Few Young Trees Are Growing Back in National Capital Region Parks, 2022 Data Show

When a forest tree dies, seedlings and saplings grow up to fill the gap, and that regeneration is how forests sustain themselves. In National Capital Region (NCR) national parks, where forests cover nearly three quarters of the land, regeneration is a concern: no park has reached the level considered healthy since monitoring began. An NPS science communication intern wrote this look at the monitoring data through 2022.

A forest plot at Catoctin Mountain Park in 2009 (left) and 2022 (right). In 2022 the plot is dominated by maple and birch seedlings. Credit: NPS.

What holds regeneration back

Large populations of hungry white-tailed deer (more than 8 per square kilometer, per a 2003 study), crowding by invasive plants and other factors keep seedlings from growing into saplings that could replace canopy trees. Over time these pressures can reduce tree species diversity and density, harming the forest and the plants and animals that rely on it. Parks reducing deer numbers include Antietam and Monocacy National Battlefields, Catoctin Mountain Park, Chesapeake and Ohio Canal and Harpers Ferry National Historical Parks, Manassas National Battlefield Park and Rock Creek Park.

The Stocking Index

The Inventory and Monitoring Network has collected long-term forest data since 2006, before deer management began in the region. Regeneration is scored with the Stocking Index, a plot-by-plot measure of regeneration potential that counts seedlings and small saplings, their size and their spread across the park, with larger saplings scoring higher because they are likelier to survive. Because seedlings take many years to become saplings and young trees, an area's score moves slowly.

A park has healthy regeneration if 67 percent of its forest plots are adequately stocked. Since monitoring began, no NCR park has reached 67 percent or even exceeded 30 percent.

Figure 1. Stocking Index, 2018 to 2022. A park needs 67 percent of plots adequately stocked (dashed line) for healthy regeneration; asterisks mark parks managing deer; error bars show 95 percent confidence intervals. NPS.

Fire at Prince William

At Prince William Forest Park, three of the five plots considered adequately stocked were burned in an accidental fire in 2006. They have a far denser understory, with many more saplings, seedlings and shrubs than unburned areas. Without those plots, the park's forest shows significant declines in seedlings and saplings.

Figure 2. Seedlings per hectare in burned (red circles) and unburned (gray triangles) plots in Prince William Forest Park. Vertical lines show 95 percent credible intervals. NPS.

Seedlings are rising in places, but that is not enough

Seedling density is increasing over time in some parks. Because the index counts both seedlings and small saplings and weights larger ones more, more seedlings take time to lift a score. The gains are promising but no guarantee: small seedlings face many threats on the way to becoming resilient saplings.

Figure 3. Change in seedlings per hectare, 2009 to 2021, parks ordered from lowest seedling density at upper left to highest at lower right. Dashed blue lines mark the year a park began deer management. NPS.

The measures also ignore which species are coming back. Native canopy trees such as oaks, maples and hickories count equally with non-natives (tree of heaven), non-canopy species (pawpaw) and species facing pests (ash trees and the emerald ash borer). Counting only canopy species likeliest to survive would leave even fewer plots stocked.

Early signs of progress

Parks with deer management show some improvement. Catoctin Mountain Park has a 19-fold increase in seedlings since management began, and two of its 49 forest plots count as adequately stocked. Rock Creek Park's seedlings have tripled and one of 19 plots counts as stocked, though that plot is heavily populated by box elder, an understory tree that will not form mature canopy. Overall, Stocking Index scores have hardly changed since the 2008 to 2011 period.

A wider study, and plans

A study by Miller and colleagues of 12 years of data from 39 eastern national parks, including NCR parks, found that 70 percent of forests have too little regeneration to replace canopy trees as they die or fall. It named overabundant deer and invasive plants as leading causes, and found deer management vital but not enough alone. Integrated management that also controls invasive plants and pests, restores targeted species and sometimes uses prescribed fire or canopy thinning is critical, with deer reduction the most effective first step. Parks with long-term (10 years or more) deer management are seeing seedlings re-establish.

Some NCR parks are exploring more. George Washington Memorial Parkway is working with Virginia Tech researchers on an Urban Forest Management Plan, and Rock Creek Park is working with its partner group, Rock Creek Conservancy, on a Forest Resilience Framework planned for the end of 2023.

Sources

• National Park Service, National Capital Region Inventory and Monitoring Network, "Forest Regeneration 2022," written by an NPS science communication intern, https://www.nps.gov/articles/000/forest-regeneration-2022.htm; rewritten in hubnx's own words. The study discussed is Miller and colleagues (2023), Ecological Applications.