Ecological dynamics

Abiotic factors
This ecological site occurs on high elevation mountain passes with deep to very deep loamy-skeletal soils derived from glacial till. Strong winds that are funneled through the pass redistribute snow from these areas, creating dry, exposed, extremely cold environments in winter, and droughty environments in summer. These dry windy conditions, cold temperatures, intense solar radiation and a short growing season limit productivity in this ecological site, promoting fell-field communities of low growing subshrubs, forbs and grasses. Species characteristic of dry rocky conditions dominate. However, loamy deep soils have higher water holding capacity than shallow soils found on adjacent ridges, allowing for species characteristic of more mesic conditions, and leading to a diverse plant community. These mesic species, for example alpine gentian(Gentiana newberryi) and Idaho bentgrass (Agrostis idahoensis) are often found in microsites where snow accumulates, such as in the lee of boulders or stones, or depressions in the soil. Sierra mousetail (Ivesia santolinoides) and spreading phlox (Phlox diffusa) dominate.

Alpine plant communities are determined by a snow cover gradient that is the product of wind exposure and topography, and by soils and geologic substrate (Billings and Mooney 1968, Burns and Tonkin 1982, Seastedt et al. 2004). Areas with exposed topography, especially with aspects that are exposed to prevailing winds, are blown free of snow for the majority of the year, for example periods of 225 or more snow-free days/year (Billings and Mooney 1968) are common. Wind through mountain passes can be extreme due to the funneling ‘Bernoulli effect’, as wind seeks a way around higher peaks on either side of the pass. These windblown sites support a fell-field community that is largely covered in surface rock fragments, and limited to low-statured, long-lived, slow-growing subshrub, forb and grass species (Smiley 1915, Kilkoff 1965, Billings and Mooney 1968, Körner 2003, Elliot and Jules 2005). These open fell-fields are characterized by a high percentage of surface rock, with sparsely distributed vegetation. Fell-field plants are less insulated than sod-forming meadows, and subject to rapid increases in soil temperature as well as rapid decreases (Billings and Mooney 1968). Subshrubs such as spreading phlox and cushion buckwheat (Eriogonum ovalifolium) are cushion forming, and forbs such as Nuttall’s sandwort (Minuarta nuttallii) and rosy pussytoes (Antennaria rosea) are mat forming, while grasses form tight tussock growth forms (Billings and Mooney 1968, Körner 2003). These adaptations trap heat, which increases photosynthetic capacity in these cold environments, and limits moisture loss due to transpiration (Billings and Mooney 1968, Körner 2003).

Disturbance/Ecological factors
Fluctuations in temperature and moisture are the primary natural disturbance impacting this ecological site, and these may temporarily alter species composition. Global climate change will likely impact this site and is discussed below, but is not included in the state-and-transition model. Anthropogenic disturbances such as trail building and trampling may impact this site, with very slow recovery due to the slow growth rate of species in this harsh environment. Sheep grazing in the late 1800’s and early 1900’s was intense and ubiquitous in the Sierra Nevada (e.g. Ratliff 1974, Vankat and Major, 1978, Benedict 1983, Ratliff 1985, Odion et al. 1988), and may have impacted this site. Impacts could include trampling and compaction of soils, trampling of sensitive vegetation, and alteration of species composition by selective grazing. Data is not available for grazing impacts on this site, and they are not included in the state-and-transition model.

Long periods of severe drought would likely lower diversity on this site, eliminating or greatly reducing species dependent on greater moisture availability. It would also likely lower production and cover overall, as even drought-adapted species may experience leaf dieback or reduce production during drought (e.g. Peterson and Billings 1982). At the time data was collected for this site annual precipitation was 65 percent of average; however, the Sierra Nevada were also in the third year of drought and evidence of dieback was apparent (see photo in community phase 1.2). Very wet years on the other hand are likely to see an increase in moisture dependent plants, and could see dieback in species adapted to dry conditions. Wet conditions caused by an increased duration in the snowpack would shorten the already limited growing season of this ecological site, causing reduced cover and production in the plant community (e.g. Franklin 2013). Wet and warm conditions without an increase in snowpack duration would likely see an increase in productivity and cover (e.g. Franklin 2013, Klimesova et al. 2013).

An ongoing pattern of warming and decreased snowpack is predicted under global climate change scenarios for the Sierra Nevada (e.g Hayhoe et al. 2004, Safford et al. 2012). 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). Alpine areas have been called bellwethers for global climate change impacts (e.g. Seastedt et al. 2004), due to a highly specialized flora and fauna that may not compete well with a lessening of harsh environmental conditions, and no higher elevations to retreat. However, unidirectional change may not be realistic. Microtopographic variation that allows for xeric and mesic species to co-occur may allow for within site or more local migrations, and confer resilience to climate change (Gibson et al. 2008, Spasojevic et al. 2013). Alternatively fell-field communities adapted to severe drought may expand downward into previously moister locations with increased warming and reduced precipitation (Spasojevic et al. 2013). Long-term studies of fell-field communities in Colorado have shown these dry systems to be fairly resilient to change, with impermanent transitions occurring in response to annual variations in climate (Johnson et al. 2011, Spasojevic et al. 2013). However, while communities are not being lost, there is an overall trend of increased diversity as species from lower elevations move up, and in time, these species may outcompete the locals (Johnson et al. 2011, Spasojevic et al. 2013).

Warmer and drier conditions could lead to an expansion of this ecological site, as it could move into habitats currently occupied by snowbank or moister communities. Warmer temperatures would likely increase the growth and productivity of at least some species in this plant community, for example granite prickly phlox in the eastern Sierra Nevada was shown to have higher growth rates during warmer periods (Franklin 2013). However increasing aridity could offset these increases. Species composition is likely to change; with species that occur in this site but that have a wider ecological amplitude than strictly alpine environments, such as cushion buckwheat (Eriogonum ovalifolium), 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, Johnson et al. 2011, Spasojevic et al. 2013). However, these increases in species richness may be offset over time by the extirpation of species that are restricted to the alpine and have no upslope environment to move to, such as rosy buckwheat (Eriogonum rosense) or marumeaf buckwheat (Eriogonum marifolium). A modeling study in the White Mountains of California predicted extinction of 10 out of 14 alpine forbs modeled with a 6 degree temperature increase, with the remaining 4 shrinking to less than 1% of their current range (Van de Ven et al. 2007). A 3 degree warming predicted extinction of 2 species, and severe range restrictions of all others (Van de Ven et al. 2007). Since the specific changes that will occur with global climate change are currently unknown for this site, climate change impacts are not included in the state-and-transition model.

All tabular data listed for a specific community phase within this ecological site description represent a summary of one or more field data collection plots taken in communities within the community phase. Although such data are valuable in understanding the phase (kinds and amounts of ground and surface materials, canopy characteristics, community phase overstory and understory species, production and composition, and growth), it typically does not represent the absolute range of characteristics nor an exhaustive listing of species for all the dynamic communities within each specific community phase.