Ozone injury to plants within the Seney National Wildlife Refuge in Northern Michigan

Annual field surveys were conducted from 1999-2004 within the Seney National Wildlife Refuge in northern Michigan to determine if ambient ozone levels at this remote location were great enough to injure refuge vegetation. Ozone injury was observed on sensitive bioindicator plants during each survey year; however, the incidence (percentage) of plants exhibiting symptoms was low and varied among species and years. Ozone-induced symptoms occurred on Sambucus canadensis (American elder), Prunus serotina (black cherry), Asclepias syriaca (common milkweed), and Apocynum androsaemifolium (spreading dogbane). The most sensitive species was spreading dogbane. In addition, ozone injury was observed on a viburnum species, tentatively identified as Viburnum nudum var. cassinoides (withe-rod). Ambient ozone has been monitored since 2002 at an EPA monitoring site within the refuge. Cumulative SUM60 ozone levels (ppb-hrs) by the end of August for each survey year were greatest in 2003, followed by 2002, and least in 2004. The annual incidence of ozone injury for the 3 years was not directly related to level of ambient ozone, but was likely confounded by environmental factors such as drought. Based on the 2004 survey, the threshold level of SUM60 ozone needed to induce visible symptoms on sensitive vegetation in this remote refuge is close to 5000 ppb-hrs.