Relevance

Cushion plants have a low, matted growth form that is typical of high elevation environments. This growth form allows them to withstand extreme climates with gusting winds, snow, and huge temperature variation (e.g. Malcolm and Malcolm 1988). The main occurrence of this plant community type in the Region is at elevations above 9,000 feet on the cluster of peaks around Freel Peak (Engelhardt and Gross 2011b). These windblown peaks support a fell-field environment that is largely covered in surface rock fragments and limited to low-statured, long-lived, slow-growing subshrub, forb and grass species. Subshrubs such as cushion buckwheat (Eriogonum ovalifolium) are cushion forming, and forbs such as Nevada podistera (Podistera nevadensis) are mat forming, while grasses form tight tussock growth forms (Billings and Mooney 1968, Bahn and 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; Korner, 2003). The Freel Peak cushion plant community supports a variety of uncommon plant species, including one of the main population centers of Tahoe draba (Draba asterophora var. asterophora). Tahoe draba is specially designated by TRPA and the U.S. Forest Service to provide this species with increased levels of protection (Engelhardt and Gross 2011a).

Human and Environmental Drivers

Climate change and trampling during recreational activities are considered the greatest threats to the community.

The rocky, loose, often steep soils of this area are highly susceptible to erosion impacts from trails and trampling and recreational use has the potential to degrade the community. 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). Protection from trampling can reverse these impacts (Bell and Bliss, 1973). Trampling of Tahoe draba in the area has been observed (Engelhardt and Gross 2011a).

Prior to the threat of climate change, high elevation cushion plant communities were considered to be a naturally stable type (Malcolm and Malcolm 1988). With climate change high elevation communities throughout the world are experiencing rapid changes (e.g. Gottfried et al. 2012) and have been identified as bellwethers for global climate change impacts (e.g. Seastedt et al. 2004).

A continental scale study of changes on all of Europe’s major mountain ranges found declines in high elevation species and increases in lower elevation species, which were correlated with increasing temperatures between 2001 and 2008 (Gottfried et al. 2012). Microtopographic variation that allows for xeric and mesic species to co-occur may allow for local migration, and confer resilience to climate change (Gibson et al. 2008, Spasojevic et al. 2013). However, areas like the Lake Tahoe Region where true alpine habitat is very limited will likely not have this resilience. Species composition is likely to change, with strictly alpine species likely to be replaced by species with wider ecological ranges. 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 zone and have no upslope environment to move to. A modelling study in the White Mountains of California predicted a six-degree Fahrenheit temperature increase would lead to the extinction of 10 out of 14 alpine forbs modelled. The remaining four species were predicted to lose 99 percent of their current range (Van de Ven et al., 2007). A three-degree temperature increase was predicted to result in the extinction of two species, and lead to severe range restrictions of all others (Van de Ven et al. 2007). California based climate models predict a nine-degree Fahrenheit increase in temperature by 2100, and more conservative models predict a two- to four-degree Fahrenheit increase in winter and four- to eight-degree increase in summer (Safford et al., 2012a). Models are more variable for precipitation, but recent models for the Sierra Nevada predict similar to slightly less precipitation. Most models predict drier summer conditions, 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).

No related projects or programs defined for this indicator.