Recovery Plan for the Northern Spotted Owl - DRAFT

II. Major Natural Disturbance Factorsof the RegionTraditional theories of disturbance in ecosystems have held that disturbancemust be a major event and must originate outside the ecosystem (i.e., be exogenous)(White 1979). We now embrace a much broader concept of disturbance,recognizing a disturbance gradient from minor to major and the endogenouscomponent of many disturbances (due either to biotic agents orecosystem states that encourage disturbance to occur). As we accept thisbroader concept, we thereby create a fuzzier image of disturbance.Disturbance is a difficult word to define. A simple definition is to "interrupt" or"break up a quiet or settled order." Forest ecosystems, we know, are not "quietor settled orders," but when do the natural, normal rhythms of the systemoscillate to a point where they become "abnormal"? There is no easy answer,but White and Pickett (1985) suggest that disturbance can be defined as ". . .any relatively discrete event in time that disrupts ecosystem, community, orpopulation structure and changes resources, substrate availability, or thephysical environment." It may be characterized by such descriptors as frequency,magnitude, extent, predictability, synergism, and timing (White andPickett 1985). It may include processes such as forest fire, wind, insect, anddisease outbreaks, ice and freeze damage, landslides, floods, and others. Disturbanceneed not be either a disaster or a catastrophe.In Pacific Northwest forests, the primary disturbance factors of concern in DCAprotection are fire, wind, insects, and disease. This report therefore focuses onthese and omits the minor disturbance factors that normally are of limitedextent and major disturbance factors such as volcanoes.I ^-HighFigure F.3. A range offire severity levelsis found within eachfire regime depictedModeratein Figure F.2. The proportion of the-s -_ _ svariousfire severity levels are shown by"' o H ~ s / the lines of the graph. Asfire severity- regimes move on the left-to-right gradientfrom low to high, the proportion of lowo / 40 4, severityfires decrease while high severity._fires increase. Moderate severityfireso /.;0 peak in the moderate severity regime.Q ., 1*/ which is the most complexfire regime in. , terms offire effects.Low Moderate HighSeverityType of fire regime- -.......................... - I..431