Fire and Invasive Plants -- Combustibility of Native and Invasive Exotic Plants

Alison C. Dibble, U.S. Department of Agriculture, Forest Service, Northern Research Station, 686 Government Rd., Bradley, ME 04411

William A. Patterson III, Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA 01003

Robert H. White, U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Madison, WI 53705-2398

The ease with which a plant fuel catches fire – its combustibility, or flammability, might differ between native plants versus invasive exotic plants that overtake their habitat. By comparing combustibility in these two groups of plants, we are seeking to improve the effectiveness of prescribed fire and the assessment of fire hazards in the Northeastern U.S. Risk of wildfire could be greater in the wildland-urban interface if invasive plants are dense and have higher combustibility than the native species. Conversely, a fire-prone ecosystem invaded by exotics might have less frequent fire return and lower severity, with consequences for fire-dependent species, e.g., federally endangered Karner Blue butterfly and its host plant, a native lupine of pitch pine forests.

With support from the Joint Fire Science Program (JFSP) we, with Mark J. Ducey of University of New Hampshire, are modifying the Rothermel fuel models to better represent conditions in the Northeast. Heat content is a missing link, especially regarding common shrubs and herbs, and some invasive exotic plants. These combustibility data can be used in BEHAVE Plus, FARSITE, and the Emissions Production Model (EPM) so that models better represent the local vegetation, and will added to the Fuel Characteristic Classification System (Cushon et al 2002), which is a clearinghouse of fuels information.

We sampled flammability of plants in a cone calorimeter (ASTM 2002, see Fig. 1) to quantify effective heat of combustion (HOC) as a measure of heat content in dried (60°C), unground leaves and twigs. We compared 14 invasives, 12 of which are exotic, to 13 native species which might be displaced in disturbed habitats. Based on five replicates per species, we found a range from 6-17 Mj/kg, which is overall lower than for green and dry plant fuels from California and Colorado.

Fig. 1 Cone calorimeter apparatus used to measure heat content in oven dried, unground leaves and twigs of 27 native and exotic plants that grow in the Northeastern U.S.

Highest average heat content was in speckled alder. Among shrubs and vines, it was relatively high in highbush blueberry, purple nightshade, common barberry, and Japanese honeysuckle, and lowest in smooth buckthorn and Oriental bittersweet. Among six herbs, rough-stemmed goldenrod had the highest heat content while Japanese stiltgrass and Japanese knotweed were lowest. Quaking aspen had higher heat content than invasive trees, while Norway maple and apple were lower than the others.

Overall, invasive plants tended to have lower heat content than native species (Fig. 2).

Fig. 2. Notched box plot summarizing effective heat of combustion in six tree species, half of which are invasive in northeastern North America and half native. Because the notched portions of the two boxes do not overlap on the horizontal plane, the groups are significantly different.

When broken out as a subset, three invasive trees (black locust -- Robinia pseudoacacia, which is native only as far north as Pennsylvania; apple -- Malus sp., and Norway maple -- Acer platanoides) are significantly LESS flammable than three native trees (Fig. 2).

Our sample is small. In January 2003 Dibble, Ducey and White applied to the JFSP to conduct a nation-wide combustibility survey of native and invasive exotic plants.

We conclude that (1) use of fire to control undesirable vegetation can be more effective if a species-by-species approach is taken to meet management objectives in a particular stand; (2) flammability also involves leaf surface to volume ratio and moisture content (which is being measured in another study), and these should be quantified to improve modeling fire behavior; and (3) comparison of combustibility data from other regions will increase our understanding of fuels in the Northeast.

References

ASTM International. 2002. Standard test method for heat and visible smoke release rates for materials and products using an oxygen consumption calorimeter. Designation E 1354-02. West Conshohocken, PA: ASTM International.

Cushon, G. H., R. D. Ottmar, D. V. Sandberg, J. A. Greenough and J. L. Key. In press. Fuel characteristic classification: characterizing wildland fuelbeds in the United States. In A. Brennan, et. al. (eds.) National Congress on Fire Ecology, Prevention and Management Proceedings, No. 1. Tall Timbers Research Station, Tallahassee, FL. http://www.fs.fed.us/pnw/fera/jfsp/fcc/FCCpaper.pdf visited 12/12/02

Richburg, J. A., A. C. Dibble, and W. A. Patterson III. 2001. Woody invasive species and their role in altering fire regimes of the Northeast and Mid-Atlantic states. Pp. 104-111 in K.E.M. Galley and T. P. Wilson (eds.). Proceedings of the Invasive Species Workshop: the Role of Fire in the Control and Spread of Invasive Species. Fire Conference 2000: the First National Congress on Fire Ecology, Prevention and Management. Misc. Publ. No. 11, Tall Timbers Research Station, Tallahassee, FL.