Measuring the health of Mt. Tam
Maintaining a healthy, vibrant and diverse Mt. Tam begins with understanding how key ecological resources are faring, and how we can better care for this iconic and beloved place.
One Tam partners and Bay Area scientists have come together to try to answer the question: How healthy are Mt. Tam's natural resources?
Why Was This Indicator Chosen?
The part of Mt. Tam that receives a marine influence (in the form of persistent summer fog) is home to maritime chaparral, a type of chaparral that is associated with several special status plant species.
Over half of the 95 species and subspecies of manzanita in California are locally endemic, occurring in lowlands adjacent to the coast and within the summer marine fog zone (Vasey & Parker, 2014). As a result of this diversity, maritime chaparral communities are recognized as one of the most diverse woody communities in the state (Sawyer et al., 2009).
Although relatively few, maritime chaparral plant species of special interest in the One Tam area of focus include:
- The rare Marin manzanita (Arctostaphylos virgata) CNPS rank 1B.2: Plants Rare, Threatened, or Endangered in California and Elsewhere - Moderately threatened in California
- Coinleaf manzanita (Arctostaphylos sensitiva) a regional endemic on Mt. Tam, and a good indicator of maritime chaparral health
Two rare wild lilacs:
- Mason’s ceanothus (Ceanothus masonii) CNPS rank 1B.2: Plants Rare, Threatened, or Endangered in California and Elsewhere - Moderately threatened in California
- Point Reyes ceanothus (C. gloriosus var. exaltatus) CNPS rank 4.3 Plants of Limited Distribution - A Watch List - Not very threatened in California
What is Healthy?
Maintain viable populations of maritime chaparral community endemics over a minimum of 90 acres.
What Are the Biggest Threats?
- Removal of fire as a key ecosystem process, resulting in vegetation succession from chaparral to forests, and a loss of the periodic fires necessary for chaparral regeneration
- Douglas-fir (Pseudotsuga menziesii) encroachment due to a lack of fire and the resulting impacts on shade-intolerant scrub and chaparral species
- Impacts from road and trail maintenance such as introduced plant pathogens, or vegetation clearing
- Threats to shallow-rooted and/or fog-dependent maritime chaparral species from drought and changes to fog patterns as a result of climate change
- Plant diseases such as the fungal pathogen Phytophthora ramorum, which causes Sudden Oak Death, and P. cinnamomi, which is particularly deadly to some manzanitas
What is The Current Condition?
The overall condition of Mt. Tam’s maritime chaparral is Poor. The number of individual plants and age classes of Mason’s ceanothus, Point Reyes ceanothus, and Marin manzanita are in Poor condition, although the overall extent of rare species is good.
What is the Current Trend?
Overall, maritime chaparral is Declining. The number of individual plants and age classes of Mason’s ceanothus, Marin manzanita, and Point Reyes ceanothus are declining, though there has been no discernable trend in the extent of rare species .
How Sure Are We?
We have High confidence in this assessment based on recent, comprehensive monitoring and survey data.
What is This Assessment Based On?
- Marin Municipal Water District rare plant maps and surveys from 1990, 2009, 2012, and 2015
- California Natural Diversity Database data for the One Tam area downloaded January 2016 (California Department of Fish and Wildlife, 2009)
- 2014 Rare Plant Population GIS data, Golden Gate National Recreation Area, updated in 2016
What Don’t We Know?
Key information gaps include:
- A genetic analysis of Mason’s ceanothus to determine if it is a viable species, or a series of semi stable or introgressing hybrids between C. gloriosus var. exaltatus and C. cuneatus var. ramulosus
- Research on germination requirements and seed life for rare species to determine if maritime chaparral that has been taken over by forest can return to chaparral after a fire
- A field study is needed to determine what plant pathogens are present
California Department of Fish and Wildlife. (2009). California Natural Diversity Database. Available from California Department of Fish and Wildlife website: https://www.wildlife.ca.gov/Data/CNDDB
Sawyer, J.O., Keeler-Wolf, T. & Evens, J. (2009). Manual of California vegetation. Sacramento, CA: California Native Plant Society Press. Available from: http://www.cnps.org/cnps/vegetation/manual.php.
Vasey, M. C. & Parker, V. T. (2014). Drivers of diversity in woody plant lineages experiencing canopy fire regimes in Mediterranean type climates, pp. 179-200. In: N. Rajakaruna, R. S. Boyd, and T. B. Harris (Eds.), Plant ecology and evolution in harsh environments. Hauppauge, NY: Nova Publishers.