Perspectives regarding 50 years of research on effects of tropospheric ozone air pollution on US forests

Tropospheric ozone (O-3) was first determined to be phytotoxic to grapes in southern California in the 1950s. Investigations followed that showed O-3 to be the cause of foliar symptoms on tobacco and eastern white pine. In the 1960s, X disease of ponderosa pines within the San Bernardino Mountains was likewise determined to be due to O-3. Nearly 50 years of research have followed. Foliar O-3 symptoms have been verified under controlled chamber conditions. Studies have demonstrated negative growth effects on forest tree seedlings due to seasonlong O-3 exposures, but due to complex interactions within forest stands, evidence of similar losses within mature tree canopies remains elusive. Investigations on tree growth, O-3 flux, and stand productivity are being conducted along natural O-3 gradients and in open-air exposure systems to better understand O-3 effects on forest ecosystems. Given projected trends in demographics, economic output and climate, O-3 impacts on US forests will continue and are likely to increase. (c) 2006 Elsevier Ltd. All rights reserved.