DIRECT AND INDIRECT EFFECTS OF ELEVATED CO2 ON WHOLE-SHOOT RESPIRATION IN PONDEROSA PINE-SEEDLINGS

We determined the short-term direct and longterm indirect effects of C O2 on apparent dark respiration (CO2 efflux in the dark) in ponderosa pine (Pinus ponderosa Dougl. ex Laws.) seedlings grown in 35 or 70 Pa CO2 partial pressure for 163 days in naturally lit, controlled-environ ment chambers. Two soil N treatments (7 and 107 ppm total N, low-N and high-N treatments, respectively) were imposed by watering half the pl ants every 2 weeks with 15/15/18 fertilizer (N,P,K) and the other half with demineralized water. Direct effects of ambient CO2 partial press ure on apparent dark respiration were measured during short-term manip ulations (from minutes to hours) of the CO2 environment surrounding th e aboveground portion of individual seedlings. Shea-term increases in the ambient CO2 partial pressure consistently resulted in significant decreases in CO2 efflux of seedling in all treatments. Efflux of CO2 d ecreased by 3 to 13% when measurement CO2 partial pressure was increas ed 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 ef fects of growth CO2 partial pressure and the direct effects of the mea surement CO2 partial pressure were found. Seedlings grown in the high- N treatment were significantly less sensitive to short-term changes in CO2 partial pressures than seedlings grown in the low-N treatment. Ap parent respiration tended to decrease in seedlings grown in elevated C O2, but the decrease was not significant. Nitrogen had a large effect on CO2 efflux, increasing apparent respiration more than twofold on bo th a leaf area and a leaf or shoot mass basis. Both the direct and ind irect effects of elevated CO2 were correlated with change's in the C/N ratio. A model of cumulative CO2 efflux for a 160-day period demonstr ated that, despite a 49% increase in total plant biomass, seedlings gr own in the high-N + high-CO2 treatment lost only 2% more carbon than s eedlings grown in the high-N + low-CO2 treatment, suggesting increased carbon use efficiency in plants grown in elevated CO2. We conclude th at small changes in instantaneous CO2 efflux, such as those observed i n ponderosa pine seedlings, could scale to large changes in carbon seq uestration.