Recovery Plan for the Northern Spotted Owl - DRAFT

A. FireThe combination and interaction of fire frequency, intensity, and extent thatoccur in an ecosystem are known as a fire regime. The fire regimes of PacificNorthwest forests span a wide gradient of variation (Agee 1981). Natural fireregimes ranged from infrequent (hundreds of years) stand replacement fires, tovery frequent (several years) low-intensity surface fires that had little effect onthe canopy trees. The fire regime is a way to synthesize the effects of fire bycombining many of the descriptors listed earlier. Although fire regimes can bedescribed on the basis of characters of the disturbance itself (Heinselman1981), or character of the vegetation (Davis et al. 1980), another way is by definingthe fire regime on the basis of fire effects, or severity (Agee 1990). Thesystem using fire severity is defined in terms of fire effects on dominant treespecies, and works well for application to owl habitat. For the Pacific Northwest,three levels of fire severity are recognized (Figure F.2): high, moderate,and low. Northern spotted owls occur in forests subjected to all three fire severitytypes.A high severity fire is one that topkills most of the vegetation in the stand (70to 80 percent plus of the basal area); a moderate severity fire topkills 20 to 70percent; and a low severity fire topkills less than 20 percent of the basal area.Fire, in a silvicultural sense, tends to thin from below, first taking smaller treesand/or those less fire-resistant (thin-barked, for example). It must be recognizedthat each regime is defined on the basis of the modal severity but thatfires of other severity levels are likely to occur as well (Figure F.3). The mostcomplex fire regimes are the moderate ones because of the mix of expected fireseverities, while the low and high fire severity regimes are generally more predictable.Management activities over decades, such as successful fire protection,can change low or moderate severity fire regimes to moderate to high severityfire regimes. Descriptions of fire regimes in later sections of the reportwill refer to these severity diagrams.Within the DCA system lightning will be the largest single source of ignition forfires. Lightning is the primary source of forest fires worldwide, with as manyas 44,000 thunderstorms occurring daily over the earth (Trewartha 1968).Thunderstorm activity is relatively mild in the Pacific Northwest. While Portland,Oregon, and Tacoma, Washington, average 5 thunderstorm days peryear, and Baker, Oregon, and Walla Walla, Washington, average 10 to 15,southeastern areas such as Mobile, Alabama, average about 75 thunderstormdays per year (Alexander 1924). However, in the southeast only 2 percent ofwildfires are lightning-caused, while in the Pacific states 37 percent of ignitionsoriginate from lightning (Taylor 1974). Some lightning storms in the PacificNorthwest are very localized and others are regional in extent. The regionalstorms can ignite hundreds of fires almost simultaneously and pose the greatesttype of fire threat to DCAs. Lightning detection systems can pinpoint wherethe lightning is striking, but crews may not arrive in time to suppress the firewhile it is small.In the past, human ignitions have been locally significant in the West Cascadessubregion, particularly Indian burning of grasslands and oak woodlands (Boyd1986, Teensma 1987, Norton 1979, White 1980). In the East Cascade subregion,Indian burning was more common in the forest (Shinn 1980, Barrett andArno 1982). Today's human ignitions (recreation, logging, debris burningcauses) are partially amenable to management actions through education(Keep Green campaigns) or by limiting access (complete or weather-relatedroad closures).432