Impact of the first recorded outbreak of the Douglas-fir tussock moth, Orgyia pseudotsugata, in southern California and the extent of its distribution in the Pacific Southwest region

Douglas-Fir Tussock Moth- and Douglas-Fir Beetle-Caused Mortality in a Ponderosa Pine/Douglas-Fir Forest in the Colorado Front Range, USA

“…Species composition and tree density and basal area levels in our study sites were similar to those found in ponderosa pine/Douglas-fir forests in other Colorado Front Range studies, particularly in north-facing aspects [22,31]. Species composition and stand structure of our heavily defoliated sites were not significantly different than in lightly defoliated sites, though other studies found that species composition and stand structure do contribute to DFTM susceptibility [32,33]. Site and stand conditions within DFTM infestations exhibit variation across the geographic extent of the insect, and our study sites are less variable in structure and species composition than either the northern Idaho [32] or southern California [33] situations, reflecting general forest conditions in our area.…”

“…Species composition and stand structure of our heavily defoliated sites were not significantly different than in lightly defoliated sites, though other studies found that species composition and stand structure do contribute to DFTM susceptibility [32,33]. Site and stand conditions within DFTM infestations exhibit variation across the geographic extent of the insect, and our study sites are less variable in structure and species composition than either the northern Idaho [32] or southern California [33] situations, reflecting general forest conditions in our area. The three studies also differ substantially in the lower size limit of trees included in the descriptive statistics, so basal area and density reported in the studies are not directly comparable.…”

“…In general, stands at lower elevations and those on ridge tops or positioned high in the slope are more likely to be infested. In southern California, stands located in areas not previously known to experience DFTM outbreaks, Coleman et al [33] also found that DFTM occurred in stands with a greater proportion of density and stocking in host species compared to non-defoliated stands. The proportion of density and basal area represented by host trees was somewhat higher in our stands (78% and 74%, respectively) than that observed by Coleman et al [33] in southern California (70% and 64%, respectively), but this probably reflects the smaller diameter limit used in our study.…”

“…In southern California, stands located in areas not previously known to experience DFTM outbreaks, Coleman et al [33] also found that DFTM occurred in stands with a greater proportion of density and stocking in host species compared to non-defoliated stands. The proportion of density and basal area represented by host trees was somewhat higher in our stands (78% and 74%, respectively) than that observed by Coleman et al [33] in southern California (70% and 64%, respectively), but this probably reflects the smaller diameter limit used in our study. Our study focused on stand conditions; examination of a more extensive affected area would be needed to assess site factors such as elevation or position on the slope and their influence on susceptibility to DFTM.…”

“…Land use changes and reduced thinning may have also have contributed to the change [33]. As it relates to the DFTM in the Colorado Front Range, a similar interpretation of species composition changes favoring DFTM is certainly plausible and needs to be studied, as disturbance patterns are expected to become more frequent and intense under a climate change scenario.…”