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We determined the short-term direct and long-term indirect effects of CO 2  on apparent dark respiration (CO 2  efflux in the dark) in ponderosa pine ( Pinus ponderosa  Dougl. ex Laws.) seedlings grown in 35 or 70 Pa CO 2  partial pressure for 163 days in naturally lit, controlled-environment chambers. Two soil N treatments (7 and 107 ppm total N, low-N and high-N treatments, respectively) were imposed by watering half the plants every 2 weeks with 15/15/18 fertilizer (N,P,K) and the other half with demineralized water. Direct effects of ambient CO 2  partial pressure on apparent dark respiration were measured during short-term manipulations (from minutes to hours) of the CO 2  environment surrounding the aboveground portion of individual seedlings. Short-term increases in the ambient CO 2  partial pressure consistently resulted in significant decreases in CO 2  efflux of seedling in all treatments. Efflux of CO 2  decreased by 3 to 13% when measurement CO 2  partial pressure was increased from 35 to 70 Pa, and by 8 to 46% over the entire measurement range from 0 to 100 Pa. No significant interactions between the indirect effects of growth CO 2  partial pressure and the direct effects of the measurement CO 2  partial pressure were found. Seedlings grown in the high-N treatment were significantly less sensitive to short-term changes in CO 2  partial pressures than seedlings grown in the low-N treatment. Apparent respiration tended to decrease in seedlings grown in elevated CO 2 , but the decrease was not significant. Nitrogen had a large effect on CO 2  efflux, increasing apparent respiration more than twofold on both a leaf area and a leaf or shoot mass basis. Both the direct and indirect effects of elevated CO 2  were correlated with changes in the C/N ratio. A model of cumulative CO 2  efflux for a 160-day period demonstrated that, despite a 49% increase in total plant biomass, seedlings grown in the high-N + high-CO 2  treatment lost only 2% more carbon than seedlings grown in the high-N + low-CO 2  treatment, suggesting increased carbon use efficiency in plants grown in elevated CO 2 . We conclude that small changes in instantaneous CO 2  efflux, such as those observed in ponderosa pine seedlings, could scale to large changes in carbon sequestration.

Direct and indirect effects of elevated CO 2 on whole-shoot respiration in ponderosa pine seedlings