Oaks Through Time: Reconstructing Historical Change in Oak Landscapes

The San Francisco Bay Area was home to some of the most celebrated valley oak lands in California, their grandeur much chronicled by early visitors and residents. Despite extensive loss, there remains substantial local interest in their preservation and restoration, necessitating an understanding of historical distribution and subsequent change. Assembling and interpreting historical and current oak data for highly modified Bay Area landscapes, such as Sonoma Valley in this study, has involved the development of a range of new techniques. The use of historical data involves many uncertainties, but through careful, site-specific integration of written accounts from the 19th century, early maps and surveys, historical landscape and aerial photographs, and modern field data, it is possible to develop a likely scenario of historical oak distribution in Sonoma Valley. This and similar studies in the Napa and Santa Clara Valleys are contributing to a detailed picture of the changes undergone by oak savannah and woodland over the last two centuries. Preliminary results indicate a highly patchy distribution within areas considered to have had relatively homogenous or continuous cover. The distribution of valley oak savannah appears to have been largely determined by microtopography, soil type, and groundwater. We have also collected age data from recently cut trees, indicating age/size relationships and suggesting that major disturbance event(s) may have occurred several decades before the arrival of Spanish and Mexican settlers. GPS-based fieldwork to identify residual heritage trees has found many more trees surviving within these modified landscapes than expected. This study has been used to assess baseline erosion rates, determine site suitability for an oak woodland mitigation project, and to assist other efforts to maintain these trees as significant components of regional landscapes into the future.