The effects of ozone on a lower slope forest of the great smoky mountain national park: Simulations linking an individual tree model to a stand model

Abstract

We used an individual tree physiology model, TREGRO, and a stand succession model, ZELIG, to extrapolate from the direct response of leaf photosynthesis to ozone exposure in individual plants to an estimate of the possible future ozone effects on a forest located in the Twin Creeks region in Great Smoky Mountains National Park (GSMNP). This forest is dominated by yellow-poplar (Liriodendron tulipifera L.). With TREGRO, we estimated whether the reduction in the supply of carbon caused by ozone exposure would prevent an individual tree from meeting its demands for carbon for growth. With ZELIG, we analyzed the effect that changes in individual tree growth have in altering the ability of trees to successfully compete during forest succession. Current ambient levels of ozone measured at mid-elevations on the west side of GSMNP were predicted to reduce the abundance of yellow-poplar by 10% over the next 100 yr. This reduction would accelerate declines that are expected because of natural successional change. An increase of ozone of 50% above current ambient conditions was predicted to decrease yellow-poplar abundance by 30%. A reduction of ozone to half-ambient levels was insufficient to prevent yellow-poplar abundance decreases. Increases in abundance in two other canopy dominants, red maple (Acer rubrum L.) and black cherry (Prunus serotina Ehrh.) were predicted from the release from competition as yellow-poplar was suppressed by ozone exposure. These increases, however, were predicted to be short-lived, with large decreases of up to 30% eventually occurring for both species. Overall basal area of the forest was predicted to be relatively unchanged by ozone exposure.