Coast redwood (Sequoia sempervirens) ecosystems are strongly influenced by the presence of summer marine fog, and variation in fog frequency is closely linked to climate variation in the NE Pacific region. Because oxygen isotope composition (δ18O) of organic matter records distinct water sources (e.g. summertime fog vs. winter precipitation) and carbon isotopes (δ13C) are typically sensitive to humidity and water status, it then follows that inter-annual variation in tree-ring isotope ratios, which are coherent across multiple sites, should preserve a potentially powerful proxy for climate reconstruction. Here we present an analysis of a 50-year time series for both δ18O and δ13C values from subdivided tree rings obtained from multiple redwood trees at multiple sites. Within-site and between-site correlations were highly significant (p < 0.01) for the δ18O time series indicating a regionally coherent common forcing of δ18O fractionation. Within-site and between-site correlation coefficients were lower for the δ13C than for the δ18O time series although most were still significant (at least to p < 0.05). The hypothesized reason for the differences in the correlation is that carbon isotope discrimination is more sensitive to microenvironmental and tree-level physiological variation than is δ18O fractionation. Stable-isotope variation in tree-ring cellulose was similar between slope, gully and riparian micro-habitats within a single watershed, implying that minor topographic variation when sampling should not be a major concern. These results indicate that stable-isotope time series from redwood tree rings are strongly influenced by regional climate drivers and potentially valuable proxies for Pacific coastal climate variability.
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