Are National Capital Area forests replacing themselves? Deer and seedling data

Forests cover about three quarters of the land in the National Capital Area (NCA) national parks, and keeping them depends on regeneration: seedlings and saplings that grow up to replace large trees as they die. A National Park Service article, written by a science communication intern with the National Capital Region Network Inventory & Monitoring program, reports the latest look using 2021 data.

Healthy regeneration after fire at Prince William Forest Park, with many oak seedlings. NPS photo.

What threatens regeneration

Forests in these parks face over-abundant white-tailed deer, new pests and pathogens, invasive plants, development of surrounding land, a lack of fire, more frequent extreme weather and a changing climate. The article names deer as one of the largest stressors: a population above about eight per square kilometer (20 per square mile) has sharply reduced regeneration by browsing, eating and damaging seedlings so they cannot grow into saplings, and over time lowering tree diversity and density. Many parks, including Antietam, Monocacy, Catoctin Mountain, Manassas, Harpers Ferry and Rock Creek, are now reducing deer.

How it is measured

The network has monitored forests since 2006, before deer management began, and its crew collects data on seedlings, saplings and mature trees at more than 400 plots. In a normal year it visits over 100 plots and completes a round of all plots every four years. COVID-19 cut operations in 2020, so the round took two years: three parks were visited in 2020 and the rest in 2021. Two measures are used:

• The Stocking Index scores each plot on the number, size and spread of seedlings and small saplings, with larger saplings scoring higher because they are likelier to survive. A park has healthy regeneration if 67 percent of its plots are adequately stocked. Since monitoring began in 2006, no NCA park has had more than 30 percent.
• Seedling density is more sensitive. It swings from year to year with seed production, deer, weather and disease, and a rise does not guarantee regeneration, since many years pass before seedlings become resilient saplings.

Figure 1. The Stocking Index, 2017 to 2021; healthy regeneration requires 67% of plots to be adequately stocked. NPS.

Figure 2. Change in seedlings per hectare, 2009 to 2021. NPS.

A caution

Neither measure shows which species are regenerating. Native oaks, maples and hickories count the same as non-natives such as tree of heaven, non-canopy trees such as pawpaw, and ash trees expected to die from emerald ash borer, so counting only canopy species likely to survive would leave fewer plots stocked.

Park by park

• Potomac Gorge sites on both sides of the river have some of the highest Stocking Index scores.
• Catoctin: seedlings have increased 13-fold since deer management began, and one of its 49 plots now counts as adequately stocked.
• Rock Creek: seedling numbers have nearly tripled, but no plot is adequately stocked yet.
• Monocacy and Wolf Trap: seedlings keep improving, but neither registers on the Stocking Index.
• Antietam: it registers on the index, but seedlings are declining, because reducing deer there is taking longer.
• Prince William Forest Park: both measures are falling. The park does not manage deer, and fires in six plots caused a burst of seedlings that will fade as they compete. Beech leaf disease, newly identified there, is expected to kill many American beech.

Overall, regeneration in the area remains a concern, and Stocking Index scores have changed very little since 2006 to 2009. The article calls the start of deer management promising and says continued effort is needed.

Sources

• National Park Service, National Capital Region Network, "Forest Regeneration 2021" (https://www.nps.gov/articles/000/forest-regeneration-2021.htm), by a science communication intern with the network; a photo of the monitoring crew is left out; rewritten in hubnx's own words.