Reconstructing Historical Patterns of Fog Water Utilization and Coastal Climate Using Tree Ring Isotope Chronologies of Sequoia sempervirens: AReport for Save-the-Redwoods League

Dawson, Todd; Johnstone, James; Roden, John

Our proposed research set-out to document redwood response to summer fog abundance and other climatic conditions using a suite of tree-ring techniques on growth rings spanning the past 50-100 years. We saw these objectives as the initial stage of a larger research program aimed at developing the use of environmental information from redwood tree-rings to reconstruct past climatic and ecological events in the coast redwood forest. We have determined that summer fog is a reliable indicator of regional climate in he northeast Pacific, and displays strong correlation to the PDO, the dominant mode of north Pacific climate variability. Fog declined 12% following the 1977 PDO shift. Temperature-based proxy indicators of fog indicate sensitivity to the summer coastinland temperature gradient in California, and also to the 20th century global temperature trend. Both factors have contributed to in inferred reduction in fog over the past century. Ring-widths show significant, albeit weak, correlations to climate such that prior winter precipitation enhances growth, and a strong coast-inland temperature gradient tends to inhibit annual growth. We find that fog inputs are detectable by both carbon and oxygen stable isotope analysis of coast redwood, and that high-resolution sampling is necessary to isolate this signal, at least for oxygen. Ongoing NSF-supported research is building upon these initial findings in several ways: 1. High resolution (10 samples/year) analysis of stable isotope variability within rings funded by NSF) has been conducted. 2. Focused ring-width analysis emphasizing northern and southern extremes of the redwood range will begin this spring in order to isolate limiting climatic variables and increase replication. (Fellowship funded by the Berkeley Atmospheric Sciences Center). 3. Expansion of fog records, both temporally and spatially, using newly-published data sets. 4. Analysis of the spatial variation of stable isotope signatures in redwood cellulose across a watershed.