Fire and the Future of the Forests at Prince William Forest Park

By Nicholas Tait, science communication intern with the National Park Service's National Capital Region Inventory and Monitoring Network, written in 2023.

A prescribed fire on the forest floor near Oak Ridge Campground in spring 2023, just west of the 2006 B-Loop wildfire. NPS/Mike Custodio.

The forest at Prince William Forest Park in Virginia is changing. Fire, and the suppression of fire for most of the past century, has altered the makeup and structure of its piedmont forest, which has also been shaped by white-tailed deer, pests, diseases and invasive species. It looks different from the forest of a hundred years ago.

The Park Service's National Capital Region Inventory and Monitoring Network began monitoring the park's forest vegetation to measure these changes, and its scientists analyzed data from 2006 to 2017 to see what fire had done.

The B-Loop Fire

On March 27, 2006, just as the network began its first season of forest monitoring, a human-caused fire started near the B-loop of Oak Ridge Campground. The B-Loop Fire burned 318 acres over nine days before firefighters put it out. It was unusually hot and destructive, killing up to 75% of canopy trees in places, and it wiped out the vegetation in six of the network's monitoring plots.

The B-Loop Fire burned 318 acres, destroying the vegetation in six forest monitoring plots. NPS.

Comparing those six burned plots with the 139 unburned plots across the park, scientists found the burned plots had burst into growth and now look markedly different:

• 3.5 times as many tree seedlings and 5 times as many shrub seedlings.
• Less deer browsing on seedlings, perhaps because thicker growth protected them, or simply because there were too many for the deer to eat.
• Young trees dominated by fire-tolerant oaks and hickories, where unburned plots are usually dominated by fire-sensitive American beech and red maple.

The fire showed that burns, prescribed or not, can have a lasting effect on a forest's structure and makeup.

Why oaks and hickories need fire

Piedmont forests of the eastern United States have long followed a predictable succession. Sun-loving pioneers such as Virginia pine and tulip poplar are among the first trees to take over cleared land. Once they form a canopy, shade-tolerant species such as oaks, hickories, red maple and black gum grow up beneath them. Over centuries of recurring fires, thick-barked oaks and hickories proved more fire-resistant than maples and black gum, and with enough burns they came to rule the canopy, forming forests that renew themselves for generations and support a wide range of wildlife. Most eastern forest life is adapted to that pattern.

In the early 20th century, Americans began suppressing fire, and fires grew far rarer in the East. Suppression favored shade-tolerant, fire-sensitive trees and set off mesophication: fewer fire-tolerant oaks and hickories, and more "mesic" species suited to cool, moist, shady, fire-free conditions, such as red maple and American beech.

After a long absence of fire, oak and hickory are declining across the park's unburned forest while mesic species keep their hold. Left alone, the mesic trees will eventually dominate the canopy, likely reducing biodiversity, and heavy deer browsing on seedlings adds to the problem. Oak forests matter to a great deal of other life. Other studies have found that:

• acorns are a crucial food for many vertebrates (Brose et al. 2014);
• birds are more abundant in oak-dominated than maple-dominated forests, probably because there is more food (Rodewald and Abrams 2002);
• oaks are the most important plant group supporting butterfly and moth diversity in North America, followed by willows and cherries, which are uncommon at Prince William, and pines, which are declining there (Narango et al. 2020);
• pure oak stands have significantly more diverse plants on the forest floor than stands with maples (Fralish 2004; Rogers et al. 2008).

Prescribed fire

A few months after the 2023 Oak Ridge prescribed burn: blackened trunks, but the forest floor already regrowing. NPS/Brolis.

Prescribed burns under controlled conditions can help reverse mesophication. They also thin dense woody growth and reduce the risk from future wildfires by burning off hazardous fuel such as dead trees and leaf litter.

The park's managers burned 46 acres between the Oak Ridge Trail and Scenic Drive in March 2023, aiming to restore fire to the forest's ecology and cut hazardous fuel, nonnative species and dense shrubs. They planned to burn the remaining 59 acres in fall 2023 or early spring 2024. An NPS fire effects team will follow the burn for 10 years; the hope is to see more oak and hickory regeneration, less deer browsing and fewer nonnative plants, as after the B-Loop Fire. Prescribed fire, together with reducing the deer population to ease browsing on new seedlings, will be crucial to keeping the forest resilient.

Sources

• National Park Service, "Fire and the Future of the Forests at Prince William Forest Park": https://www.nps.gov/articles/000/fire-and-the-future-of-the-forests-at-prince-william-forest-park.htm
• 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. https://doi.org/10.36967/2296913
• Studies cited: Brose, Dey and Waldrop 2014 (USDA Forest Service Gen. Tech. Rep. NRS-135); Fralish 2004 (Gen. Tech. Rep. SRS-73); Narango, Tallamy and Shropshire 2020, Nature Communications 11:5751; Rodewald and Abrams 2002, Forest Science 48; Rogers et al. 2008, Ecology 89(9).