Decline in long-term growth trends of white oak

Abstract

White oak (Quercusalba L.) tree-ring collections from 89 locations throughout much of its range, from Connecticut to North Carolina to Iowa, were examined for evidence of growth decline initiated in the 1950's. The expected trend of annual basal area increments, based on pre-1950 growth, appears to be linear, with the slope varying among collections relative to site quality. Growth decline, defined as departure of actual growth below that expected, was identified in 40 of 60 collections judged to have not been affected by local site histories. The percentage of collections showing decline was essentially the same in the Northeast, the Midwest, and the Southeast. Onset of decline began during a relatively narrow time window from the mid-1950's to the early 1960's. Since then the growth trend has been linear and appears unrelated to changes in regional sulfur and nitrogen oxide emissions. It is postulated that events or conditions, unique to the 1950's, initiated a growth rate change in about two thirds of the white oak stands examined. Initiation of the decline appears to be unrelated to tree age, geographic location, site quality, climatic trends, or regional emissions of sulfur and nitrogen oxides.