 * Critical Finding * THE SNEP STUDY AREA CLIMATE * Insect Species Found Only in the Sierra * Land Ownership and Reserve Allocation in the Sierra Nevada The Sierra Nevada of the Future
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* The SNEP Study Area
The core area boundary for the Sierra Nevada Ecosystem Project was the area containing
the headwaters of twenty-four major river basins and extending through the foothill
zone on the west side and the base of the escarpment on the east side (figure 1.2).
No single boundary adequately defines all the ecological components, but watersheds
are in many ways the most discernible and to many biota the most meaningful ecological
units in the Sierra. At the request of Congress, a larger study area for the project
included portions north of the physiographic Sierra Nevada and extensions beyond the
core area to the south and east. Appropriate adjustments to these boundaries were
considered in SNEP analyses pertinent to the needs of each issue.  FIGURE 1.2 (ACTUAL VIEW 20K) Boundaries of the core Sierra Nevada ecoregion, the study area, and the twenty-four river basins used by SNEP in its assessments. Climate
Major climate change has occurred at millennial, decadal, and annual scales in the
history of the Sierra Nevada (figure 1.3). The regional climate developed from warm,
wet, tropical conditions about 65 million years ago through a cycle of at least eight
major glacial and interglacial periods of the last million years to the winter-wet,
summer-dry pattern of the last 10,000 years. These climatic periods have greatly
influenced vegetation, animals, and human populations; their effects are observable
today and influence how people manage resources. For instance, two extensive droughts, each
lasting 100 to 200 years, occurred within the last 1,200 years. During the cold phase
of the Little Ice Age (about a.d. 16501850), glaciers in the Sierra Nevada advanced
to positions they had not occupied since the end of the last major ice age more than
10,000 years ago. The period of modern settlement in the Sierra Nevada (about the
last 150 years), by contrast, has been relatively warm and wet, containing one of
the wettest half-century intervals of the past 1,000 years. Many of the forests that stand today
were established under different climatesgenerally wetter onesfrom the present regime.  FIGURE 1.3 (ACTUAL VIEW 8K) Global temperatures (relative changes based on oxygen isotopes) at four time scales. (From volume II, chapter 4. Reprinted by permission of the Society for Range Management.) The current Sierran climate is dominated by a mediterranean pattern of a cool, wet winter followed by a long dry period in summer. High yearly variability in temperature and precipitation is also characteristic. Because of the influence of the Pacific Ocean and storm tracks from the west, strong climatic gradients develop with elevation from west to east. At foothill altitudes, summer hot, dry climates predominate; as elevation increases, so does precipitation. Winter storms are moisture-laden and release enormous precipitation on the west slope. In winter, snow covers the landscape to about 6,0008,000 feet. The transition zone of rain to snow is an important determinant of vegetation types, stream dynamics, and human settlement. The Sierra summits wring water from the winter storms and summer convection systems, leaving the eastern flank progressively drier each mile east (figure 1.4). From moist mountain ecosystems at the Sierran crest, the transition to semiarid desert near Bishop, for example, can occur in less than two horizontal miles. The west shore of Mono Lake, at the base of the Sierran escarpment, receives an average of 12 inches of rain annually, whereas the eastern edge, lying in Great Basin steppe, receives only 6 inches. Strong gradients of aridity also exist from north to south along the Sierran axis as a result of the location of jet stream and subtropical high pressure cells.  FIGURE 1.4 (ACTUAL VIEW 27K) Mount Tom and the steep eastern escarpment of the Sierra Nevada, with pinon woodlands at the base. (Photo by Deborah L. Elliott-Fisk.) Climatic and geological forces are the royal architects of Sierra Nevada ecosystems. Water, wildfire, plants, fauna, and humans are highly dependent on regional climate and local weather. Organisms must adjust (migrate, adapt) or die as climate changes. The current patterns of vegetation, water flow and abundance, and animal distribution in the Sierra are determined largely by cumulative effects of past and present climates. Human development in the Sierra has proceeded during a temporary period of relatively wet, warm climate. Patterns of human settlement, perceptions of wildfire, design of water delivery systems, predictions of water availability, future forest and urban planning, and aesthetic expectations about forest condition (size, composition, health of forests) are based largely on conditions of this anomalous climate period. One implication of a longer view of climate is, for instance, that the droughts of the mid-1970s and mid-1980s were actually not droughts at all, relative to the century-long dry periods that have been common in recent Sierran climate history. 
 
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