Crown Scorch Height
Crown scorch height represents the vertical distance above the ground that lethal scorching of the needles or leaves in the canopy of a forest or shrubland ecosystem has occurred as a result of the heat generated from a surface fire.
The degree of heat scorch and specific plant characteristics, such as size (e.g., tree height), dictate in part the likelihood of tree mortality in many species (Fernandes et al. 2008; Woolley et al. 2012), although other factors like growing season or time of year, for example, play a role (Wade and Johansen 1986; Burrows 1997). Thus, crown scorch height is an important consideration in developing a prescribed burn prescription and also a criterion for salvage logging following fire in many forest types (Peterson and Ryan 1986; Hood et al. 2007).
The temperature reached at a given height in the convection column or plume above a surface fire in a forest depends on the energy release rate, as quantified by fireline intensity (Byram 1959), ambient air temperature, and the in-stand wind speed. The maximum crown scorch height represents the point above the ground surface where an air temperature (assumed to be equivalent to the needle or leaf temperature) of ~60 °C is reached, which is generally regarded as the lethal temperature for crown foliage (Byram 1958), although, in fact, a time-temperature relationship exists as opposed to a distinct temperature threshold (Alexander 1998). For a lethal temperature of 60 °C, the needle exposure time is considered to be around 30 s (cf. Nelson 1952). This is roughly the nominal flame-front residence time for surface head fires in many forest types (e.g., Lawson 1972; Wotton et al. 2012).
Van Wagner’s Equations
Equation 1 was developed with data from 13 experimental fires carried out in Eastern Ontario, Canada, with IB and hs varying from 67 to 1255 kW/m and 2 to 17 m, respectively. Eight of the fires were in a stand comprised of red pine (Pinus resinosa) and eastern white pine (P. strobus), two in jack pine (P. banksiana), one in northern red oak (Quercus rubra), and the remaining two in a red pine plantation. The experimental fires were carried out under moderate to high ambient air temperatures (23–31.5 °C) and light in-stand winds (2.3–4.7 km/h). All of the fires were conducted on flat ground. The results agreed with established theory that hs varies with the 2/3 power of IB (Thomas 1963).
As depicted in Fig. 2, hs and stem-bark char height are two physical, post-fire indicators of fire behavior. Several investigators have, for example, used the inverse of Eq. 1 (Alexander and Cruz 2012a, b) to obtain a rough approximation of IB from field measurements of hs in the absence of on-site observation of a fire’s behavior. Trees are however not perfect sensors. The limitations of this method include hs exceeding the height of small trees and not reaching the crown base height of tall trees (Martinson and Omi 2008).
Equations 1, 2, and 3 were parameterized by fitting a linear model through the origin to transformed data. The models produced an r2 of 0.98. Van Wagner (1973) was to note that such a degree of fit was probably somewhat fortuitous in view of the rough nature of the data.
Within the US wildland fire management community (Hood 2007c), the corresponding units for IB, hs, U, and both TL and Ta are Btu/(s-ft), ft., mi/h, and °F respectively. For a list of conversion factors, see Van Wagner (1978).
Other Model Relationships Between Fireline Intensity and Crown Scorch Height
Van Wagner (1973) was the first to formally derive and publish an empirical relation between IB and hs. As a result, there has been an overwhelming tendency within the wildland fire community to regard Van Wagner’s (1973) equations as universal in nature (Alexander and Cruz 2012a). For example, a list of 11 other hs – IB equations, including the experimental data range in the variables associated with these equations, is presented in Alexander and Cruz (2012a). Seven of the 11 equations also employed the 2/3 power of IB.
Graphical representation of the 12 hs – IB equations (Alexander and Cruz 2012a, b) shows considerable variation among relationships. This is believed to be due to the differences in tree species, stand structure, surface and ladder (e.g., bark) fuel characteristics, measurement methodologies, type of ignition or firing pattern, and the combustion environment (Alexander and Cruz~2012a).
None of the hs-IB models developed to date are suitable for cases of sloping terrain. They would all tend to underpredict hs (Rothermel 1985). However, higher than expected scorch heights can be expected to occur at ridge lines (Rothermel~1985).
Relevance to Wildfires and Wildland-Urban Interface Fires
Crown scorch of the overstory is a significant outcome of both prescribed fires and wildfires burning beneath forest canopies within and outside the wildland-urban interface (Scott et al. 2014). Maximum crown scorch heights are often used to define a burning prescription as mentioned earlier, with predicted crown scorch height being compared to stand characteristics to determine whether a burning prescription is acceptable or not (Reinhardt and Ryan 1988; de Ronde et al. 1990; Fernandes et al. 2012). The degree of crown scorching is often used as the primary indicator of prescribed burn success in certain forest ecosystems (Wade and Lunsford 1988). In the case of wildfires, postburn hs assessments are utilized in judging fuel treatment effectiveness (Cram et al. 2006; Martinson and Omi 2008; Safford et al. 2012), and simulated values are useful in long-term fire and fuel management planning (Salazar and Bradshaw 1986; Stephens 1998).
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