Ozone exposure thresholds and foliar injury on forest plants in Switzerland

Canton Ticino in southern Switzerland is exposed to some of the highest con centrations of tropospheric ozone in Europe. During recent field surveys in Canton Ticino, foliar symptoms identical to those caused by ozone have bee n documented on native tree and shrub species. In Europe, the critical ozon e level for forest trees has been defined at an AOT40 of 10 ppm.h O-3 (10 p pm.h accumulated exposure of ozone over a threshold of 40 ppb) during dayli ght hours over a six-month growing season. The objective of this study was to determine the amount of ambient ozone required to induce visible foliar symptoms on various forest plant species in southern Switzerland. Species w ere grown within eight open-top chambers and four open plots at the Vivaio Lattecaldo Cantonal Forest Nursery in Ticino, Switzerland. Species differed significantly in terms of the ppb.h exposures needed to cause visible symp toms. The most to least symptomatic species grown within open-plots in this study rank as Prunus serotina, Salix viminalis, Viburnum lantana, Rhamnus cathartica, Betula pendula, Rumex obtusifolius, Sambucus racemosa, Morus ni gra, Prunus avium, Fraxinus excelsior, Rhamnus frangula, Alnus viridis, Fag us sylvatica and Acer pseudoplatanus. Similar rankings were obtained in the non-filtered chamber plots. The ranking of species sensitivity closely fol lows AOT values for the occurrence of initial symptoms and symptom progress ion across the remainder of the exposure season. Species that first showed evidence of foliar injury also demonstrated the most sensitivity throughout the growing season, with symptoms rapidly advancing over ca. 25-30% of the total plant leaf surfaces by the end of the observation period. Conversely , those species that developed symptoms later in the season had far less to tal injury to plant foliage by the end of the observation period (1.5 to < 5% total leaf area injured). The current European ambient ozone standard ma y be insufficient to protect native plant species from visible foliar injur y, and many more native species may be sensitive to ozone-induced foliar in jury than are currently known. (C) 2000 Elsevier Science Ltd. All rights re served.