Ecological dynamics

Abiotic Features:

This ecological site occurs on steep headwater to first order drainageways in colluvial valleys. Stream channels are absent, small rills or a Rosgen A or Ba channel type. As presently mapped, this riparian complex has high variability. It is dominant in one mapunit, and associated with the higher order Aquic Xerorthents component (1 to 2 percent composition) in 38 mapunits. The headwater and first order streams are the most common system, so are the focus of this ecological site. Lower elevation reaches, with Order 2 and 3 streams, are currently mapped as this ecological site but should be developed as another ecological site. These areas are not adequately described in this ESD.

Hydrologic factors:
This riparian ecological site is composed of several components that may or may not occur side by side along the drainageway. It is primarily dependent upon subsurface flow, but may also naturally have a small Rosgen A or Ba channel below the sloped meadow. At the headwater sections of this riparian complex water discharges from seeps or springs in concave swales, or at bedrock faults. Water tables in the wettest areas are near the surface throughout most of the summer. Spring snowmelt can create sheet flooding across the drainageway and associated sloped meadows. Lower in the drainageway, or in steeper sections, small A or Ba channels develop.

Disturbance Factors:

During the mid-1800’s approximately 80 percent of the forests in Tahoe were clear-cut during the Comstock era (Elliott-Fisk et al. 1996). Tree removal increased erosion on adjacent mountain slopes and increased sediment supply to these systems. Removal of trees would also have reduced water loss to evapotranspiration and increased water flow into the streams. The side effects of the logging era on the stream morphology are unknown, but likely increased sediment supply and stream volume in these systems . Davis (DAVIS (1996) reports increased erosion between 1850 and 1900 based on analysis of cores from Lake Tahoe. The erosion was attributed to logging, grazing and road development during this period. During the same period, there was an increase in the algae Pediastrum, and an increase in sedge (Carex spp.) pollen, possibly due to increased nutrient loading into the lake and tributaries.

The 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 (Elliott-Fisk et al. 1996). Cattle concentrate in riparian habitats because of access to water and forage, shade and gentle landscapes (Kie and Boroski 1996). Cattle may have affected the present composition of vegetation by selectively grazing palatable species such as grasses, and trampling susceptible species. Grazing can affect channel morphology by removing bank stabilizing vegetation such as willows, and by trampling stream banks when accessing the stream.

Beavers were once thought to be non-native to the Sierra Nevada, but carbon dating of old beaver dams has shown that beavers were in the Sierra Nevada since AD 580 (James and Lanman 2012, Landman et al. 2012). Beaver trapping and extirpation efforts eliminated beavers from the higher Sierra Nevada by mid-1800s. Beavers were reintroduced into the Lake Tahoe Basin from 1939 to 1949 (Beier and Barrett 1987) after 100 years or more of absence. Nine beavers were introduced from the Snake River in Idaho to the Truckee River. Since then populations have expanded to many watersheds around Lake Tahoe. In 1987 there were 0.72 colonies (3.5 beavers) per km of stream along the Truckee River (Beier and Barrett 1987).

These small streams do not provide the deep shallow water preferred for making beaver ponds, but beaver cut thinleaf alder and willow from these areas, and carry them to their dams typically downstream from these sites.

Ecological sites associated with lotic stream systems are developed using channel evolution models. Stream systems are dynamic and continually evolve to reach a stable equilibrium. Streams develop identifiable stages of development based on channel morphology (Rosgen 1997). These stages are identified using a state and transition model based on stream evolution models.

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.