A method of estimating direct and indirect effects of Armillaria root disease and other small-scale forest disturbances on canopy gap size

Abstract

The ecological and economic impacts of individual forest pathogens on forest stands are often difficult to measure because of indirect impacts arising from their interactions with other agents. Root diseases especially interact with many elements of the forest. This study uses a method called path analysis to estimate direct and indirect effects of Armillaria root disease and other small-scale disturbances on a pristine Pinus ponderosa stand in the Black Hills of South Dakota. Indirect effect was defined as an outcome where one small-scale disturbance alters the effect that another has on canopy gap size. Results indicate that key factors affecting gap size include Armillaria root disease, bark beetles, weak pathogens, ice/snow damage, lightning, and wildfires, presented in order of relative importance. Interactions occur between Armillaria and lightning, beetles, wind, and fire; between bark beetles and wind, fire, and weak pathogens; and between wind and fire. Armillaria and beetles both tend to occur where damage by wind is absent. Armillaria root disease was predicted to have the largest overall effect on gap size, which is mostly due to its direct effects, and not resulting from its interactions with other disturbances. Path analysis generates a predictive model based on covariation among disturbances and offers a way of quantifying the effects of these associations.