Ecological dynamics

Abiotic factors
This ecological site occurs above timberline on windswept ridges, shoulders and mountain peaks between 9,000 and 12,000 feet. Soils are derived from granitic parent material, and are shallow to moderately deep over paralithic granitic bedrock, with a sandy skeletal particle size class. Wind exposure, cold temperatures, high solar radiation, a short growing season, coarse granitic soils with low water holding capacity, and slope instability limit the vegetation community to a very sparse cover of dwarf alpine subshrubs, perennial forbs and grasses. Vegetation composition is variable, with low constancy across sites, but rosy buckwheat (Eriogonum rosense), Nevada podistera (Podistera nevadensis), granite prickly phlox (Linanthes pungens), and fewseed draba (Draba oligosperma) are among the more frequently occurring species. Alpine ridges, especially in the Sierra Nevada with a Mediterranean climate with dry summers, are extremely xeric environments due to frequent high winds and high solar radiation (Chambers 1997, Körner 2003, Elliot and Jules 2005). Where soils are coarse with little soil development, as in much of the alpine Lake Tahoe area, drought stress is particularly acute (Smiley 1915). The vegetation that can withstand these harsh conditions is limited to low-growing, long-lived, slow-growing forb and grass species (Smiley 1915, Körner 2003, Elliot and Jules 2005). Subshrubs such as rosy buckwheat and granite prickly phlox exhibit a prostrate growth form, and forbs such as Nevada podistera and fewseed draba are cushion or mat forming, while grasses form tight tussock growth forms (Körner 2003). These adaptations trap heat, which increases photosynthetic capacity in these cold environments, and limits moisture loss due to transpiration (Körner 2003). More upright forbs, which include Pacific hulsea (Hulsea algida) and dwarf alpine Indian paintbrush (Castilleja nana) are often densely covered in hairs, which serve the same functions.

Scree and talus slopes are inherently unstable environments (e.g. Perez 2012), where plants are subject to both burial and erosion, both of which also limit production and cover on this ecological site.

Disturbance/Ecological factors
Slope erosion from recreational impacts, climatic fluctuations, including both reduced and heavy snowpack, and climate change are the primary disturbances impacting this ecological site. Although lightning strikes are common in these exposed environments, fuel loads are too low to carry a fire.

Slope instability on these rocky, loose soils means that this site is highly susceptible to erosion impacts from trails and trampling. Even light trampling can trigger significant downslope rock movement, which decreases plant production and cover (Bell and Bliss 1973). The erosion of nutrient and moisture poor soils with low propagule availability in these low cover environments may cause significant damage to sensitive, slow-growing plant communities (e.g. Chambers 1997). However, protection from trampling can reverse these impacts (Bell and Bliss 1973).

Increased snowpack shortens the already limited growing season of this ecological site, causing reduced cover and production in the plant community (e.g. Franklin 2013). Reduced snowpack and warmer temperatures increases the growth and productivity of at least some species in this plant community. Granite prickly phlox in the eastern Sierra Nevada was shown to have higher growth rates during warmer periods (Franklin 2013), and decreased snowpack has been shown to increase plant performance in similar habitats in Europe (Klimesova et al. 2013).

Currently, these fluctuations fall within the range of variability expected for the reference plant community of this ecological site, and are not included as a separate community phase in the State-and-Transition (STM) model. However, an ongoing pattern of warming and decreased snowpack, which has been predicted under global climate change scenarios for the Sierra Nevada (e.g. Hayhoe et al. 2004, Safford et al. 2012), will likely have more significant impacts on this ecological site, shifting it into an altered state. Since these are theoretical impacts based on climate and range shift modeling, or on patterns observed in other locations, and the specific impacts on the species that occur in this site is unknown, an altered state is not included in the STM. Production and cover of this ecological site are likely to increase with warming, and species composition is likely to change. Species that occur in this site but that have a wider ecological amplitude than strictly alpine environments, such as granite prickly phlox, cushion buckwheat (Eriogonum ovalifolium), Shasta knotweed (Polygonum shastense), western needlegrass (Achnatherum occidentalis), and squirreltail (Elymus elymoides) are likely to increase in production and cover. Species richness may increase from species moving upslope, as has already been demonstrated in other alpine environments (e.g. Bahn and Körner 2003). However, these increases in species richness may be offset by the extirpation of species that are restricted to the alpine and have no upslope environment to move to, such as rosy buckwheat, Pacific hulsea, Nevada podistera, Tahoe draba (Draba asterophora), and dwarf alpine penstemon. A modeling study in the White Mountains of California predicted extinction of 10 or 14 alpine forbs modeled with a 6 degree temperature increase, with the remaining 4 shrinking to less than 1% of current range (Van de Ven et al. 2007). A 3 degree warming predicted extinction of 2 species including, and severe range restrictions of all others (Van de Ven et al. 2007). Recent California based climate models predict a 9 degree F increase in temperature by 2100, and more conservative models predict a 2 to 4 degree F increase in winter and 4 to 8 degree increase in summer (Safford et al. 2012). Models are more variable for precipitation, but recent models for the Sierra Nevada, predict similar to slightly less precipitation. Most models agree that summers will become drier, since more of the precipitation is predicted to come as rain, and snow melt-off will occur earlier in spring (Hayhoe et al. 2004, Safford et al. 2012).

Conifer invasion of formerly persistent snowfields has been documented during warm periods during the past century (Millar et al. 2004), and ongoing warming could see tree invasion in this site. Krummholz whitebark pine (Pinus albicaulis) communities exist at slightly lower elevations or more sheltered topographic positions to this site; milder conditions will likely lead to an expansion of these krummholz patches. Krummholz trees exhibit leader growth during warm periods (Millar et al. 2004), and would likely eventually transition to upright growth forms with continued warming. With a 2 to 6 degree warming, species such as Sierra lodgepole pine (Pinus contorta var. murrayana), or mountain hemlock (Tsuga mertensiana) may move into this ecological site. A 9 degree warming shift over the next 85 years could make conditions favorable for upper montane species to establish. Species such as Jeffrey pine (Pinus jeffreyi) and California red fir (Abies magnifica) could survive with the longer growing season and warmer temperatures for seedling germination and leader growth.