- 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.