Measuring Positive, Negative, and Null Impacts of Forest Disturbances: A Case Study Using Dwarf Mistletoe on Douglas Fir

Not all disease activity causes an impact. Not all impacts are negative. The aim of this study was to examine a method that could conceptually specify when impacts occur and that could quantify both negative and positive disease impacts. For this study, dwarf mistletoe (Arceuthobium douglasi) of Douglas fir (Pseudotsuga menziesii) in southwestern Oregon was used as a case study. The method uses six variables for forest growth, mortality, and stand structure, and six categorical disease severity scores. The impact model displays stands as points in multidimensional scaling space, where relative position is determined by values of the six stand variables. Positions in this two-dimensional space change when stand characteristics change. Differences associated with disease severity could be traced as trajectories, and impact was quantified using the length and direction of these trajectories. This multivariate impact assessment method was contrasted to impact assessments based on single variables. Methods based on multiple variables offer a useful way of characterizing impact on multiple objectives. The model indicates that dwarf mistletoe has positive, negative, and neutral impacts and that these could be illustrated and quantified using this method.