of the divide, and the East Side drops off sharply to the Great Basin. On this steep eastern escarpment, altitudinal vegetation zones overlap extensively, making them less apparent than on the West Side. North of Lake Tahoe the main crest is flanked on both sides by other ridges and the elevation of the crest itself is lower, making the distinction between the western and eastern Sierra less obvious. The Kern Plateau in southeastern Tulare County is an area that does not fit neatly into the zones described below. It includes ecological elements from both sides and is an area where species normally associated with the East Side (e.g., Pinyon Jay) occur on the West Side, and where other species are found at much higher altitudes than elsewhere in the Sierra (e.g., Lawrence’s Goldfinch).
In this book we have recognized seven major ecological zones: Foothill, Lower Conifer, Upper Conifer, Subalpine, Alpine, East Side, and Desert (see Table 1; Map 3). Note that the elevation ranges are approximate and overlap considerably. Local differences in slope, soils, rainfall, and other factors alter the exact range of any ecological zone. As discussed in this chapter, most of these zones include several distinct bird habitats. Sierra watersheds and key locations are shown in Map 4. Common and scientific names of all plant species are provided in Appendix 4.
MAP 2 Elevational zones
MAP 3 Ecological zones
MAP 4 Watersheds and key locations mentioned in the text
TABLE 1 Approximate elevations and total area of Sierra ecological zones
FOOTHILL ZONE
Annual Grasslands
North <500 to 2,000 feet; South <500 to 2,500 feet
Many travelers pass through annual grasslands without registering them as “habitat.” This open (less than 10 percent tree cover), gently rolling terrain is parched to a golden brown in summer but transforms to vivid green in late fall through early spring. Patches of this habitat in the Sierra are found in eastern Tehama and Butte Counties and westernmost El Dorado County, but the largest expanses are found from western Tuolumne County south into Kern County. Because no naturalists were present in pre-European times to document the conditions, we do not know what plant species dominated this landscape, and there is considerable controversy about whether perennial grasses or forbs were the most abundant plants. Even before European settlement, Native Americans had been managing these areas with fire for thousands of years. In any case, they are now dominated by introduced grasses brought by European settlers. A high diversity of indigenous plants still survive in vernal pools or intermixed with the non-native species. In many areas woodland and chaparral were cleared to create grazing land and are now annual grasslands. In spring, wildflowers still flourish in the foothills, and specialized blooms form rings around the receding waters of vernal pools. Recent research has shown that cattle grazing—by far the dominant land use in this habitat—is actually beneficial for most native vernal pool plants and most grassland birds, at least partly because grazing keeps aggressively invasive plants like star thistle and medusa head in check and prevents thatch buildup that inhibits growth of many native plants.
Wintering Horned Larks and American Pipits flock together in grazed pastures and plowed furrows, and Savannah Sparrows forage in the deeper grasses, each species constantly wary of the risk of a Prairie Falcon attack. Annual grasslands provide abundant food and cover for high numbers of rodents and other small mammals and therefore support an impressive variety of raptors with winter migrants augmenting resident Red-tailed Hawks and American Kestrels and species like Ferruginous and Rough-legged Hawks visiting only from fall through early spring. In spring, American Kestrels, Western Kingbirds, and Loggerhead Shrikes stake out territories on fence lines or on “tombstones” of ancient metamorphic rock that rise abruptly here and there. The lovely songs of Western Meadowlarks can be heard almost any month of the year. On hot afternoons Turkey Vultures float lazily above low ridges and hillsides.
FIGURE 1 Foothill zone
Oak Savannas
North < 500 to 3,000 feet; South <500 to 2,500 feet
Savannas dominated by blue oak occupy more than a million acres of the Sierra foothills. These habitats have 10 to 30 percent tree cover and vary from grasslands with a few widely spaced trees to denser stands that may also support interior live oaks, California buckeyes, and occasional gray pines. The extent of oak savanna in the foothills has been reduced through conversion to developed and agricultural land uses, woodcutting, and historical efforts to improve grazing land by clearing trees. Range managers have now learned that having oaks on the land actually improves the forage for cattle by allowing the shaded grasses to retain water.
This quintessentially California landscape is threatened by a widespread lack of natural regeneration of oaks. Although the impacts of grazing have certainly affected the ability of these trees to reproduce, it has been shown that simply removing grazers does not aid regeneration. Other factors, like competition with nonnative plants for scarce water (mainly at the seedling or sapling stages), are also critical to oak survival. In fact, removing grazers entirely can inhibit oak regeneration because of the deep thatch and explosion of invasive weeds that usually follows. Encouraging results have been seen with a combination of protection of seedlings and saplings from grazing and management of competing vegetation, followed by a well-managed grazing regimen. Western Scrub-Jays have historically been the main agents of oak regeneration because acorns they bury but fail to recover have a good chance of germinating; since acorns do not roll uphill, jays and squirrels must move them there. Oaks have evolved a boom-and-bust cycle of acorn production to guarantee that in boom years there will be far too many acorns for the jays to consume.
Oak savannas provide perching and nesting sites for several species of raptors as well as for a stunning variety of songbirds like Lark Sparrows, Western Kingbirds, Bullock’s Orioles, and Western Meadowlarks. As the oaks age or die, they provide essential nesting habitat for a variety of cavity nesters such as Acorn and Nuttall’s Woodpeckers, Oak Titmice, Ash-throated Flycatchers, White-breasted Nuthatches, Bewick’s Wrens, and both Violet-green and Tree Swallows. Some winters, large numbers of richly colored Lewis’s Woodpeckers visit oak savannas.
Oak Woodlands
North 1,500 to 4,500 feet; South 2,000 to 5,500 feet
Above the grasslands and oak savannas, denser oak groves of more than 30 percent tree cover crowd the hillsides of the western Sierra. In addition to blue and interior live oaks, three other species dominate: valley oaks, California black oaks, and canyon live oaks. All are long-lived and sprout from their stem bases when cut or top-killed by fire. Otherwise, there are considerable differences in their ecology. Valley oaks are large deciduous trees that dominate some riparian forests and open woodlands on fertile, deep soils. Similar to blue oaks, valley oaks suffer from limited recruitment of saplings. Blue oaks tolerate thinner soils and drier conditions than valley oaks and are the most likely oak to be found in uplands well away from drainages. Interior and canyon live oaks are evergreens and vary in growth from low shrubs to large trees. In steep ravines and river canyons these oaks cling to rocky slopes, while toyon, California bay laurel, and redbud grow in cooler glades below. Gray pines, with their wispy, gray green needles, grow along with the oaks in the northern and central Sierra and in the extreme south.
In
