Flammability of plants contribute to fire behavior in natural ecosystems. As urban development continues within and near fire-dependent ecosystems in the southern United States, the flammability of plants in those ecosystems may influence the survival of human-built structures during wildfire. To assess the importance of plant flammability in the Wildland-Urban Interface (WUI), we compared flammability components—ignitability, sustainability, combustibility and consumability—of native understory species commonly found in pine flatwood and hardwood hammock ecosystems in the southern United States and used in landscape plantings.

During the summer of 2002, six species from five pine flatwood sites—Gaylussacia dumosa, Ilex glabra, Lyonia ferruginea, Vaccinium myrsinities, Myrica cerifera, and Serenoa repens—and six species from five hardwood hammock sites—Callicarpa americana, Ilex opaca, Quercus nigra, Vaccinium arboreum, Myrica cerifera, and Serenoa repens—were harvested for biomass analyses. Plant components were separated into live and dead foliage, accumulated litter on and under the plant, and small (<6 mm diameter) and large (>6 mm diameter) twigs, branches, and stems. Foliar biomass was further analyzed for leaf area, volatile solids, and energy content.

Statistical analyses revealed differences between ecosystem types and among species. Understory plants in pine flatwoods have higher ignitability (lower moisture content), sustainability (higher fuel loading), combustability (higher energy content), and consumability (higher proportion of biomass as fine fuel). In addition, the risk of plants being exposed to wildfire is greater in pine flatwood ecosystems because of the deep litter layer and dense understory compared to hardwood hammock ecosystems. Understory species in hardwood hammocks contain more total biomass because they contain more large stems, whereas fine fuel biomass was the same between ecosystems.

Serenoa repens, a species common to both ecosystems, had relatively low foliar energy content per gram, but individual plants present a high hazard due to the great amount of biomass. Ilex glabra is also hazardous to WUI structures because it has high foliar energy content and a great amount of foliar biomass per hectare in flatwoods. Callicarpa americana plants present the lowest fire hazard to WUI structures compared to all other species studied. Results will assist the development of regionally specific guidelines for firewise landscaping.