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How healthy are Mt. Tam's natural resources?



Grasslands | Photo by William Follette
Condition: Caution
Trend: Declining
Confidence: Low

Why Was This Indicator Chosen?

Grassland ecosystems are dominated by both perennial and annual herbaceous plants, with little to no trees or shrubs. Dominant native grassland species in the One Tam area of focus include purple needlegrass (Stipa pulchra), blue wild rye (Elymus glaucus), clovers (Trifolium spp.), California oatgrass (Danthonia californica), and red and blue fescue (Festuca rubra and F. idahoensis), among others.

California native grasslands are among the most endangered ecosystems in the country occupying less than 1% of their historic extent (Noss & Peters, 1995). Perennial grasslands provide ample carbon storage below ground in extensive root systems (Potthoff et. al., 2005) and some species of native grasses can live for hundreds of years (Marty et al., 2005).

Nearly 90 percent of California’s rare plant species listed in the Inventory of Rare and Endangered Species in California (Skinner & Pavlik, 1994) may be found within California grasslands, in addition to 30 percent of the threatened and endangered wildlife species (over 40 percent of terrestrial animals) (CDFW, 2016). American badgers (Lay, 2008) and grassland-nesting birds (Rao et al., 2008) rely on large patches of grassland for reproduction and forage.

What is Healthy?

The persistence of large, intact, and native species-rich blocks, which are needed to support grassland-dependent plant and wildlife species that are sensitive to edge effects and fragmentation. Because grassland habitats have decreased dramatically in extent over the last 100 years, both statewide and on Mt. Tam, preservation or expansion of grassland acreage is desirable.

Good examples of this vegetation type can be found on Pine Mountain, in scattered patches along Highway 1, and adjacent to Bootjack Creek below Mountain Theatre.

What Are the Biggest Threats?

  • At nearly all grassland sites, non-native species make up the majority of the plant cover, which has caused a loss of native species diversity, changes in nutrient cycling and hydrology, and shifts in invertebrate abundances (MMWD, internal data; CNPS, 2006; Steers, 2013; Ford & Hayes, 2007)
  • Transition to scrublands, woodlands, and/or forest in the absence of fire, grazing, or other landscape-scale disturbance
  • The potential effects of climate change, including frequent drought conditions and increased evapotranspiration under warmer temperatures
  • Lack of episodic disturbances from fire and from grazing by antelope, tule elk, and black-tailed deer, which has resulted in the loss of native species productivity, diversity, and the loss of grasslands themselves as they convert to woody dominated communities
  • Atmospheric nitrogen deposition, which can cause increased annual grass biomass, an accumulation of thatch, and losses of native biodiversity (Molinari & D’Antonio, 2014)

What is The Current Condition?

The overall condition of Mt. Tam’s grasslands is Fair. This reflects a good amount of acreage, but a cautionary condition for patch size and significant concern about levels of non-native species invasion.

What is the Current Trend?

Overall, grasslands are Declining, primarily due to non-native plant species invasion and the associated loss of native species diversity.

Map of Grassland Loss on MMWD lands (AIS, 2008; MMWD, 2016)
Map of Grassland Loss on MMWD lands (AIS, 2008; MMWD, 2016)


How Sure Are We?

We have Low confidence in this assessment because although grasslands have been mapped on all jurisdictions, much of the available information is dated. Data on community composition and native grass species cover are also limited.

What is This Assessment Based On?

  • Marin Municipal Water District (MMWD) vegetation maps from 2004, 2009, 2014 (AIS, 2015)
  • Marin County Parks vegetation map 2008 (AIS, 2008)
  • National Park Service vegetation map, NPS 2013 study (Steers & Spalding, 2013)

What Don’t We Know?

Key information gaps include:

  • Change-over-time assessment of total acres of grasslands and patch size
  • Comprehensive grassland composition data from a permanent plot network spanning all agency jurisdictions that would reveal changes is grassland species over time




Aerial Information Systems, Inc. (2008). Marin County Open Space District Vegetation Photo Interpretation and Mapping Classification Report, prepared by Aerial Information Systems, Inc. for Marin County Parks.

Aerial Information Systems, Inc. (2015). Summary Report for the 2014 Photo Interpretation and Floristic Reclassification of Mt. Tamalpais Watershed Forest and Woodlands Project, prepared by Aerial Information Systems, Inc. for the Marin Municipal Water District.

California Department of Fish and Wildlife. (2016). State & Federally Listed Endangered & Threatened Animals of California. Available from:

California Native Plant Society (2016). A Manual of California Vegetation Online. Retrieved from

Ford, L. D., & Hayes, G. F. (2007). Northern coastal scrub and coastal prairie. In M. Barbour, T. Keeler-Wolf, & A. A. Schoenherr (Eds.), Terrestrial Vegetation of California (3rd ed.) (pp.180-207). Berkeley, CA: University of California Press.

Lay, C. (2008). The status of the American Badger in the San Francisco Bay Area. (Master’s thesis). Paper 3623. Available from:

Marty, J. T., Collinge, S. K., & Rice, K. J. (2005). Responses of a remnant California native bunchgrass population to grazing, burning and climatic variation. Plant Ecology, 181, 101-112. Available from:

Molinari, N. A. & D'Antonio, C. M. (2014). Structural, compositional and trait differences between native- and non-native-dominated grassland patches. Functional Ecology, 28, 745-754.Available from:

Noss, R.F., Peters, R. L. (1995). Endangered ecosystems: a status report on America's vanishing habitat and wildlife. Retrieved from

Potthoff, M., Jackson, L. E., Steenwerth, K. L., Ramirez, I., Stromberg, M. R., & Rolston, D. E. (2005). Soil biological and chemical properties in restored perennial grassland in California. Restoration Ecology 13, 61-67. Available from:

Rao, D., Gennet, S., Hammond, M., Hopkinson, P., & Bartolome, J. (2008). A Landscape Analysis of Grassland Birds in a Valley Grassland-Oak Woodland Mosaic. (United States Department of Agriculture General Technical Report PSW-GTR-217). Pacific Southwest Research Station: U.S. Department of Agriculture, Forest Service. Retrieved from Forest Service website:

Schoenherr, A. A. (1992). A Natural History of California. Berkeley, CA: University of California Press.

Skinner, M.W. & Pavlik. B.M. (1994). Inventory of Rare and Endangered Vascular plants of California (5th ed.). Sacramento, California: California Native Plant Society. Available from:

Steers, R. J., & Spalding, H. L. (2013). Native component grasslands of the Marin Headlands. Natural Resource Technical Report NPS/SFAN/NRTR—2013/832. National Park Service, Fort Collins, Colorado.