Why Was This Indicator Chosen?
Mt. Tam’s iconic, sweeping grasslands serve as habitat for numerous plants and animals, and contain tremendous biodiversity. 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's native grasslands are among the most endangered ecosystems in the country, at 1% of their historical extent, and Mt. Tam preserves some of the best examples of remnant grassland ecosystems in the region (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 desired condition for grasslands is the persistence of large (30 acres or above), intact, native-rich blocks of this vegetation type, which supports grassland-dependent plant and wildlife species sensitive to edge effects and fragmentation. Good examples can be found on Pine Mountain, in scattered patches along Highway 1, and adjacent to Bootjack Creek below Mountain Theater.
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 climatic water deficit 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)
- Non-system (unofficial) trails contribute to erosion, localized soil compaction, and invasive plant spread
What is The Current Condition?
The overall condition of Mt. Tam’s grasslands is Fair. An overall decline in mapped grasslands was detected at a rate of 30% between the data sources used for the 2016 grasslands chapter and the 2018 Fine Scale Vegetation Map. Some of this change occurred after of the 1994 mapping, some is attributable to new mapping rules, and some may be attributable to grassland succession to woody types. Eleven patches of grassland at 30 acres or more in size were observed in 2022, down from 19 in 2016. This chapter establishes a new baseline for future comparisons. If vegetation mapping rules continue to evolve, comparisons may be made through separate analyses using aerial imagery (Startin, 2022). In addition, we have only limited data about the composition of native species present in Mt. Tam's grasslands.
What is the Current Trend?
The trend is unknown as of our 2022 update. The 2018 Marin County Fine Scale Vegetation Map provided new information on grassland extent and composition to support this chapter update. Notably, it documented significantly fewer acres of grassland than were included on earlier maps. A change in geographic boundaries for the One Tam area of focus also changed grassland extent. Differences in this map and previous efforts meant that we had to establish a new baseline from which a trend may be inferred in the future.
How Sure Are We?
We have Low confidence in this assessment because available data sources between 2016 and 2022 are not comparable, which prevents a full assessment of metrics. In addition, we have only limited data about the composition of native species present in Mt. Tam's grasslands.
What is This Assessment Based On?
- Marin Water vegetation maps (2004, 2009, 2014; AIS, 2015)
- Marin County Parks vegetation map (2008; AIS, 2008)
- National Park Service vegetation map and study (2013)
- Redwood Creek grassland assessments (2017)
- One Tam grassland assessments (2018)
- Marin Countywide Fine Scale Vegetation Map (2018; GGNPC et al., 2021)
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
resources
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