Interactions between fire, fungi, bark beetles and lodgepole pines growing on the pumice plateau of central Oregon are described. Mountain pine beetle (Dendroctonus ponderosae) outbreaks occur mainly in forests that are 80-150 years old with a mean diameter of about 25 cm and weakened by a fungus, Phaeolus schweinitzii. The outbreak subsides after most of the large diameter trees are killed. The dead trees fuel subsequent fires which return nutrients to the soil, and a new age class begins. The surviving fire scarred trees are prone to infection by the slow fungal disease and about 100 years later these trees are then susceptible to bark beetle attack.

At least once a year the mountain pine beetle searches for lodgepole pines that provide a suitable habitat for a new brood. After attacking females feed, they produce an attractant pheromone that causes beetles to aggregate and, during outbreaks, to usually mass attack the "focus" tree. Near the completion of mass attack, incoming beetles are repelled and initiate attacks on adjacent "recipient" trees. An understanding of this "switching" process is useful for prescribing measures that minimize beetle damage.A mathematical model was developed to (1) describe beetle aggregation, (2) predict the relation of tree susceptibility and switching to changes in beetle density, (3) provide a structure for current knowledge, and (4) pose questions for further research. The model indicates that a high population density ensures mass aggregation and consequently successful tree colonization and switching. The model also indicates that the number of beetles attracted per attacking beetle differs from tree to tree, possibly depending on resin quality and production and/or the local flying density of beetles. Field and model results indicate that tree size appears to affect the repellence of beetles, suggesting that the attack density or the visual attractiveness of large trees is a factor. Further research could be directed at our assumptions on host resistance, repellence, pheromone emission rates, threshold concentrations, navigation, and pheromone dispersion.

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After light ground fires smolder through climax lodgepole pine (Pinus contorta var. murrayana) stands, Dendroctonus ponderosae attacked a high proportion of unscorched and lightly fire‐damaged trees; Ips pini preferentially attacked the moderately to heavily fire‐damaged trees. Discriminant analysis showed that percent root kill and tree diameter correctly classified 74% of the D. ponderosae infested trees; I. pini attacks were negatively correlated with tree diameter and positively correlated with percent basal kill and these two variables correctly classified 84% of I. pini infestations. Percent root kill was the most important fire related variable associated with combined Ips and/or Dendroctonus attacks. The influence these light fires have on lodgepole pine stand dynamics is also discussed. Zusammenfassung Borkenkäferbefall von Lodgepole‐Kiefern nach einem Waldbrand in South Central Oregon Nach einem leichten Bodenschwelbrand in Klimax‐Beständen der Lodgepol‐Kiefer (Pinus contorta var. murrayana) befiel Dendroctonus ponderosae einen hohen Anteil der angebrannten und leicht feuergeschädigten Bäume. Ips pini dagegen befiel bevorzugt die mittel bis stark vom Feuer geschädigten Bäume. Die Diskriminanzanalyse zeigte, daß der Prozentsatz getöteter Wurzeln pro Baum und der Stammdurchmesser 74% der von D. ponderosae befallenen Bäume einwandfrei klassifizierten. I. pini‐Angriffe waren mit dem Stammdurchmesser negativ, mit der basalen Abtötung positiv korreliert, und beide Variablen klassifizierten 84% der von diesem Borkenkäfer befallenen Bäume. Der Prozentsatz abgetöteter Wurzeln war die wichtigste der mit dem Feuer verbundenen Variablen, die mit beiden Borkenkäferarten korreliert war. Der Einfluß solcher leichten Brände auf die Dynamik von Lodgepol‐Kiefer‐Beständen wird diskutiert.