Site and stand variables influencing red oak borer, Enaphalodes rufulus (Coleoptera: Cerambycidae), population densities and tree mortality

Three research areas in the Ozark National Forest, Arkansas, were chosen to investigate relationships of site and stand conditions to northern red oak, Quercus rubra L., mortality attributed to red oak borer, Enaphalodes rufulus (Haldeman), a native wood-boring beetle. Fixed vegetation plots were installed in each area on five topographic positions: north, south, east, and west-facing benches and on ridges. Red oaks were classified into three red oak borer infestation history classes, low, moderate, and high, based on crown condition and red oak borer basal emergence holes. Three red oak borer population variables, number of current generation galleries, previous generation heartwood galleries, and emerging adults, were extrapolated to the stand-level based on infestation history class and within-tree red oak borer density data derived from whole and partial-tree sampling during the 2003 red oak borer cohort. Strong explanatory site and stand variables for borer population densities and northern red oak mortality were number of northern red oak, research area, topographic position, tree species richness, and tree species diversity. Number of northern red oak was positively correlated with increased borer densities and oak mortality whereas richness and diversity were negatively correlated. Yearly re-evaluations of 527 red oaks from 2002 to 2005 revealed that 8% improved in health relative to red oak borer infestation, 48% remained the same, 31% declined, and 13% died. Numbers of living trees and healthy trees decreased in some research areas and on some topographic positions. Results revealed spatial variability in tree and stand variables, red oak borer densities, and oak mortality and elucidated stand factors associated with this insect outbreak and oak mortality event.