American pika, Belding's ground squirrel, the yellow-bellied marmot, and the endangered Sierra Nevada bighorn sheep. This zone can be viewed up close by hiking or climbing into the high elevations of the Sierra. Eastern biotic zones The four highest eastern biotic zones are the same as the western zones, but at a higher elevation, due to less precipitation. The elevation of these zones in the Central Sierra are: Alpine zone: and above Subalpine forest: Upper montane forest: Lower montane forest: (heavily dominated by Jeffrey pines). In the Owens Valley, the Foothill Woodland Zone is replaced by a Pinyon-Juniper Woodland Zone, characterized by single-leaf pinyon pines and sierra junipers. The underbrush contains big sagebrush (Artemisia tridentata) and blackbrush (Coleogyne ramosissima). Jeffrey pines may occur along streams. Notable animals in this zone include the pinyon jay and the desert bighorn sheep. The Pinyon-Juniper Woodland Zone extends down to elevation. Below , there is not enough precipitation to support trees. The zones below this elevation are the Sagebrush Scrub Zone, Saltbush Scrub Zone, and the Alkali Sink Zone. These zones are distinguished by soil salinity. Threats Exotic Plants in Yosemite National Park Yosemite National Park has documented more than 130 non-native plant species within park boundaries. These non-native plants were introduced into Yosemite following the migration of early settlers in the late 1850s. Natural and human-caused disturbances, such as wildland fires and construction activities, have contributed to a rapid increase in the spread of non-native plants. A number of these species aggressively invade and displace the native plant communities, resulting in impacts on the park's resources. Non-native plants can bring about significant changes in park ecosystems by altering the native plant communities and the processes that support them. Some non-native species may cause an increase in the fire frequency of an area or increase the available nitrogen in the soil that may allow more non-native plants to become established. Many non-native species, such as yellow starthistle (Centaurea solstitialis), are able to produce a long tap root that allows them to out-compete the native plants for available water. Bull thistle (Cirsium vulgare), common mullein (Verbascum thapsus), and Klamath weed (Hypericum perforatum) have been identified as noxious pests in Yosemite since the 1940s. Additional species that have been recognized more recently as aggressive and requiring control are yellow starthistle, sweet clovers (Melilotus spp.), Himalayan blackberry (Rubus discolor), cut-leaved blackberry (Rubus laciniatus) and periwinkle (Vinca major).