Documentation of prehistoric vegetation for a small coastal prairie in the north coast range, Humboldt County, California, using opal phytolith analysis and repeat photography

This study documented the prehistoric vegetation of a small coastal prairie in the northern Coast Range, Humboldt County, California. Two primary methods were used, repeat photography and opal phytolith analysis. Tree core analysis also provided information on historic vegetation conditions of the study site. A historic orthophoto, 1942, was overlain with a recent orthophoto, 2000, which determined conifer encroachment had occurred over the last 58 years, but was not uniform spatially. Intersection of the1942 prairie polygon and 2000 prairie polygon created four vegetation zones used for opal phytolith sampling. Zone GG was grass-dominated in 1942 and remains grass-dominated today. Zone GT was grass-dominated in 1942 and is now tree-dominated. Zone TG was tree-dominated in 1942 and is now grass-dominated. Zone TT was tree-dominated in 1942 and remains tree-dominated today. Opal phytolith concentrations were categorized for each vegetation zone by morphological shape indicating festucoid-type and panicoid-type grasses. Average opal concentrations for each morphological type were statistically significantly different between vegetation zones using a nonparametric Kruskal-Wallis test. High variability between sample points within and between vegetation zones indicates a dynamic vegetation history for the study site. Vegetation boundaries created from aerial photographs do not designate prehistoric vegetation boundaries. Low concentrations of dumbbell opals found in all vegetation zones indicate that panicoid grasses did not dominate this site prehistorically. However, the presence of dumbbell phytoliths in all zones show that native grasses have occurred at the site at some time in the past. It is likely that Danthonia ca/ifornica, a dumbbell-producing native currently very sparse at the study site, was more widespread prehistorically and also present in the current forest zones. My first hypothesis, that the study site is being encroached by forest vegetation is supported by repeat aerial photography and opal phytolith analysis. My second hypothesis, that the study site was prehistorically part of a larger prairie is not supported by opal phytolith analysis. Prairie vegetation occurred at the site but was not homogenous and probably not stable over the long term. Rather, it appears that the study site was prehistorically a temporal and spatial vegetation mosaic.