A Comparison of Presettlement Vegetation and Fire Regimes with Current Patterns in Oak Woodlands and Shrublands of SW Oregon

Fuel reduction efforts on BLM lands in southwestern Oregon are motivated by the need to reduce fire hazard and restore and rehabilitate ecosystems. Successful ecosystem restoration depends, in part, on understanding the target: what ecosystem model is considered natural and healthy for a given area? Oak (Quercus garryana) woodlands and shrublands are two of the most characteristic ecosystems in interior valleys of southwest Oregon, and extensive acreages within these systems are treated annually for fuel reduction. However, these are also two of the least understood ecosystems in the region. We know little about their presettlement attributes, responses to disturbance, or successional relationships. This study was intended to provide insight into historical patterns of vegetation in interior valleys of southwest OR, dating back to the time of initial settlement by Europeans, and into current vegetation patterns and stand structures for chaparral and oak woodlands. We analyzed changes in vegetation between the pre-fire suppression era and the present, examining in particular the direction and magnitude of change in vegetation that is considered to be fire dependent, particularly oak woodlands and shrublands. Results advance our understanding of the relationships of plant communities to fire, and also provide insight into the degree to which current fuel reduction prescriptions reflect past burn regimes. Several sources of historical information on vegetation and fire were used, including: (1) General Land Office (GLO) surveys, the majority of which were completed early in the settlement period ( 1854-1858) [we relied most heavily on these sources of information]; (2) Homestead Patent Application maps (~1905 – 1915); and (3) Donation Land Claim surveys (mid-1800’s). Standardized methods of interpreting these sources were used in this work. Resultant maps of presettlement vegetation were digitized, allowing comparison with recent aerial photographs and with GIS layers of current information on vegetation, soils, and other site attributes. Proportions of the landscape falling in various vegetation types were compared quantitatively between the past and the present and transitions between vegetation types were assessed quantitatively in relation to site environment and disturbance history. The digitized data on historical and current fire and vegetation will provide valuable baseline information against which future changes in vegetation or disturbance regime can be assessed. We also carried out analyses of age and size structures for oaks and chaparral shrubs (Arctostaphylos viscida and Ceanothus cuneatus), and analyzed these structures in relation to site characteristics and known fire history. While oak woodlands and chaparral are characteristic features of the landscape in southwestern Oregon, almost nothing is known about their structure or stand dynamics or the drivers of those dynamics. Our work contributes to filling that large information gap, and the research also provides further insights into fire regimes over the past century. Results will enhance the ability of land managers to focus on fuel reduction prescriptions that can be justified both on the basis of hazard reduction and ecosystem restoration and to communicate their rationale to the concerned public