Volatile Compounds

Volatile compounds (also termed secondary chemicals, secondary plant compounds, extractives) have high heats of combustion, low heat capacity, and volatility that increase a fuel’s flammability and facilitates combustion (Shafizadeh et al. 1977, Susott 1980). In the early stages of combustion, volatile compounds are released by the fuel through pyrolysis. These gases then combust and ignite less flammable components (other volatiles, celluloses, hemicelluloses, phenolics). Volatile compounds include terpenes (mono-, sesqui-, di-, tri-, and polyterpenes), fats, waxes, and sugars. The content in a single species changes through a year; volatile concentrations increase as foliage ages (Hough 1969).

For many southeastern species, volatile content is a prevailing variable in their flammability. Conifer species contain terpenes and waxes that lower their ignition temperature and cause them to combust more rapidly. Pines generally contain between 20 and 25% volatiles on a wet weight basis. Many southeastern shrub fuels contain volatile compounds that enhance flammability, notable among these is gallberry (Ilex glabra), wax myrtle (Myrica cerifera), Rhododendron, titi (Cyrilla racemiflora), and the invasive melaleuca (Melaleuca quinquenervia) among others. Volatiles in gallberry foliage constitute 45% of its wet weight, and other southeastern species have similar amounts of these compounds. Sand pine foliage extractives peak during the soring dry period when their foliage moisture troughs, this interaction leading to seasonal patterns in ignition and combustion (Shafizadeh et al. 1977, Burgan and Susott 1991).

See also: Plant Flammability.