Regional And Watershed-Scale Coherence In The Stable-Oxygen and Carbon Isotope Ratio Time Series in Tree Rings Of Coast Redwood (Sequoia Sempervirens)

Abstract

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.