 SCOPE AND BACKGROUND OVERVIEW OF SIERRA NEVADA ECOSYSTEMS AND ASSESSMENT STATUS MANAGEMENT STRATEGIES FOR ECOSYSTEM SUSTAINABILITY INSTITUTIONAL INTEGRATION OF ASSESSMENTS AND SOLUTIONS
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rev. Jun. 17, 1996 |
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Climate 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 recent 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. Major climate change has occurred at millennial, decadal, and annual scales in the history of the Sierra Nevada. 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 over 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 regimes. The current Sierran climate is dominated by a mediterranean pattern of a cool, wet winter followed by a long hot and dry period in summer, with high yearly variability in temperature and precipitation. Precipitation increases and average temperature decreases with increase in elevation. 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. From moist mountain ecosystems at the Sierran crest, the transition to semiarid desert can occur in less than two horizontal miles. 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. Water Given strong seasonal Mediterranean patterns, high annual variability of climate, natural aridity of the eastern flanks, and the constant thirst of plants, animals, and burgeoning human communities adjacent to the Sierra, water remains a subject of intense competition for all Sierran biota. Water partitions the Sierra into twenty-four readily discernible river basins or watershed units. Streams, creeks, and temporary waters define subwatersheds at increasingly smaller scales within these areas. Watersheds at each scale are important to a diverse aquatic biota, including fishes, amphibians, invertebrates, and plants. Aquatic and their associated riparian systems are the most altered and impaired habitats of the Sierra. At middle and low elevations, fish diversity of pre-contact streams was high compared with the present. Chinook salmon and steelhead once ran in most of the major Sierran streams but now have been nearly eliminated from the range due to dams and impoundments, which profoundly alter stream-flow patterns and water temperatures. Decline in other native fish species is also evident, especially at lower elevations. The best indicators of health of the aquatic system may be the group of organisms least knowninvertebrates. These small creatures are rarely seen by most people but are central to the functioning of aquatic ecosystems and represent the majority of species diversity. Some species are highly specialized, occurring only in a few wetlands, springs, or small streams. Extensive and abundant populations of frogs and salamanders once inhabited most Sierran streams, lakes, and wet meadows. Frogs are now missing from many of these habitats. More than 4,000 lakes in the high Sierramost of them naturally fishlessonce supported a diversity of aquatic amphibian and invertebrate species. Non-native fish, introduced for sport fishing, now dominate most of these high Sierran lakes and have radically transformed aquatic ecosystems at the expense of native amphibians and invertebrates. Despite apparent protection of natural resources by wilderness and other reserve area designations, native aquatic biota have suffered extensive local extinctions and are threatened rangewide. Plants and Vegetation The Sierra Nevada today is rich in vascular plant diversity, with more than 3,500 native species of plants, making up more than 50% of the plant diversity of California. Hundreds of rare species and species growing only in the Sierra Nevada (en-demics) occupy scattered and particular niches of the range. Three native species are believed to be extinct from the range, whereas hundreds of non-native species now occur in the range that were not present before Euro-American settlement. Vegetation, or the assemblage of plants growing together in an area, is a dominant element of Sierran ecosystems, for ecological functions that plants engage in (e.g., soil aeration) and as habitat and sustenance for other organisms. The distribution of wildlife is closely associated with the distribution of vegetation, and the same is true for less visible and less familiar forms of life such as fungi, bacteria, and insects. The major vegetation zones of the Sierra form readily apparent large-scale elevational patterns. Unlike aquatic systems, whose dominant Sierran pattern is defined by east-west watersheds, primary vegetation types of the Sierra form north-south bands along the axis of the Sierra. Major east-west trending watersheds that dissect the Sierra into steep canyons form a secondary pattern of vegetation in the Sierra. Diversity of regional and local plant species in the Sierra Nevada is highly influenced by climate, elevation (temperatures), and soil type, and eighty-eight primary vegetation types are recognized. Only part of the Sierran landscape is forested, the rest being meadow, chaparral scrub, woodland, savanna, canyon land, alpine habitat, bare rock, and water. The boundaries of the Sierran floristic province differ from boundaries defined by geology, watersheds, aquatic diversity, or wildlife, especially at the northern and southern edges of the range. Of all the Sierran vegetation types, the foothill plant communities have supported the most native biodiversity and highest human populations during the last few centuries. Now these are most at risk of loss by conversion to human settlement. On the west side, forest types change from ponderosa pine to mixed conifer to firs with increasing elevations. On the east side, forest types change from piñon pine and juniper to Jeffrey and ponderosa pines and an east-side version of mixed conifer. Straddling the crest is a zone of subalpine and alpine vegetation. Each vegetation type in the Sierra is in itself a mosaic. Small changes in topography, differences in soil and rock characteristics, and the history of disturbance (fire, storm blowdown, insect and pathogen activity, avalanche) contribute to the complex mixture of patches that characterizes Sierran forests. Plant patterns vary not only from place to place in the Sierra but also over time. This complexity at the local scale makes it difficult to map vegetation, to generalize relationships of structure to function, and to assess forest conditions. Characteristic structure and function develop in Sierran forests as they age. Under aboriginal conditions, fires and other disturbance events regularly burned entire stands of trees, leaving openings that passed through continuous but distinctive phases as they aged. This succession of a forest through time between major disturbances is important for plants and animals that use different stages as habitat. Different ecological functions develop with successional phase in a forest. From seedling colonists to mature forest stands, forests develop in structural complexity and species composition until they reach a stage known as late successional, or more popularly, old growth. We know most about late successional/old-growth attributesand the relationships of structure to ecological functionin middle-elevation conifer forests, specifically mixed conifer, red fir, and east-side pine. A dominant feature in middle-elevation forests is the spatial variability that develops as a result of succession in Sierran forests. In these and other vegetation types, wildfire was a frequent characteristic of pre-contact conditions. The vagaries of fire, from low to high intensity, small to large areas, contribute to the great variability that typifies Sierran middle-elevation forests. Each stand passes through its own history, thus developing a distinctive structure. Various events (tree fall, windfall, avalanche, fire hot spots, insect outbreak) create small and large openings in some areas, whereas other areas maintain standing trees (alive and dead) despite disturbance. Patches develop a characteristic structure in their abundances of large, old trees (relicts left after ground fires); multiple age classes of live trees; mixtures of dominant species; snags and downed woody debris of different sizes and degrees of deterioration; closed crown canopy; and layers of vegetation. Collectively the forests containing these patches are highly heterogeneous. The image evoked popularly by the term old growth , that is, extensive uniform stands of even-aged, old trees, although descriptive of some Pacific Northwest forests, does not fit the complex and heterogeneous Sierran forests. Depending on forest type, about 19% of the current distributions of middle-elevation conifer forest types are presently in high-quality late successional condition. National parks contain the major concentrations of these forests, and, proportionately, have about four times as much forest in late successional condition as the Sierran national forests (for west- and east-side mixed conifer, white fir, and red fir). Despite alteration of the national park forests due to fire suppression, forests in the parks represent the best available benchmark for presettlement amounts of late successional forest at these elevations in the Sierra. The most commercially valuable forest types, namely, west-side mixed conifer and east-side pine, are the most deficient in high-quality late successional forests. These types have had the longest and most intense histories of timber harvest. Despite timber harvest, fires, livestock grazing, and mining, there is still a high level of continuity in middle-elevation forest landscapes. The forest cover at these elevations is relatively continuous, and most forested stands have sufficient structural complexity to provide for at least low levels of late successional forest functions. Fragmentation of forests through patch clear-cutting has been much less common in the Sierra than on federal forest lands in the Northwest. Though forest continuity is high, forest structure has been greatly simplified relative to pre-contact conditions. Over the past decade, Sierra Nevada conifer forests have experienced widespread, locally severe levels of mortality caused principally by bark beetles infesting trees stressed by drought, overdense stands, and pathogens. Pine and fir forests in the Tahoe Basin and along the eastern slope of the Sierra have been especially affected, although heavy losses in firs have occurred in central-western forests. Along the western boundary of the Sierra, air pollution stress contributes to extensive mortality. Although fire suppression and some forestry practices are implicated in the die-off, outbreaks of similar extent are recorded to the beginning of the twentieth century and appear to be the natural condition. The oak woodlands, grass savannas, and riparian communities of the Sierra Nevada foothills are the most ecologically transformed terrestrial ecosystems in the range. These communities have been converted at an alarming rate over the last century, first for rangeland clearing and more recently for residential and industrial developments. In the mid-nineteenth century, the perennial herbaceous understory in these communities was virtually replaced by introduced annual Eurasian grasses and herbs whose life history traits differ greatly from those of native species, and those traits create major transformations in ecological function that ripple through the ecosystem. Most areas have been grazed heavily for many years or converted to agriculture. Local firewood collection has reduced the abundance of large, old trees, snags, and fallen logs. Riparian habitats in the foothill zones have suffered proportionately greater reduction than those elsewhere in the range, with species reduction and total removal of vegetative cover in many places. 
 
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