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Succession functions of forest pathogens and insects : ecosections M332a and M333d in northern Idaho and western Montana : summary

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  • We analyzed the effects of pathogens and insects on forest succession in the absence of fire or management, addressing a number of related questions: 1. What is the rate of change in such forests? 2. How significant are the roles of pathogens and insects in the forest change? 3. How do pathogens and insects influence forest succession? Vegetation change was measured using a geographic information system (GIS) analysis method that overlaid 1935-era and 1975-era maps of sample subcompartments on national forest land in two ecoregions in northern Idaho and western Montana. This 40-year period was, coincidentally, the time in which white pine blister rust became epidemic and in which fire suppression policies were implemented. Stand hazard ratings were used to classify stand susceptibility to insects and most pathogens; root disease severity was rated from aerial photographs. We considered an insect or pathogen to be a cause of successional change when the following conditions were met: the insect or disease hazard or severity rating for a cover type/structure stage class was high or moderate; a transition from one class to another was consistent with the expected function of the agent; and the change was not explained by advancing succession in the absence of pathogen or insect influence. We found high rates of change from pathogens and insects in forests that had no evidence of recent active management or fire. More than 90 percent of the sample stands changed to a different cover type, structure stage, or both during the 40-year period. Insects and pathogens were associated with 75 percent or more of that change. Root pathogens, white pine blister rust, and bark beetles were the cause of most of the observed changes. The most significant pathogen and insect influences on cover type were to accelerate succession of western white pine, ponderosa pine, and lodgepole pine to later successional, more shade-tolerant species. The effects on structure were to reduce stand density or prevent canopy closure. Grand fir, Douglasfir, and subalpine fir were the predominant cover types at the end of the period, and were highly susceptible to root diseases, bark beetles, fire, and drought. The trend toward mature, dense, climax forest is projected to decrease substantially during the next 40 years, with greater accumulations occurring in low-density mature and younger pole-sized stands that result from root disease- and bark beetle-caused mortality. Our results underscore the relevance of pathogens and insects to forest planning and forest management. The introduction of white pine blister rust has drastically and perhaps permanently altered succession in this once-significant type. In the absence of fire or management, native pathogens, and insects continue to bring about change in forest composition and structure. This change is different from that produced by fire, as early seral species are usually not regenerated as a result of pathogen or insect activity. The ecological outcomes of pathogen and insect activities are sometimes desirable and sometimes not desirable. We should consider whether or not their effects create desired conditions for the landscape in deciding whether or not to alter their influence through management. This information on long-term effects of pathogens and insects on succession can be used to address forest health in forest plans, to analyze alternative actions, and to more accurately communicate outcomes of those alternatives to various stakeholders. We found that pathogens and insects can have large effects on forest succession. The economic impacts of pathogens and insects have been well documented; with this analysis, we have begun to understand and quantify their successional effects.
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