Estimating Vegetation Reference Conditions by Combining Historical Source Analysis and Soil Phytolith Analysis at Pepperwood Preserve, Northern California Coast Ranges, U.S.A

The vegetation on many Californian landscapes changed substantially after the late eighteenth century arrival of European settlers. Restoring vegetation to pre-settlement status requires an understanding of reference conditions, the range of variability of vegetation and the trajectory of vegetation change through time. We used parallel, independent studies of historical evidence and soil phytoliths to increase temporal depth and estimate reference conditions at 39 sites in a mosaic of grassland–savanna–woodland–chaparral vegetation at Pepperwood Preserve, in California's northern Coast Ranges. Historical evidence consisted of mid-1800s General Land Office survey records and repeat aerial photographs. Survey points were relocated and survey descriptions compared with modern vegetation; vegetation type had not changed on 16 of 19 sites. Aerial photographs showed increasing tree cover and decreasing grassland from 1942 to 2000. Phytolith analysis, including soil phytolith percent weight and absolute counts of grass short cells, was used to estimate long-term vegetation. Sites containing greater than 0.30% soil phytolith weight and greater than 90,000 rondel short cells/g soil were classified as grassland/savanna; sites below the 0.30% threshold with less than 32,000 and 32,000-90,000 rondels/g soil were classified as forest and woodland, respectively, while chaparral had low (<0.70) rondel/elongate ratios. Phytolith-based classification of long-term vegetation indicated that vegetation type has not changed on 30 of 39 sites. Both historical and phytolith evidence indicated most of the vegetation landscape at Pepperwood has been long-term stable. Historical source analysis and phytolith analysis are highly complementary approaches that should be routinely performed to provide accurate estimates of site reference conditions prior to commencing vegetation restoration.