Relevance

Grass Lake lies within the Upper Truckee Critical Aquatic Refuge (CAR; a U.S. Forest Service designation) on the southern boundary of the Lake Tahoe Region. It was established as a U.S. Forest Service Research Natural Area (RNA) in 1991. Grass Lake is roughly 250 acres in size and has long been considered the largest and best example of a Sphagnum fen in the Sierra Nevada (Bittman 1985). Sphagnum fens are peat-forming wetlands that form when stable hydric soils allow a rate of organic matter production that is greater than the rate of decomposition, which over millennia leads to an accumulation of peat (Patterson and Cooper 2007, Weixelman and Cooper 2009). In environments with low summer precipitation like the Sierra Nevada, fens are sustained by groundwater input rather than precipitation. They are important sites of groundwater discharge and may serve as indicators of shallow aquifers (Cooper 1990).The conditions required for fens are very limited in mountain ecosystems, and fens occupy only 0.1 to 0.2 percent of the landscape in the Sierra Nevada (Wolf and Cooper 2015). Because fens form slowly over thousands of years, they are not easily restored once destroyed (Cooper et al. 1998) and they provide an important record of prehistoric climate and vegetation (Wolf and Cooper 2015). Fens have been identified by the U.S. Forest Service (SNEP 1996) and in the Tahoe Science Plan (Manley et al. 2010a) as among the most sensitive habitat types in the Sierra Nevada. Fens are hotspots of biodiversity that support rare plants, insects, and animals. Vegetation in all wetland types, including fens, marshes and meadows, plays an important role in recycling nutrients, trapping eroding soil, and filtering out pollutants such as nitrates (Cooper and Wolf 2006). In addition, fens figure prominently in nearly all scenarios of carbon dioxide-induced global climate change because they are major sinks for atmospheric carbon (Chimner and Cooper 2002). In the 2010 Lake Tahoe Region Fen Assessment, Grass Lake received the highest conservation significance rank of any fen in the Lake Tahoe Region with a score of 30 out of 40 (Sikes et al. 2011). Elements that contributed to the high ranking include its large size, its status as a Natural Research Area, the presence of rare plants and vegetation associations, high species diversity, low levels of disturbance, and a high likelihood of persistence. This high score combined with a qualitative assessment of management and recreation led to a rating of “at or better than target” in 2012 (TRPA 2012).

Human and Environmental Drivers

Any event or activity that disturbs the hydrologic regime or nutrient levels of a fen or causes drying or changes in plant composition is a threat to the function of that fen (Patterson and Cooper 2007, Weixelman and Cooper 2009). Activities in general that threaten fens in the Sierra Nevada include timber harvest, mechanical fuel reduction treatments, road and trail construction, livestock trampling, off-road vehicles, ground and surface water pumping, and water pollution (Cooper and Wolf 2006). All are regulated and managed in the Tahoe Region, and the RNA status protects Grass Lake from these activities. Recreational use is light, and the impacts from cross-country skiing in the winter are likely to be negligible. Runoff from State Route 89 has likely been a source of water pollution, but recent road improvements for stormwater management were designed to divert surface road flow away from Grass Lake. Hydrologic change, which will likely be exacerbated by climate change, is predicted to be the largest threat to the Grass Lake community (Christensen 2013). Recent California based climate models predict a nine-degree Fahrenheit increase in temperature by 2100, and more conservative models predict a two- to four-degree Fahrenheit increase in winter and four- to eight-degree increase in summer (Safford et al., 2012a). Models are more variable for precipitation, but recent models for the Sierra Nevada predict similar to slightly less precipitation. Most models predict drier summer conditions, since more of the precipitation is predicted to come as rain, and snow melt-off will occur earlier in spring (Hayhoe et al. 2004, Dettinger 2005, Safford et al. 2012, Drexler et al. 2013). In the Lake Tahoe Region, these changes appear to be happening at an accelerated pace (Coats 2010). Snowpack is the dominant source of groundwater recharge (Earman et al. 2006), and since Sierra Nevada fen sustainability depends on groundwater, these climatic changes pose a severe threat (Drexler et al. 2013). Further, Sierra Nevada fens have relatively shallow peat depths, which make them highly susceptible to drying effects from increased temperature and/or reduced moisture (Drexler et al. 2013). Drexler et al. (2013) found that five Sierra Nevada fens had shrunk by 10 to18 percent between 1951 and 2010, while at the same time mean minimum air temperature had increased and snowpack longevity and April 1 snow water equivalent had decreased. Hydrologic monitoring and climate change modeling scenarios show that a rain dominated precipitation regime would likely lead to desaturation of approximately half of Grass Lake, which would cause aerobic decomposition of peat near the fen’s edges and in the western and eastern portions (Christensen 2013). Increasing temperature would accelerate the rate of decomposition.

Transportation Indicators

• Acres of Habitat Restored or Enhanced
• Parcels Inspected for Defensible Space