Stand and individual tree characteristics associated with historical red oak borer (Enaphalodes rufulus (Haldeman) Coleoptera: Cerambycidae) infestation within the Ozark and Ouachita National Forests

Many oak decline events have been reported within the past century in the Eastern U.S., and important causal factors often differ slightly among them. Coincident with a recent decline event in upland oak-dominant forests of Arkansas, Missouri, and Oklahoma was an unexpected outbreak of a native secondary invader, Enaphalodes rufulus (Haldeman), red oak borer. A large range in estimates of oak mortality throughout affected forests was presumably due to variation in species composition, where oak-dominant areas experienced the greatest mortality. We chose eight sites across the Ozark and Ouachita National Forests similar both topographically and by oak dominance to determine if other stand or tree characteristics were important factors in variation of E. rufulus infestation across these forests. We created an estimate of the E. rufulus population level at each site during the recent outbreak using counts of exactly dated larval gallery scars within a subset (108) of all Q. rubra L., northern red oak, sampled (976). We used classification trees, followed by multinomial regression to determine important tree and stand characteristics. These models indicated that trees which died during the recent outbreak were smaller and apparently suppressed, and likely weakened physiologically, perhaps rendering them more susceptible to E. rufulus. At the stand level, E. rufulus populations were higher at sites with lower Q. rubra basal area. This is likely a result of greater Q. rubra mortality at these sites during the borer outbreak in the early 2000s.