Rotations for high-severity and moderate-severity fire in low/mid-elevation conifer forests of western North America.

Data from General Land Office or mapped data over large areas (>25,000 ha) over >70 or more years prior to fire exclusion.1Study area was dominated by mixed conifer and Jeffrey pine and minimally logged. Fire exclusion only began in the 1970s and has had only a modest impact [138]. Thus, historical and current rates are assumed to be comparable.2Analysis of Leiberg’s mapping of high-severity fire areas within unlogged mixed-conifer Sierran stands is found in [139]. According to Leiberg [51], most such fire occurred prior to 1850. In addition, he stated “If the many small lots [3The numerator was estimated at 100 years based on ponderosa pine in this region [140], whose growth would surpass 30 cm dbh, rendering mixed and high-severity effects indistinguishable (see [35: Table 1]). This calculation is conservative because tree growth to 30 cm dbh in moister forests is faster than 100 years.4High- and mixed-severity fire consistent with a definition of >70% and 20–70% basal area mortality, respectively, was identified from overstory canopy percentage, the overstory size class, the understory size class, and the fire tolerance of the cover type (see [35: Table 1]). Large patch sizes of historical high severity fire (100s to >5000 ha) from this work are reported in [17].5The estimate is from the span of years over which fire effects were distinguishable, using forest structure evident in the Government Land Office historical survey data, divided by the fraction of the forested landscape in which those fires occurred [56]. Rotations for high-severity fire are determined by dividing the observation period (the period of time over which fire effects are distinguishable by stand structure) by the percentage of the landscape experiencing high-severity fire. The methods were found to have 14.4–23% accuracy compared to plot sampling.6High- and mixed-severity fire, consistent with a definition of >70% and 20–70%, respectively, were identified from overstory canopy percentage, the overstory size class, the understory size class, and the fire tolerance of the cover type (see [35: Table 1]).7High-severity consistent with a definition of >70% basal area mortality [35] was identified having a percentage of small trees >50% and a percentage of large trees [56], [57], [73],8Estimated from the length of General Land Office section lines intercepted by moderate- and high-severity fire. Accuracy tests using the length of section lines intercepted by modern moderate- and high-severity fire yielded a relative error of 15.6%.