Visible foliar injury of red spruce seedlings subjected to simulated acid mist

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Abstract
Two‐year‐old red spruce seedlings [Picea rubens Sarg. syn. P. rubra (Du Roi) Link] were subjected to 6 simulated acid mist treatments (pH 2.5, 2.7, 3.0, 3.5, 4.0, 5.0) in a replicated experiment using open‐top chambers. Acid mist solutions containing equimolar (NH4)2SO4, and HNO3 were applied twice weekly for 22 weeks, each application being equivalent to 2 mm of precipitation. Visible symptoms of foliar damage were observed on the 3 most acidic mist treatments (pH 2.5, 2.7, 3.0). The inputs of nitrogen, sulphur and acidity in the most acidic treatment were 55, 42, 1,3 kg ha‐1, respectively, over a 10 week period. The plants subjected to the pH 2.5 treatment were found to be most severely damaged with approximately 40% foliar necrosis after 10 weeks of treatment. On approximately 80% of seedlings, necrosis was confined to current year needles only. These damaged needles were initially light brown or light orange in colour turning a deeper red 3 to 5 weeks after initial necrosis. Percentage foliar damage was linearly related to concentration (of NH4+, NO3‐, SO42– and H+) with 62% foliar damage in the pH 2.5 treatment after a 22–week period. Spray application stopped in December 1987. Observations during the following spring showed that the pH 2.5 and pH 3 treatments induced earlier Hushing, requiring 60 day °C less thermal time than the pH 5–0 treatment. In 1988, this decrease in thermal requirement was equivalent to flushing 11 days earlier. There was no evidence of acid mist treatments inducing bud mortality.