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Intra-annual variation in the stable oxygen and carbon isotope ratios of cellulose in tree rings of coast redwood (Sequoia sempervirens)

Fog is a seasonally variable hydrologic input to coast redwood (Sequoia sempervirens, D. Don) ecosystems with a stable isotopic composition (δ 18O, δ2H) that is distinct from rainfall. The intra-annual variation in tree ring cellulose δ18O of coast redwood was measured to determine what portion of the tree ring should be sampled to best target fog signals. Ten years of rings were subdivided into ten equal units for three trees at two sites in northern California. Intra-annual variation in cellulose δ18O was as high as 3‰ with a consistent pattern of enriched (in 18O) values from the annual ring boundary and depleted values from the central portion of the ring. These patterns show a strong coherence between replicate trees at the same site as well as significant correlations between individuals at sites over 250 km apart. Intra-annual variation in stable carbon isotope ratios (δ13C) was as high as 2‰ and produced a pattern with depleted (in 13C) values from the ring boundary and enriched values from the central portion of the ring. Although these δ13C patterns were not as strongly correlated between replicate trees as the patterns in δ18O variation, they did produce significant correlations with the variation in fog frequencies recorded at local airports. This study highlights the importance of quantifying intra-annual variation in tree ring stable isotope signals as a guide to further investigations on historic variability from long chronologies especially if the signal of interest is seasonally variable (eg, fog).