Severity and frequency of SO₂-induced leaf necrosis on seedlings of 57 tree species

Container-grown seedlings or rooted cuttings of 57 tree species common in the northeastern United States were tested during 1977-80 to establish the severity and frequency of SO₂-induced leaf necrosis for each species. Plants were exposed to 0.9 ppm SO₂ (2,400 μg m^-3) for 2 or 4 hr in controlled-environment chambers. Species were grouped into four sensitivity classes. For seven species considered highly sensitive, more than 40 percent of the individuals exhibited symptoms, and more than 4 percent of the leaf surface on a typical individual was necrotic on average. At the other extreme, eight species did not develop symptoms. The results are consistent with a direct relationship between a species's tolerance to SO₂ and its shade tolerance.