Volume 1/Chapter 1/Sierra Nevada Ecosystems

* Critical Finding

Introduction

Rock and Soil

* The SNEP Study Area

Climate

Water

PLANTS AND VEGETATION

* Ecosystems

* Insect Species Found Only in the Sierra

Animals

Humans In The Sierra

Social Institutions

* Land Ownership and Reserve Allocation in the Sierra Nevada

The Sierra Nevada of the Future

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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 (endemics) occupy scattered and particular niches of the range. The assemblage of plants growing together in an area creates characteristic vegetation types. Vegetation is a dominant element of ecosystems, for plant diversity, for ecological functions plants engage in (e.g., soil aeration, microclimate alteration), and as habitat and sustenance for other organisms. The architecture of each vegetation type creates habitat suitable for some species and unsuitable for others. 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 as well as vegetation types in the Sierra Nevada are highly influenced by climate, elevation (temperatures), and soil type. From an aerial perspective, it is obvious that only part of the Sierran landscape is forested, the rest being meadow, chaparral scrub, woodland, savanna, canyon land, alpine habitat, bare rock, and water. As might be expected, 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.

At the lowest elevation on the west side, interfingering with the Central Valley grasslands and chaparrals, are the foothill woodland vegetation types. These woodlands are unique to California, although not to the Sierra (they extend around the Central Valley), and include several deciduous and evergreen oaks as well as foothill pine. Tree cover here ranges from open savannas to lush riverside forests. 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.

Intermixed with the foothill woodlands are a large group of dense shrublands called chaparral. Although chaparral vegetation looks similar throughout the range, there is great variation in species composition from one location to another. Many factors determine the location of chaparral types, but generally they are restricted to rocky soils with low fertility. The mediterranean climate is an overriding environmental factor in the ecology of Sierran chaparrals, including the climates promotion of frequent burning in intense wildfire. The boundary between chaparral and forest is dynamic and determined partly by wildfire. Shrublands on the east side of the Sierra Nevada are the Great Basin sagebrush steppe and bitterbrush vegetation types, which begin near the base of the eastern escarpment and extend across the vast expanse of the Great Basin. These arid shrublands have much less species diversity than west-slope chaparrals.

Depending on latitude, a broad conifer zone begins at elevations between 1,000 and 3,000 feet on the west and between 3,000 and 5,000 feet on the east side. Ponderosa pine (mixed with hardwoods) dominates the lower western montane zones, whereas at lower elevations on the east side, piñon pine and juniper, then Jeffrey and ponderosa pine forests occur. Above these zones on the west side is the commercially important mixed conifer forest type (figure 1.6), typified by varying mixtures of Douglas fir, ponderosa pine, white fir, sugar pine, and incense cedar. On the eastern front, this mixed conifer zone is less diverse, and species mixes vary more from place to place than on the west side.

FIGURE 1.6 (ACTUAL VIEW 36K)

Mixed conifer forest with giant sequoia, Kings Canyon National Park. (Photo by Constance I. Millar.)

With increasing elevation , the mixed conifer zone gives way to a fir beltfirst white fir, then predominantly red fir. The location of this shift in forest type depends on the transition from rain to snow, which varies with elevation at a particular latitude, shifting uphill farther south in the range. The fir zone is less extensive on the east side; south of Lake Tahoe, only a few pockets exist. Trees become shorter and more scattered with increasing elevation. The subalpine zone is a mixture of vegetation types and distributions, ranging from clusters of dense hemlock or lodgepole pine to open forests of limber pine or western white pine, to sparse, mostly rock-slope types containing whitebark pine, foxtail pine, and western juniper. Above this zone is alpine vegetation adapted to the cold, dry conditions of the highest Sierran elevations; trees give way to low shrubs and finally cushion-plant communities that grow among rock crevices in a zone of ice and wind.

As one drives or hikes through the Sierra, it is obvious that each vegetation type 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 of the Sierra, wildfire, which was a frequent characteristic of presettlement conditions, has been an architect and important ecological agent of forest and stand structure. 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 spot, 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 , i.e., extensive uniform stands of even-aged, old trees, although descriptive of some Pacific Northwest forests, is inappropriate to the complex and heterogeneous Sierran forests.

The forests of the Sierra are part of the river of change in the mountain range. Many of the current vegetation distributions have been in place locally for only a few thousand years. At shorter intervals within that time, changes in individual distribution have occurred. For instance, during the Little Ice Age of the last centuries, tree lines dropped and forest densities and wildfire patterns changed; during the warm centuries of the last millennium, many species grew in different locations from their current sites, wildfires burned in different patterns, and water flows and lake levels were very low. During the glacial-interglacial periods, most vegetation zones shifted altitudinally up and down by as much as 1,600 feet; throughout the millennia before the ice ages, vegetation types of the region were vastly different from anything we see in the Sierra now. Today Sierran forests show the effects of decades of fire suppression, which has changed the character of many forests even in places otherwise minimally influenced by humans, such as the national parks.