Temporal and spatial variation of oak growth–climate relationships along a pollution gradient in the midwestern United States

The Kalman filter procedure was used to evaluate temporal variation in associations between physiologically based climate indices and radial growth of black oak (Quercusvelutina Lam.) and white oak (Quercusalba L.) at seven similar sites along the Ohio River corridor acidic-deposition gradient. Physiological response variables were derived by a model that used daily weather data to estimate effects of climate on growing season net photosynthesis and woody respiration. Correlations between oak radial growth indices and physiological response variables deteriorated over the period of record (1900–1987) at all seven study sites; there was no spatial association between the deterioration and the acidic-deposition gradient. This deterioration of growth–climate correlations was temporally associated with decreased growing season temperature at all seven sites; no consistent temporal trend was found for growing season precipitation. The effects of decreasing temperature on modeled physiological response variables included increased net photosynthesis and decreased woody respiration. These results suggest that recent assessments of relationships between acidic deposition and forest condition in the Ohio River region have been done during a time period of relaxed climatic stress and may have underestimated pollution–climate stress interactions.