The Marin Municipal Water District’s (MMWD) 18,500 acre Mt. Tamalpais Watershed in Marin County, CA has the dubious distinction of being one of the earliest and most extensively Phytophthora ramorum impacted zones in California. Rapid die-offs of tanoaks (Notholithocarpus densiflorus var. densiflorus) were first documented in 1995.With funding support from the US 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. This 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 inform 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 and where is SOD likely to spread? (3) What is the status of natural regeneration in SOD-impacted stands? An additional benefit from this project was revision the of SOD-impacted portions of the 2004 vegetation map for the Mt. Tamalpais Watershed to more accurately reflect novel stand conditions that have emerged in the wake of disease.
Results/Conclusions
Analysis of true color aerial imagery of the watershed indicates the spatial extent and severity of SOD-related tree mortality expanded between 2004 and 2009 from 8750 acres to 10,700 acres. This represents 83% of all habitat on the watershed with a principal component of tan oaks, coast live oak, black oak (Q. kelloggii), canyon live oak (Q. chrysolepis), or Shreve’s oak (Q. parvula var. shrevei). The largest un-impacted stand in 2009 was 150 acres in size. In both 2004 and 2009, canopy mortality was most pronounced in tanoak-dominant assemblages in the western Bolinas Ridge portion of the watershed. Much of the expansion between 2004 and 2009 is due to a spatial increase in coast live oak mortality in the southern and eastern portions of the watershed. Type conversions from one recognized National Vegetation Classification (NVC) association to other recognized or to novel associations were detected for 2075 acres of habitat where tanoak ceased to be a primary component. Understory stand conditions have changed significantly over the last 20 years, particularly with regard to shrub densities and fuel loading. MMWD is currently completing analysis of its 2015 repeat of this mapping and classification exercise. The District is also revising its vegetation management and wildfire risk reduction program to address changing conditions.