This dissertation presents the results of stable isotope (carbon and oxygen) analyses of ∼16,000 year old Douglas-fir (Pseudotsuga menziesii) wood from Santa Cruz Island off the coast of southern California and centennial scale pollen analysis of ODP core 893A from the adjacent Santa Barbara Basin for the period of ∼18,000-10,000 calendar years before present (cal. yr. B.P.).

Douglas-fir trees grew on Santa Cruz Island ∼16,600 cal. yr. B.P., 100 km south of the species' modern range limit (Griffin and Critchfield, 1972). The fossil wood has significantly more positive carbon and oxygen stable isotope ratios than modern Douglas-fir wood from Point Reyes in coastal northern California, which suggests that the trees on Santa Cruz Island used fog as a water source.

To complement the stable isotope analysis, I analyzed the pollen from Ocean Drilling Program core 893A from the Santa Barbara Basin. The dominance of juniper (Juniperus), pine (Pinus), and sage (Artemisia) at ∼16,600 cal. yr. B.P. suggests that when the Douglas-fir trees grew on Santa Cruz Island the climate in the Santa Barbara region had lower temperatures and less precipitation than today. Temperature began to increase after 16,000 cal. yr. B.P. and winter precipitation increased slightly after 15,000 cal. yr. B.P. and then more substantially after 13,000 cal. yr. B.P. Earlier studies (Heusser, 1995, 1998; Heusser and Sirocko, 1997) suggested that from 30.000–10,000 cal. yr. B.P. shifts in vegetation in southern coastal California correlated with rapid climate change, such as the Younger Dryas, in other regions. While the diagram presented in this study shows a shift in dominant taxa from ∼18,000–10,000 cal. yr. B.P., the changes in pollen are not synchronous with the abrupt changes in the oxygen isotope records from the same core or from the North Atlantic (Hendy et al., 2002).