Five-week-old aspen seedlings and 8-week-old jack pine seedlings were exposed to a 3-h SO2fumigation episode (peakensp;=ensp;0.78ensp;mu;Lensp;Lminus;1; meanensp;=ensp;0.30ensp;mu;Lensp;Lminus;1) 0, 2, or 5 times per week. Intermittent fumigation of both species for up to 8 weeks had no consistent, significant effect on aboveground biomass or on net assimilation rate and leaf resistance. A significant decrease (as much as 47) in biomass of fumigated jack pine seedlings was found after 6 weeks of SO2exposure, but after 7 weeks the decrease was not significant. In contrast, a significant increase in biomass of fumigated aspen seedlings was found, but only at 6 weeks, and apparently related to increased leaf retention. Fumigation caused transient decreases in net assimilation rate of both species. The maximum decrease in net assimilation rate measured just after an SO2episode was 13 for aspen and 35 for jack pine, with a mean decrease for both of 12. During SO2fumigation, aspen net assimilation rate decreased as much as 38 and leaf resistance increased up to 47. Both aspen and pine seedlings recovered from these transient SO2effects, and there was no evidence for increased sensitivity to fumigation with time. After exposure to a single SO2episode in the laboratory, field-collected branches of mature aspen showed a 30 decrease in net assimilation rate and a 50 increase in leaf resistance, from initial values similar to those of the greenhouse seedlings.
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