Influence of humidity and temperature during exposure on SO₂-induced leaf necrosis of Virginia pine and river birch

River birch and Virginia pine seedlings were exposed to 0.8 or 1.0 ppm SO₂ (approximately 2100 and 2600 μg m^-3, respectively) for 4 h at temperatures of 16, 24, and 32°C and at relative humidities of 60, 75, and 90%, in all combinations. Virginia pine seedlings exhibited increased SO₂-induced leaf necrosis with increasing temperature and with increasing humidity. For greenhouse-grown Virginia pine seedlings, the relationship between leaf necrosis and vapor pressure deficit was linear with a common negative slope but with different intercepts for each temperature. For Virginia pine seedlings grown outdoors, the relationship between leaf necrosis and vapor pressure deficit was also linear, but both slope and intercept changed with temperature. For river birch, temperature did not greatly affect the overall level of injury; instead, the response to humidity was negligible, linear, or quadratic, depending on temperature and on the conditions under which seedlings were grown. The effects of temperature and humidity on injury to Virginia pine are consistent with an explanation based on changes in leaf conductance; however, such an explanation cannot easily account for the observed response of river birch.