Carbon isotope variations in a plantation of Sitka spruce, and the effect of acid mist

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Intra- and inter-tree variations in 13C/12C ratios were studied within a single clone plantation of 20-year-old Sitka spruce, some of which were treated with mist simulating acidic cloud water. For groups of trees of similar height and the same treatment, sampled at the same whorl height, δ13C values for current year needles showed variations (1 SD) of between 0.2 and 0.7‰. The variations reflect the seasonally averaged influences, on intercellular CO2 concentrations, of slight variations in the microhabitat within a group. For a typical intra-group variation of 0.4‰ one may be able to distinguish between groups whose mean intercellular CO2 concentrations differ by only 8 ppm. Acid misting resulted in a lowering of δ13C values by c. 0.7‰ (significant at the P≤0.05 level). This reflects higher intercellular CO2 concentrations for acid misted trees, which can be interpreted in terms of their having assimilation rates c. 10% lower than those of control trees, and might explain the observed reduction in stem growth for acid-misted trees. Without careful attention to sampling strategy, however, these small inter-tree δ13C variations can be easily masked by the much larger intra-tree variations with height. Large gradients of increasing needle δ13C with height, of c. 0.5‰ m-1, were observed in two untreated trees of different total height. The gradient was similar for both trees so, though δ13C values of both trees were identical close to their leaders (-27‰), the taller tree displayed much lower values close to the ground (-31‰). The gradients are believed to reflect lower light levels close to the ground, rather than the accumulation of respired CO2 in the atmosphere. The different height response of stems versus needles, reflected by an increase in δ13Cstems-δ13Cneedles with height (for cellulose), is discussed in terms of stem photosynthetic recapture of internally respired CO2.