(Picea mariana [Mill.] B .S .P.) stand in which the head fire rate of advance was 6.6m/min and flame depth averaged 3 m. The backfire spread at a rate of 0.5 m/min with a coresponding fire-front depth of 0.8m.Thus, the computed residence times from Eq. 4 are 0.45 min or 27 s and 1.6 min or 96 s, respectively.