Many forests in the northeastern U.S. have been invaded by non-native, undesirable vegetation, potentially leading to changes in fuel load and fire behavior. Common forest invasives include shrubs such as smooth buckthorn (Frangula alnus), Japanese barberry (Berberis thunbergii)and Asian honeysuckles (Lonicera spp.); vines such as Asian bittersweet(Celastrus orbiculata); trees such as locust (Robinia pseudoacacia); and grasses including Japanese stiltgrass (Microstegium vimineum) and vernal sweetgrass (Anthoxanthum odoratum). Though not always considered fire-prone, the region has seen numerous wildfires over the past century, including a 1947 fire in Maine that burned more than 200,000 acres in rural and small town areas and destroyed more than 800 year round homes. This same region has seen substantial population increases, creating a dense wildland-urban interface characterized by fuel beds often with a novel, invasive component.

At 12 sites on federal, state and private land in Maine, Maryland, Massachusetts, New Hamphshire, New Jersey, New York, Vermont, and Virginia, we compared fuels in invaded stands to those in nearby uninvaded stands. Using standard methods, we collected data to better represent forests of our region in BEHAVE fuel models. Each site had five plots in invaded and five in uninvaded conditions. Total planar intercept length for a condition was between 500-1800 ft at each site.

No single trend characterized all sites, and three fire-adapted pitch pine sites differed from the other sites, which were in hardwoods, mixed woods and spruce-fir (Picea rubens - Abies balsamifera). Uninvaded conditions had significantly more nonwoody litter at three sites, and slightly more at most sites. 1-hour timelag fuels, in tons per acre, were greater in invaded conditions at three sites, and less at one of the pitch pine sites. 10-hour fuels were greater in invaded conditions at one site, and less at the same pitch pine site as the 1-hour results mentioned above. Duff depth was less in invaded conditions at seven of the 12 sites, and greater at one pitch pine site. Fuel depth was greater in the invaded stand at Acadia National Park in Maine.

Invasive shrubs greatly increase the density of live fuels. However, the greatest potential for changed fire behavior appears to be at sites where the invasive plants included dense grass cover. In the absence of control, long-term changes in fire regime are possible at these sites. Grasses are not abundant in uninvaded forests of this region, but in invaded stands they represent a fine fuel that could carry a wildfire in a dry year. Where invasive grasses grow under black locust in what was formerly a pitch pine forest, the restoration of pitch pine to dominance will require concerted effort.