequately stocked.

Percent of plots adequately stocked in each park. NPS figure.

Which trees come back

Deer browse some species, such as white ash, in preference to others, such as American beech, pawpaw or the invasive tree of heaven, which can change a forest's structure and the habitat it offers. At Catoctin, where browsing was once so heavy that almost no seedlings survived, seedlings have risen 13-fold and are dominated by white ash. Ash made up almost 75 percent of seedlings in the early years of deer management, and over the most recent four years, other species are starting to come back and ash is about 62 percent.

A young white ash seedling at Catoctin Mountain Park, a species deer prefer to browse. NPS/Loncosky.

The ash resurgence is promising, but ash saplings are likely to die from the emerald ash borer, so other species are more likely to form future canopies, and watching species diversity matters as much as raw counts. Forest covers about three quarters of the land in the area's national parks, and the brief says the lack of regeneration remains a concern: Stocking Index scores have changed very little since 2006 to 2009. Deer management has made a promising start, but continued effort is needed.

Sources

• National Park Service, "Forest Regeneration 2020" (https://www.nps.gov/articles/000/forest-regeneration-2020.htm), public domain as a work of the U.S. government; data as of the 2020 monitoring. The page's list of journal references and its video are not reproduced.
• National Park Service; rewritten in hubnx's own words.