A time to ignition–temperature–moisture relationship for branches of three western conifers

Abstract

The effect of convection column air temperature and live needle moisture content on ignitability of tree branches was verified and quantified by exposing branches of three conifer species to a hot-air convection column, at temperatures between 400 and 640 °C, and measuring time to ignition. The three species were ponderosa pine (Pinusponderosa Laws.), Douglas-fir (Pseudotsugamenziesii var glauca (Beissn.) Franco), and lodgepole pine (Pinuscontorta Dougl.). The experiment was repeated monthly over the course of a year, taking advantage of the natural fluctuation of live needle moisture content. Three multiple regression equations for the prediction of time to ignition with air temperature and needle moisture as the independent variables were developed.

Assessing the probability of crown fire initiation based on fire danger indices

“…Table 1 summarizes basic descriptive statistics of the independent variables selected for analysis. Foliar moisture content, canopy base height, and wind speed are three fire environment variables that theoretically are major factors controlling the onset of crowning and have already been used in modeling this phenomenon (Van Wagner 1977a, Xanthopoulos 1990, Alexander 1998Cruz et al 2002). The range and deviation around the mean for these variables suggest a balanced distribution suitable for the statistical analysis to be pursued in the present study.…”

“…From a fire management perspective, knowledge of the onset of crowning is of critical importance from a safety and tactical standpoint, yet few research attempts have been directed at explaining and/or developing models to predict this feature of wildland fire behaviour (Van Wagner 1977a, Xanthopoulos 1990, Alexander 1998, Cruz 1999. This lack of attention probably can be explained by (1) the unknowns in fundamental combustion and heat transfer theory in wildland fuels, (2) the difficulty in replicating such phenomena in a laboratory setting, and (3) the difficulty of instrumenting outdoor high-intensity experimental fires in order to quantitatively describe the influence of relevant variables, important physical processes, and their outcomes.…”

“…Although this model has been embraced by many members of the wildland fire research community in North America (e.g., Bessie and Johnson 1995, Agee 1996, Scott 1998, Graham et al 1999, and currently integrates various types of decision aids and decision-support systems for fire management (e.g., Alexander 1988, Forestry Canada Fire Danger Group 1992, Finney 1998, Scott and Reinhardt 2001, it has never been subjected to a formal model evaluation. It possesses several theoretical and practical limitations, namely: (1) its heat of ignition formulation assumes that all the moisture must be driven off before ignition (Xanthopoulos 1990, Alexander 1998, whereas the occurrence of simultaneous drying and pyrolysis (Saastamoinen and Richard 1996) is expected under high energy fluxes such as the ones verified at the onset of crowning; (2) the proportionality constant has been assumed to be universal, although recent research (Alexander 1998) indicates that it is not the case-it is expected that it is dependent on combustion characteristics as determined by surface fuel bed structural properties and fuel moisture dynamics (Cruz 1999); (3) the model is based solely on convective theory, whereas the radiative component might predominate in some burning conditions; (4) the use of Byram's (1959) fire intensity variable requires the estimation of rate of fire spread and fuel consumption from other models, inducing an error propagation problem and increasing the uncertainty of the model system.…”

Convection Ignition of Live Forest Fuels

“…The study of ignition and subsequent flame spread of forest fuels has largely been performed using dried and dead fuels and only a few works were found that examined live fuels [12,13,14,15,16]. Dimitrakopoulos and Papaioannou [12], Jervis et al [13], McAllister et al [14], all studied ignition due to radiative heating.…”

“…Dimitrakopoulos and Papaioannou [12], Jervis et al [13], McAllister et al [14], all studied ignition due to radiative heating. Xanthopoulos and Wakimoto [16] and Fletcher and coworkers ( [15, 17, and 18]) were the only studies found that convectively heated live fuels to ignition. However, the study in [12] was primarily focused on developing an empirical correlation for the ignition time rather than investigating the physical processes involved in ignition.…”