Effects of short‐term ozone exposure on the carbon economy of mature and juvenile Douglas firs [Pseudotsuga menziesii (Mirb.) Franco]

Effects of short‐term ozone exposure of mature trees were compared with those of seedlings. Both 25‐yr‐old Douglas firs [Pseudotsuga menziesii (Mirb.) Franco] and 3‐yr‐old Douglas fir seedlings were exposed to 0, 200 and 400 μgm⁻¹ozone for about 1.5 wk and then labelled with ¹⁴CO₂ to study the effect on net photosynthesis, translocation and partitioning of assimilates. In the seedling experiment, two identical growth cabinets with separated shoot and root compartments were used for ozone fumigation and ¹⁴C‐labelling. Seedlings were harvested and ᴴC contents in the needles (subdivided into starch, ethanol‐soluble and residue fractions), branches, roots, root/soil respiration and soil residue were determined, together with the total starch content of the needles. Ozone increased the retention of ¹⁴C‐photosynthates in the needles. Translocation of carbon to the branches and roots seemed to be inhibited by the highest ozone treatment. The increased ¹⁴C‐retention was mainly recovered in the residue fraction of the needles. Total starch content of the needles decreased in the highest ozone treatment. In the experiment with mature trees, terminal shoots were fumigated with ozone and labelled with ¹⁴C using three small branch chambers. Carbon distribution was studied after harvest of the branches. Total ¹⁴C contents in the needle fractions and the branches were determined, together with the total starch content of the needles. Ozone was found to inhibit net ¹⁴CO₂ assimilation as well as translocation of ¹⁴C‐labelled assimilates from exposed needles to the branch. No effects of ozone were found on the partitioning of ¹⁴C among the starch, ethanol‐soluble, and residue fractions of the needles, although amounts in the ethanol‐soluble fraction tended to increase after exposure to ozone. The results indicate that mature and juvenile Douglas firs respond in a similar way to short‐term ozone exposure with regard to carbon translocation. If this similarity also applies to other species, then results from phytotron experiments on ozone and translocation of carbon might gain importance with respect to extrapolation to forest ecosystems.