Short- versus long-term effects of elevated CO2 on night-time respiration of needles of Scots pine (Pinus sylvestris L.)

Dark respiration rate in the night (R-D) was measured in five-year-old Scot s pine (Pinus sylvestris L.) trees grown for two years under ambient (AC) a nd elevated (AC + 400 mu mol mol(-1) = EC) CO2 concentrations in open top c hambers. Two needle age classes (i.e., current-year and one-year-old) were measured at AC and EC in both AC- and EC-grown pines. Additionally differen t chemical characteristics were determined on the needles, such as nitrogen (N), carbon (C), starch, and soluble sugar concentrations as well as speci fic leaf area. The direct, short-term and indirect, long-term effects of EC on R-D for the two needle age classes were examined. R-D was expressed on a per needle area, needle mass, N, C, and C/N bases. Direct effects were on ly pronounced in the AC treatment where inhibition of R-D was found at EC i n both current- and one-year-old needles. Indirect effects were only signif icant in one-year-old needles where a decrease was found in the EC grown tr ees as compared with AC ones when R-D was expressed per unit needle mass, C , or C/N. R-D per unit needle area and needle N were not sensitive to long- term EC, in any needle age class. Long-term EC treatment also influenced th e response of the two needle age classes. One-year-old needles from the EC treatment had significantly lower R-D than current-year needles, but no suc h response was observed in the AC treatment. Our experiment reemphasised th e importance of expressing R-D on different bases for a correct interpretat ion of the responses to EC. Moreover, we showed that different needle age c lasses can respond differently to a CO2 enrichment.