West Coast vegetation shifts as a response to climate change over the past 130,000 years: geographic patterns and process from pollen data

We compiled pollen sequences from lake and offshore cores at least 6,000 years old (6 ka) for the Mediterranean and Marine ecoregions of the US West Coast. Principal Component Analysis highlighted vegetation differences in core-tops, the Holocene Thermal Maximum (6 ka) and Last Glacial Maximum (LGM, 19 ka). Core-top and HTM ordination produced clusters that reflected geographic clusters in the Sierra Nevada, and the Pacific Northwest coast. Little change in these clusters between 6-0 ka suggested that vegetation communities in coastal and alpine settings persisted, despite warmer global temperatures. PCA outliers reflect distinct pollen assemblages that often were isolated sites or bordered the Great Basin. During the LGM, greater shrub and herb presence in the Marine ecoregion interior indicated enhanced aridity, while conifer presence in coastal and Southern California indicated moist conditions. Qualitatively, tree taxa from the Last Interglacial (~130–120 ka) showed how vegetation shifted over 6–10 kyr from alder, to oak, then redwood, a successional pattern that began again at the Late Glacial (~15 ka). In future West Coast pollen studies, sampling and chronologic control at centennial resolution will enable further study of more time periods and rates of vegetation change in response to climate.