Ecological dynamics

Abiotic factors
This ecological site is a complex of vegetation communities associated with depressional basin peatlands and mound peatlands. Peatlands, or fens, form when stable hydric soils with an aquic soil moisture regime allow a rate of organic matter production that is greater than the rate of decomposition, which over very long periods of time, leads to an accumulation of peat (Patterson and Cooper 2007, Weixelman and Cooper 2009). Peat accumulates very slowly, taking thousands of years to develop (Patterson and Cooper 2007). Basin fens develop in topographic depressions with a groundwater source, or that receive surface runoff from basin edges (Weixelman and Cooper 2009). Mound fens are raised areas where peat accumulates due to a source of upwelling of water (Weixelman and Cooper 2009).

Ecological/Disturbance factors
The fen system supports a complex of soils and vegetation communities. In the center is open water supporting aquatic vegetation including yellow pond lily (Nuphar lutea) and pondweeds (Potamogeton ssp.). At the water's edge is a floating or stable sphagnum mat. This mat is dominated by sphagnum moss species. The fen vegetation changes in relation to depth and stability of the peat layer. In some cases this can be seen in concentric rings extending from the remaining open water. More stable surfaces support dwarf bilberry, bog laurel, mountain willow, and sedges and forbs. This mat may support unusual plant species such as the carniverous round leaf sundew (Drosera rotundifolia) and the mud sedge (Carex limosa). Fen margins, which are seasonally inundated and have soils with more mineral horizon development, support emergent plants dominated by sedges (Carex ssp.). Trees such as Sierra lodgepole pine (Pinus contorta var. murrayana) may invade and retreat from the fen and fen margins (Bartolome et al. 1990). Meadows associated with the fen system have drier soils with more mineral horizon development, and support a diverse tufted hairgrass (Deschampsia caespitosa) – forb community. Discharge slopes with very deep histic soils may be present, and support a forb community dominated by California false hellebore (Veratrum californicum var. californicum).

Fens in the Sierra Nevada are highly important for the biological diversity they support, and they are also important carbon sinks (Weixelman and Cooper 2009). Fens are naturally highly stable systems although fluctuations in size, location, and plant community composition may occur over time (e.g. Erman 1976, Bartolome et al. 1990). However, fens are highly vulnerable to anthropogenic disturbances including hydrological diversions, grazing, and off-road vehicle use (Patterson and Cooper 2007, Weixelman and Cooper 2009). Hydrological alterations such as ditches, dams and roads that reduce water supply to the fen creates aerobic conditions that stop the accumulation of peat and allow non wetland species to establish (Cooper et al. 1998, Patterson and Cooper 2007, Austin 2008). Removing the diversion and re-establishing water flow can re-establish anaerobic conditions and restore the fen (Cooper et al. 1998, Patterson and Cooper 2007). Grazing and off-road vehicle use can compact peat, expose bare ground, and lead to erosion and gullying that depletes fen water supply and allows for the establishment of non wetland species as above (Weixelman and Cooper 2009).

Historical land management has impacted fens in the Lake Tahoe Basin. For example, Osgood Swamp was drained in the 1960s, but after acquisition by the U.S. Forest Service the water table was restored (Holst 2000). The water table then dropped in response to drought in the 1990s, allowing Sierra lodgepole pine (Pinus contorta var. murrayana) to invade, but beaver activity assisted by management again restored the water table and the meadow (Holst 2000). All fens were likely impacted by historic grazing, as all meadows in the Lake Tahoe Basin were grazed in the past by cattle and sheep. Over 13 dairy farms were active in the basin, and most meadows were fenced for cattle grazing. Sheep roamed the mountains, and denuded much of the forbs and grasses (Elliot-Fisk et al. 1996). The impacts on the basin fens have been described as minimal however (Holst 2000).

The peatlands in the Lake Tahoe basin area have developed by gradually filling in relict glacial lakes. Grass Lake by Luther Pass in one example of this process. Slowly over time sphagnum has grown from the edge of the meadow towards the center. As it grows, it inhibits water flow creating an anoxic environment. Low oxygen and nutrient availability create an environment suitable for the growth of sphagnum mosses. As the mosses continue to grow, they create a thick floating mat of fiber called peat. If you walk on this layer it shakes and rolls like walking on a waterbed. When this peat layer becomes thick enough, or reaches the bottom of the lake, Sierra lodgepole pine and mountain willow (Salix eastwoodiae) can establish. The vegetation changes in relation to depth and stability of the peat layer.

Although every peatland is different within this survey area, they tend to follow a similar pattern. In the center is open water with yellow pond lily (Nuphar lutea) and pondweeds (Potamogeton spp.). At the water's edge is the floating sphagnum mat. On this mat several unusual plant species survive, such as the carniverous round leaf sundew (Drosera rotundifolia) and the mud sedge (Carex limosa). Emergent plants, such as Northwest Territory Sedge (Carex utriculata) are found bordering the mat in the shallow water. Beyond the Northwest Territory sedges is a wet grassland plant community dominated by tufted hairgrass (Deschampsia caespitosa).

The reference state is typically the pre-settlement, most successionally advanced and hydrologically stable community phase (numbered 1.1), and the community phases that result from natural and human disturbances. Community phase 1.1 is deemed the phase representative of the most successionally advanced pre-European plant/animal community including beaver activity and hydrologic conditions that influence its composition and production. Because this phase is partly determined from reconstruction and/or historic literature, some speculation is necessarily involved in describing it.

All tabular data listed for a specific community component within this ecological site description represent a summary of one or more field data collection plots taken in modal communities within the community component. Although such data are valuable in understanding the community component (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), they do not represent the absolute range of characteristics or an exhaustive listing of all species that may occur in that phase over the geographic range of the ecological site.