Why Was This Indicator Chosen?
Open-canopy oak woodlands on Mt. Tam are characterized by the presence of long-lived acorn-producing trees from the genus Quercus. Although open-canopy oak woodlands have many tree species in common with mixed hardwood forests, the lower density and patchier distribution of trees create a distinct habitat structure for both herbaceous plants and wildlife. Understory species also include a distinct and more varied array of grasses, sedges, and forbs than closed-canopy forests (Evans & Kentner, 2006).
Oak woodlands in California support 1,400 species of flowering plants and over 300 species of vertebrates—more species than any other habitat type in the state (Mayer et al., 1986). On Mt. Tam, open-canopy oak woodlands can be used as an indicator of forest disease, fire regimes, and habitat quality for a number of oak-dependent birds (Rizzo et al., 2003; Holmes et al., 2008; Cocking, 2014). Lace lichen (Ramalina menziesii), which is California’s state lichen, primarily grows in open-canopy oak woodlands and is a good indicator of air quality (Sharnoff, 2014).
What is Healthy?
The maintenance of the full spatial extent of this vegetation type, the persistence of a discontinuous canopy dominated by trees from the genus Quercus, and discontinuous shrub and herbaceous layers dominated by native species.
Good examples of this type can be found in the Bon Tempe/Lake Lagunitas area and in the Cascade Canyon Preserve.
What Are the Biggest Threats?
- More than 90 percent of open-canopy oak woodlands on Marin Water lands is impacted by Sudden Oak Death (SOD) (AIS, 2015)
- A loss of fire to maintain an open canopy structure, limit the development of a shrub layer, and prevent the establishment of Douglas-fir (Pseudotsuga menziesii), and reduce the build-up of fuels and the risk of high-intensity wildfires with the potential to kill mature oaks
- High densities of deer browsing pressure on broadleaf tree seedlings and young saplings, leading to a depressed rate of new tree recruitment (Beschta, 2005; Ripple & Beschta, 2008)
- Acorn predation by introduced wild turkeys (Meleagris gallopavo)
- Impacts from invasive species such as French broom (Genista monspessulana) and non-native grasses which can reduce habitat function and biodiversity
- Fire suppression on Mt. Tam has shifted some areas from oak woodland to Douglas-fir conifer forest, changed oak woodland stand structure, and increased fuel loads.
What is the Current Condition?
Open-canopy oak woodlands on Mt. Tam are in Fair condition overall. The 2018 Fine Scale Vegetation Map (GGNPC et al., 2021a) shows that there are 1,594 acres of oak woodlands in the area of focus, 99% of which have at least 25% hardwood cover. Only nine acres fall below the 25% threshold, indicating that the condition of hardwood cover is good. However, 73% of oak woodlands have hardwood cover greater than 60%, leaving just 418 acres of what we consider to be open-canopy woodlands. When compared to closed-canopy hardwood forests, the open-canopy acres are even fewer (just under 370) when the additional canopy cover of conifers in these stands is taken into consideration. In addition, non-native, invasive species remain a significant concern.
The 2018 Marin Countywide Fine Scale Vegetation Map (GGNPC et al., 2021a) is the product of Marin County’s first simultaneous, multi-agency vegetation mapping effort to use a single consistent methodology across multiple jurisdictions. The quality and consistency of these data make the new map a foundational resource for calculating current baseline acreages of open-canopy oak woodlands in the One Tam area of focus. Looking ahead, future comparisons against the 2018 Fine Scale Vegetation Map should have greater accuracy and confidence levels.
What is the Current Trend?
Overall, these communities are Declining, primarily as a result of SOD and invasive species.
How Sure Are We?
Changes in mapping techniques and comparison areas reduce our general confidence in trend determination. However, we have relatively high confidence in the quality of mortality and hardwood-cover data and the level at which these are represented on all One Tam partner agency lands in the area of focus. This new dataset covers 100% of Mt. Tam’s oak woodlands when measuring 2018 mortality and hardwood-cover attributes, and 90% of oak woodlands when measuring hardwood decline from 2014 to 2018. We therefore have moderate confidence in comparing the results from the two assessment periods (2016 and 2022).
What is This Assessment Based On?
Aerial Surveys and Mapping:
Standardized 2004–2014 County Parks/Marin Water vegetation map (GGNPC et al., 2021b)- 2018 Marin County Fine Scale Vegetation Map (GGNPC et al., 2021a)
- Marin Water broom mapping from 2010 draft vegmgmt_polys_9_3, and 2013 and 2018 broom remapping
- Marin Water 2015 photo interpretation of SOD-affected forest stands (AIS, 2015)
- Marin Water, Marin County Parks, California State Parks, and National Park Service weed records from both the Calflora database and internal records
What Don’t We Know?
Key information gaps include:
- A measure of the diversity of native species so that we can try to maintain species richness at the reference condition for this community type
- Data on the age structure of native trees to determine if new trees are being recruited at a rate that is sufficient to maintain the total acres and structural integrity of open-canopy oak woodlands over time
- Greater understanding of the habitat function and value of open- vs. closed-canopy oak woodlands
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