Branch- and Plant-level Flammability

Branch-level flammability

Branch flammability is influenced by both particle flammability and the arrangement of particles into leaf and stem structure. Leaf thickness, surface area-to-volume ratio, and particle density affect flammability at the leaf and stem level.

Time until ignition at 750^o C was directly related to leaf thickness of foliar samples from 32 species (Montgomery and Cheo 1971). In the same study, surface area-to-volume ratio was inversely related to ignition time for the same samples (Montgomery and Cheo 1971).

Heat transfer, in the form of radiation, conduction, and convection, is affected by surface area. In fuels with high surface area-to-volume ratios, heat is transferred faster to the interior causing more rapid combustion (Rundel 1981). In addition, fuels with higher surface area-to-volume ratios can exhibit more rapid water loss, indirectly increasing flammability (Rundel 1981). For examples of species with high surface area-to-volumes, see Surface area-to-volume ratios of Southern Species.

The amount of mass per volume of particles, or particle density, also influences heat transfer, thereby affecting flammability (Rundel 1981). Particle density affects the type of ignition, whether spontaneous (indirect heat source) or pilot-ignited (direct heat source). Lower particle density fine fuels are more likely to spontaneously ignite in the absence of a pilot fire (Brown 1970).

In general, a leaf attached to a plant contributes to fire behavior differently than a similar leaf immediately after being dropped from a plant. In a study assessing the use of the limiting oxygen index method in measuring foliar flammability, mature leaves and freshly fallen leaves from 10 tree and shrub species were tested (Mak 1988). Results showed that the freshly fallen leaves required less oxygen to ignite and sustain burning than the mature leaves of the same plant (Mak 1988). It is unclear how the chemical makeup of the leaves differed.

Methods for testing flammability at the leaf or stem level include muffle furnace tests (Montgomery and Cheo 1971), cone calorimetry (White et al. 1996), and the limiting oxygen index method (Mak 1988).

Plant-level flammability

Horizontal and vertical arrangement of leaves and branches on a plant can affect its overall flammability. Of 10 plant-level characteristics, Etlinger (2000) found that mass and moisture content of foliage were the most important predictors of flammability. Additional information on the flammability of southern plant species is presented in Comparing flammability of southern plants.

Measurements of flammability at this scale have been done with an intermediate scale biomass calorimeter with a line burner ignition source, which is able to measure the heat released from the combustion of an entire plant (Etlinger 2000).

Research limitations of plant flammability research

Measurements of flammability are complex, as they include both internal and external properties, and they can be made using a variety of techniques and equipment. Also, flammability characteristics have been studied to different extents by various methods, are not equally important to plant flammability, nor are they all independent of one another (Shafizadeh et al. 1977, Etlinger 2000, Francis 2000). An accepted methodology is necessary for determining the flammability value of plants that incorporates the entire plant structure, but measures only the most influential characteristics (Frommer and Weise 1995, University of California FPL 2001). In addition to providing needed information to WUI residents engaging in firewise landscaping, quantification of flammability characteristics can contribute to the development of more ecosystem-specific and Wildland Urban Interface (WUI)-specific fire behavior models (Hough and Albini 1978, Rehm et al. 2001). To be able to rank the relative flammability of southern species, more empirical data must be collected.