Dendrochronological parameters of northern red oak (Quercus rubra L. (Fagaceae)) infested with red oak borer (Enaphalodes rufulus (Haldeman) (Coleoptera: Cerambycidae))

Oak-dominated forests in northwestern Arkansas have recently experienced an oak mortality event associated with an unprecedented outbreak of a native insect, the red oak borer, Enaphalodes rufulus (Haldeman). To determine whether prior drought was associated with increased E. rufulus infestation level of Quercus rubra L. trees, we employed a suite of dendrochronological measurements from Q. rubra in affected forest stands. We examined patterns of tree growth and physiological indicators of stress including α-cellulose δ13C, intrinsic water-use efficiency (Wi) and photosynthetic discrimination (Δ) in Q. rubra from 1954 to 2002. Basal area increment measurements revealed that Q. rubra with lower levels of E. rufulus infestation experienced approximately 20% greater stem growth than Q. rubra more severely infested for ∼17 years prior to the onset of high rates of Q. rubra mortality. This trend emerged immediately following a severe drought in 1979–1981 and continued through 2002, suggesting that drought influenced E. rufulus infestation levels of some Q. rubra trees. Indicators of tree carbon and water relations (δ13C, Wi, and Δ) revealed no relationship with degree of E. rufulus infestation, except for Δ in 2000, when physiology likely was responding to the advanced stage of decline in heavily infested trees. δ13C, Wi, and Δ responded to drought periods as predicted early in the study period, but lost their sensitivity to drought after differences emerged in stem growth rate with degree of infestation. Though reduced stem growth in heavily infested trees suggests these trees were negatively influenced by E. rufulus infestation, the lack of infestation response of δ13C, Wi, and, for much of the study period, Δ, suggests that infestation had negligible influence on C–water relations. The study highlights how δ13C and δ13C-based parameters may not indicate variation in tree C–water relations when forest disturbances are associated with reductions in tree growth rates.