Fire Ecology in Virginia

the National Park Service uses fire to maintain historic viewsheds, with open fields rather than forests, at Manassas National Battlefield Park
Source: National Park Service, Wildland firefighter uses drip torch to start prescribed burn at Manassas National Battlefield Park (November 21, 2019)

Before the Europeans arrived, it was common for small fires to travel through Virginia forests every 3-9 years. Lightning would spark a fire in the leaf litter, burning the leaves and twigs on the ground. Regular fires prevented the accumulation of enough fuel for a fire to become hot enough to kill healthy pine trees.

The species composition of the woods was determined in part by fire. In dry areas, a species like maple with thin bark could be killed by regular while chestnuts, oaks, hickories, and pines survived. In riparian areas where wet soils limited the ignition of fires, thinner-barked trees could thrive.

creeping ground fire moving down slope at Shenandoah National Park during the Lewis Mountain Fire
Source: Shhenandoah National Park, Creeping ground fire (2006)

Virginia's most fire dependent tree is the longleaf pine. Its apical buds are surrounded by needles during the initial growing stage, protecting it from fire. Longleaf pine out-competed other trees in southeastern Virginia, where natural fires were common, due to its fire resistance.. Longleaf pines created "the forest that fire made."

Native Americans and the early European explorers would commonly travel through recently-burned forests, where the undergrowth had been removed but the bark of the trees only lightly scorched. Fires created a more-open savannah than modern forests, and favored oaks and pines:¹

Young oaks, by growing large roots and sprouting repeatedly after top-kill, would have persisted through multiple cycles of burning until they were able to bolt to a more fire-resistant size during the occasional fire-free interval of abnormal length, i.e., a length on the order of 10–15 years. Burning also favored yellow pines, particularly on relatively dry, infertile sites with sandy, well-drained soils on ridgetops and south- or west-facing slopes. On these sites, post-fire vegetation recovery was slow, and fires often burned with greater severity than elsewhere.

Fire essentially operated as a filter on tree establishment within fire-prone sites by destroying the seedlings of fire-sensitive species.

excessive buildup of fuel in a hilltop forest in Fauquier County

Trees are always vulnerable to storms. An entire tree may be broken by a windstorm, but most often just some branches break off. In that case, the tree will normally survive, because wound tissue grows to restore a protective covering that keeps insects, fungi, and bacteria out of the living phloem and xylem cells of the tree. Trees are also vulnerable to damage by fire, caused most often in nature by lightning. The broken limbs fall to the forest floor and gradually rot, recycling the nutrients into the soil.

In the last century, fire suppression as advocated by Smoky Bear has allowed "dead and down" timber to accumulate. In some places, the woods have accumulated an unnatural fuel load. A wildfire could burn much hotter due to the additional fuel, and have a greater-than-normal impact. The additional heat and the longer burning time could penetrate the bark that normally protects a tree, killing the cells and causing the tree to die.

Virginia state law prohibits open burning within 300 feet of woodland, brushland or fields containing dry grass or other flammable material, from February 15-April 30 except between 4pm-midnight. This includes campfires in the woods - 30 percent of Virginia's wildfires are caused by open burning.²

According to the Virginia Department of Forestry:³

Virginia's wildfire season is normally in the Spring (March & April) and then again in the Fall (October & November). Why do you think this is so? The answer is that during these times the relative humidities are usually lower, winds tend to be higher, and the fuels are cured to the point where they readily ignite. Also hardwood leaves are on the ground providing more fuel, and allowing sunlight to directly reach the forest floor, warming and drying the surface fuels.

Fire activity fluctuates during each month and also varies from year to year. During years Virginia gets adequate rain and snow keeping fire occurance low. Lack of moisture during other years means extended periods of warm, dry, windy, days and therefore increased fire activity.

the Rocky Mount Fire in April, 2016 burned over 10,000 acres in Shenandoah National Park, and sent smoke towards Harrisonburg
Source: National Aeronautics and Space Administration (NASA), Rocky Mount Fire, Virginia

More than just the woods are at risk. Homes have been built in the woods, and at the edge of the woods. Fires can start at the "wildland urban interface" from human activity - especially burning of trash by homeowners who live far from an urban garbage collection route. In addition, wildland fires that might be of minor risk are now threats to high-value rural homes and the people who live in them, at the end of long dirt roads with no alternative exit route.

The 2001 Communities at Risk report identified 855 developments in Virginia that could be affected by a wildland fire:⁴