Determining landscape-scale changes in forest structure and possible management responses to Phytophthora ramorum in the Mt. Tamalpais watershed, Marin County, California

The Marin Municipal Water District's (MMWD) 7487 ha Mt. Tamalpais watershed in Marin County, California has the dubious distinction of being one of the earliest and most extensive areas impacted by Phytophthora ramorum in California. Rapid die off of tanoaks (Notholithocarpus densiflorus (Hook. & Arn.) Manos, Cannon & S.H. Oh) were first documented in 1995.With funding support from the U.S. Department of Agriculture Forest Service, MMWD initiated an assessment of landscape-scale changes in forest structure and understory floristics relative to P. ramorum spread in the Mt Tamalpais watershed. The assessment looked at changes in the extent and severity of diseased stands over a 5-year period as well as changes in understory vegetation. Three specific questions were addressed to support the development of a response strategy: (1) What sudden oak death (SOD)-related changes have already occurred? (2)What future SOD-related impacts are likely, or where is SOD likely to spread? (3) What is the status of natural regeneration in SOD-impacted stands? An additional benefit of this project was revision of the SOD-impacted portions of the 2004 vegetation map for the Mt. Tamalpais watershed to more accurately reflect stand conditions in 2009.