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Home Water Aquatic Species and Communities

Grass Lake

Status and Trend

Interpretation and Commentary

Status: At or Somewhat Better Than Target
Trend: Little or No Change
Confidence: Moderate

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  • 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 Basin. 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 rely on groundwater input rather than precipitation. They are important sites of groundwater discharge, and may serve as indicators of shallow aquifers (Cooper 1990). Fens form slowly over thousands of years and thus, they are not easily restored once destroyed (Cooper et al. 1998). Fens have been identified by the U.S. Forest Service (SNEP 1996; Weixelman and Cooper 2010)and in the Tahoe Science Plan (Hymanson and Collopy 2010) as among the most sensitive habitat types in the Sierra Nevada. Fens are hotspots of biodiversity that support rare plants, insects, and small and large mammals. 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).
  • Standard - Provide for the non-degradation of natural qualities of any plant community that is uncommon to the Basin or of exceptional scientific, ecological, or scenic value. The Threshold Standards shall apply, but not be limited to, 1) the deep-water plants of Lake Tahoe, 2) Grass Lake, 3) Osgood Swamp, 4) Hell Hole, 5) Upper Truckee Marsh, 6) Taylor Creek Marsh, 7) Freel Peak Cushion Plant Community, and 8) Pope Marsh.
  • Indicator - The status and trend determination is based on a qualitative assessment of the natural qualities of a plant community. Natural qualities of a plant community include the current plant species assemblage, the health, age, and ecological condition of those plant species, and the condition of the hydrologic regime.
  • Status – Although Grass Lake is located near a major state route, the wet conditions and unstable sphagnum substrate deter hikers and cyclists. Recreational use is mainly limited to cross-country skiing in the winter. The RNA status protects the site from off-road vehicles, grazing, and water diversions. LTBMU management actions have focused on the installation of vegetation monitoring plots within Grass Lake, but the data is not yet available (Engelhardt and Gross 2011a). Fuels reduction treatments in the surrounding area include 100 acres that were mechanically thinned on the west side of Grass Lake in 2008, and about 200 acres are planned for thinning on the opposite side of the highway (USDA 2011b). Extensive roadwork on Highway 89 was completed in 2011, with the central objective to safeguard water quality in Grass Lake and Lake Tahoe.

    Recent Threshold Evaluations have assessed the status of Grass Lake as in attainment based on qualitative evaluations of recreation impacts and management actions, rather than any direct measurements of factors that contribute to the integrity of the community (TRPA 2001; TRPA 2007c). Aquantitative system for ranking the ecological integrity and quality of fens in the Sierra Nevada has recently become available (Sikes et al. 2011), and was used to assess the attainment status of the fen at Grass Lake.In the 2010 Lake Tahoe Basin Fen Assessment, Grass Lake received the highest Conservation Significance rank of any fen in the Lake Tahoe Basin with a score of 30 out of 40. 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 the qualitative assessment of management and recreation presented here indicates that the natural qualities of the site are being maintained and that the threshold is “at or better than target.”

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  • Trend Since this is the first time the status of Grass Lake has been assessed based on the Conservation Significance ranking, a trend analysis of that ranking is not possible. No data is available on possible impacts or improvements in the condition of Grass Lake from the surrounding fuel reduction treatments or road improvements. Recent Threshold Evaluations have concluded that the trend was stable (TRPA 2001; TRPA 2007c). However, limited quantitative data is now available that will likely be used in future evaluations. In 2004/2005 the U.S. Forest Service conducted targeted mapping of two sensitive moss species (Meesia triquetra and Sphagnum spp.) at Grass Lake (Engelhardt and Gross 2011d). Mosses (bryophytes) are good indicator organisms of environmental change due to their relatively simple structures and their sensitivity to various environmental parameters. The targeted moss mapping plots were re-sampled in 2009, and the data indicates that the area occupied by moss declined by approximately 30%, and a 1-25% decline in cover was observed. However, these rapid declines may simply represent natural hydrologic variability; 2005 had greater peak discharge and base flow compared to 2004 and 2009. The sensitive response of fen communities to hydrologic variability means that quantitative changes in trend will require a long-term dataset collected across the full spectrum of hydrologic conditions. Since these data are not yet available, the trend in the condition of Grass Lake is based on the best available information and is assessed as “little or no change.”

  • Confidence – The confidence in the status as assessed by the Conservation Significance ranking was high but the confidence in the trend analysis was low because of insufficient data. Therefore, confidence in the status and trend at Grass Lake is “moderate.”
  • 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 (Weixelman and Cooper 2009). Activities that threaten fens in the Sierra Nevada include timber harvest, mechanical fuel reduction treatments, road and trail construction, stock trampling, off-road vehicles, ground and surface water pumping,and water pollution (Cooper and Wolf 2006). 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 were designed to divert surface road flow away from Grass Lake.Hydrologic change, which could be exacerbated by climate change, is predicted to be the largest threat to this community.
  • Monitoring Approach – Two different monitoring approaches have recently been implemented at Grass Lake. As part of the Region 5 Fen Assessment program, a total of 135 potential fens, including Grass Lake, have been assessed within the Lake Tahoe Basin Management Unit since 2006 (Sikes et al. 2011). Of these, a total of 47 locations have been confirmed as fens. In addition to this inventory, the U.S. Forest Servicecollaborated with the California Native Plant Society in 2010 to developa quantitative system for ranking the ecological integrity and quality of fens (Sikes et al. 2011).Using this ranking system, surveyors objectively score a fen on eight different criteria on a five-point scale. The criteria include such factors as rarity, biodiversity, impacts, accessibility, and uniqueness. The Conservation Significance rank is the sum of scores for each criterion, and has a maximum value of 40 points. In 2010, the Conservation Significance of the 47 confirmed fens in the Tahoe Basin ranged from a low of 18 to a high of 30.

    The second monitoring approach ispart ofthe U.S. Forest Service Region 5 Range Monitoring Program designed to quantify changes in the ecological condition of wetland plant communities (Weixelman et al. 2003). The protocol is designed to classify meadows and wetlands according to dominant plant species, elevation, and site moisture characteristics, and then use a customized quantitative ecological condition scorecard for that meadow type. The user assigns an ecological condition of low, moderate, or high based on plant species composition, the presence of different plant functional groups, and other hydrogeomorphic variables. The protocol provides information on the environmental conditions necessary to support certain rare species, and the monitoring design allows for quantitatively tracking rare species abundance. In 2004, ten plots and ten permanent photo points were established at Grass Lake (Engelhardt and Gross 2011a). The moss mapping plots were installed at the same time (Engelhardt and Gross 2011d).All plots were re-visited in 2009/2010, but the data have not yet been analyzed. These quantitative data will likely form the basis of future Threshold Evaluations.

  • Monitoring Partners – U.S. Forest Service Lake Tahoe Basin Management Unit, California Native Plant Society


  • Monitoring Plan
  • Conceptual Model


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Map showing location of Grass Lake and surrounding area.

Additional Info


Additional Information

Last Updated on Tuesday, 02 October 2012 12:36