GROWTH AND PHYSIOLOGY OF NORTHERN RED OAK - PRELIMINARY COMPARISONS OF MATURE TREE AND SEEDLING RESPONSES TO OZONE

Considerable progress has been made during the past decade in the deve lopment of mechanistic models that allow complex chemical, physical, a nd biological processes to be evaluated in the global change context. However, quantitative predictions of the response of individual trees, stands, and forest ecosystems to pollutants and climatic variables re quire extrapolation of existing data sets, derived largely from seedli ng studies, to increasing levels of complexity with little or no under standing of the uncertainties associated with these extrapolations. Co nsequently, a project designed to address concerns associated with sca ling from seedling to mature tree responses was initiated. During the 1990 and 1991 growing seasons, mature northern red oak (Quercus rubra L.) trees and seedlings were exposed to subambient, ambient, and twice ambient ozone (O3) concentrations. The initial focus of the study was to identify possible trends and obvious differences between mature tr ees and seedlings, both in terms of growth and physiology and in respo nse to O3. Generally, mature trees exhibited a greater decrease in pho tosynthesis rates over the growing season than did the seedlings. Ozon e treatments had no consistent effect on gas exchange rates of seedlin gs, but the twice ambient O3 treatment resulted in reduced photosynthe sis rates in the mature tree. Despite no effect of O3 on seedling gas exchange rates, total seedling biomass was significantly less at the e nd of the 1991 growing season for those seedlings exposed to twice amb ient O3 levels. Disproportionate reductions in root biomass also resul ted in reduced root to shoot ratios at elevated O3 concentrations.