Interactive effects of acidic deposition, drought, and insect attack on oak populations in the midwestern United States

The objective of this study was to determine if acidic deposition predisposes oaks to decline after drought. Associations were evaluated between oak decline and mortality after drought and (1)pre-drought growth; (2)tree size, age, and competitive status; (3)attack by twolined chestnut borer (TLCB); (4)acidic deposition; and (5)soil acidity. This study was conducted in the Ohio River region of the United States. White oak (Quercus albaL.), black oak (Quercus velutina Lam.), and the locally dominant oak (chestnut oak (Quercus prinusL.) or post oak (Quercus stellata Wangenh.)) were studied at six similar sites along an acidic deposition gradient during the period 1988-1992. Prevalence of severe branch dieback and mortality was low ( < 10%) at all sites, but fewer oaks in Illinois were in the 0-10% dieback class than in other states. Also, fewer oaks in Illinois than Ohio exhibited immediate recovery after the 1988 drought. Prevalence of TLCB attack was similar across all sites; >85% of dead trees had been attacked, but <10% of living trees were damaged. Most dead oaks had lower pre-drought radial growth than survivors of the same species. Branch dieback, growth decline, and mortality were more prevalent in the black oak population than the other oak species. There was no evidence that acidic deposition predisposed oaks to post-drought decline.