Carbon isotope discrimination in photosynthetic bark

We developed and tested a theoretical model describing carbon isotope discrimination during photosynthesis in tree bark. Bark photosynthesis reduces losses of respired CO₂ from the underlying stem. As a consequence, the isotopic composition of source CO₂ and the CO₂ concentration around the chloroplasts are quite different from those of photosynthesizing leaves. We found three lines of evidence that bark photosynthesis discriminates against ¹³C. First, in bark of Populus tremuloides, the δ¹³C of CO₂ efflux increased from -24.2‰ in darkness to -15.8‰ in the light. In Pinus monticola, the δ¹³C of CO₂ efflux increased from -27.7‰ in darkness to -10.2‰ in the light. Observed increases in δ¹³C were generally in good agreement with predictions from the theoretical model. Second, we found that δ¹³C of dark-respired CO₂ decreased following 2-3 h of illumination (P<0.01 for Populus tremuloides, P<0.001 for Pinus monticola). These decreases suggest that refixed photosynthate rapidly mixes into the respiratory substrate pool. Third, a field experiment demonstrated that bark photosynthesis influenced whole-tissue δ¹³C. Long-term light exclusion caused a localized increase in the δ¹³C of whole bark and current-year wood in branches of P. monticola (P<0.001 and P<0.0001, respectively). Thus bark photosynthesis was shown to discriminate against ¹³C and create a pool of photosynthate isotopically lighter than the dark respiratory pool in all three experiments. Failure to account for discrimination during bark photosynthesis could interfere with interpretation of the δ¹³C in woody tissues or in woody-tissue respiration.