Examples of Southern Fuels with High Surface Area-to-volume Ratios

Many fuels typical of southeastern ecosystems are highly flammable due to their high surface area-to-volume ratios (Brown 1970). In particular, long-needle pine litter and grasses have very high surface areas and are responsible for carrying surface fires in many southern ecosystems. This page provides examples of plant species with high surface area-to-volume ratios.

Grasses, Sedges, Rushes

Grasses and their grass-like allies (sedges and rushes) have extremely high surface area-to-volumes and are highly flammable fuels that carry surface fires in many southeastern ecosystems. Notable examples of these flammable fuels are:

• wiregrasses (Aristida stricta and others),

• bluestems and broom sedges (Andropogon and Schizachyrium spp.),

• giant cane (Arundinaria gigantea),

• panic grasses (Panicum and Dichanthelium spp.),

• muhly grasses (Muhlenbergia spp.),

• the invasive cogon grass (Imperata cylindrica), and

• sedges (particularly sawgrass, Cladium jamaicense) and rushes.

Conifers

Conifers in the southeastern US contain a large group of species with highly flammable litter (Fonda 2001) due to the high surface area-to-volume of conifer needles.

• Long-needle pines have the highest surface area-to-volume ratios of conifers. Examples are: longleaf pine (P. palustris), south Florida slash pine (P. elliottii var densa), slash pine (P. elliottii var elliottii), loblolly pine (P. taeda), pond pine (P. serotina).

• Short-needle pines have lower surface area-to-volume ratios but are still flammable. Examples include: shortleaf pine (P. echinata), Ocala sand pine (P. clausa var. clausa), Choctawhatchee sand pine (P. clausa var. immuginata), spruce pine (P. glabra), Virginia pine (P. virginiana), Table Mountain pine (P. pungens), eastern white pine (P. strobus), and pitch pine (P. rigida).

• Other southeastern conifers have reduced surface area-to-volume, but still much higher than small woody fuels (10- and 100-hour timelag fuels) and many hardwood foliage fuels. Examples include: eastern redcedar (Juniperus virginiana), southern redcedar (J. silicicola), Atlantic white-cedar (Chamaecyparis thyiodes), bald cypress (Taxodium distichum), pond cypress (T. ascendens), eastern hemlock (Tsuga canadensis), Florida torreya (Torreya taxifolia), Fraser fir (Abies fraseri), and red spruce (Picea rubens).

Forbs, shrubs, palms, vines, ferns, lichen, and bromeliads

Other southeastern fuels that are notable for their surface area-to-volume and resulting flammability are forbs, palms, lichens, ferns, vines and bromeliads.

• There is a great diversity of forbs and shrubs with high surface area-to-volume ratios and make up a large portion of the available fuels in several southeastern ecosystems.

• Palm leaves have high surface area-to-volume and are highly flammable; saw-palmetto (Serenoa repens), cabbage palm (Sabal palmetto), and scrub palm (Sabal etonia) are notable examples.

• Lichen fuels (foliose lichens; the reindeer “mosses” Cladonia and Cladina spp. and draped Usnea spp.) have extremely high surface area-to-volume, and combust readily where they dominate patches in pine sandhill and sand pine scrub ecosystems (Brown 1970).

• Understory ferns (particularly bracken fern, Pteridium aquilinum) also have high surface area: volume and combust readily in southeastern surface fires.

• Several draped vines (many Smilax spp. and yellow jessamine, Gelsemium sempervirens) have high surface area-to-volume and contribute to surface fires and localized crown torching.

• Spanish moss (Tillandsia usneoides) and Tillandsia recurvata can be important firebrand fuels, contributing to long-distance spotting (Clements 1976) and spill-over.

Hardwoods

Hardwood foliage has a great diversity of surface area-to-volume, so flammability varies drastically in these fuels. Generally speaking, hardwood litter can burn under dry conditions, but is much less flammable than needle litter and standing dead herbaceous fuel.

• Among hardwoods, most oaks [notably turkey oak (Quercus laevis), post oak (Q. stellata and Q. margaretta), and white oak (Quercus alba)], many other deciduous tree and shrub species [black cherry (Prunus serotina), blackgum (Nyssa sylvatica), hawthorns (Crataegus spp.), and heaths) have high surface area-to-volume and are the most flammable.

• Other deciduous hardwoods and most evergreen hardwood species [e.g., live oaks (Quercus virginiana and Q. geminata) and southern magnolia (Magnolia grandiflora)] have lower surface area-to-volume and require more pre-heating for ignition and combustion.

There are other species-specific characteristics that determine flammability in addition to surface area-to-volume ratios, such as the concentration of organic volatiles. For information on the flammability of individual plant species that take into consideration all of these characteristics, see Plant Flammability.