Department of Biological Sciences, Fort Hays State University Instructor: Mark Eberle                  Course Homepage

Sierra Nevada Forests Calaveras Big Tree SP California

On a map, the Sierra Nevada of California appears to be a southern extension of the Cascade Mountains in Washington and Oregon. Initially, they both were formed by the same geological process of plate subduction that can create an inland chain of volcanoes, as discussed in the webpage for Mount Rainier. However, the volcanic origins of these 2 mountain ranges were widely separated in time. When the volcanoes of the Sierra Nevada were active, the coast of Oregon and Washington was farther east, and the volcanic chain was located in present-day Idaho. Back in California, the oceanic plate subducting below the Sierra Nevada was eventually overridden by the continental plate. With no more subducting plate to melt and fuel the volcanoes, the major volcanic activity in the Sierras stopped. Over millions of years, erosion leveled the Sierran peaks, but subsequent geological tensions produced alternating north-south bands of underlying rocks that rose or collapsed throughout the present-day Great Basin, an area that extends eastward from the Sierra Nevada to the Wasatch Range of eastern Utah. Erosion has exposed granitic batholiths (such as the often photographed peaks of Yosemite Valley) that were deposited underground while the Sierras were still volcanically active (information from summary by Whitney, 1979:40-81). The Cascade Mountains, which extend from British Columbia south to Lassen Peak in northern California, still include active volcanoes, because the oceanic plate continues to slide under the continental plate along the coast in this region.

As in the southern Rocky Mountains (e.g., Snowy Range), fires in Sierran forests are more frequent than in the coastal rain forests, but the thick, resin-free bark and high branches of the sequoia help to protect it from the flames. Natural fires occurred every few decades and regularly removed debris and low-growing vegetation that would otherwise continue to accumulate and eventually carry a fire to the crowns of the taller trees (summary by Johnston, 1994:118). Prescribed burns are now used in several parks to reduce the likelihood of a crown fire and to assist in the establishment of young sequoias (described below). Although they are adapted to survive fires, it is difficult for the oldest sequoias to support growth at the top of the tree, which often dies, possibly after being struck by lightning. The narrow spire that remains (photograph below, left) is eventually knocked off by wind, lightning, or heavy snow. However, the branches on the oldest sequoias are massive.

For such a large tree, the cones of sequoias are quite small (see photograph below, left) and require 2 years to mature, although they might stay on the tree longer than this. Fire is the most important agent in seed release and germination; the heat dries the cones and causes them to open, releasing seeds onto a seedbed prepared by the fire. The fire also temporarily removes white fir and other plants that shade the seedlings. Another important agent in the propagation of sequoias is a long-horned beetle (Phymatodes nitidus), which lays its eggs in the green cones. The larvae feed on the fleshy cone scales, eventually releasing the seeds. Douglas squirrels (chickarees; Tamiasciurus douglasi) also might play a role in planting sequoia seeds. These squirrels cut many of the cones from the trees and bury them for winter food. They only eat the fleshy cone scales, though, not the tiny seeds. Adequate soil moisture, especially throughout the summer, is the most important factor in the successful establishment of sequoia seedlings; thus, the 2 sequoia groves in Calaveras Big Tree State Park are located near small streams. Once established among the giants of the forest, sequoias most often die from toppling. This usually happens when fire, saturated soil, undercutting by a stream, or disease weaken the support provided by the roots and lower trunk on one side of the tree (summary by Johnston, 1994:114-122).

The sequoia is not the only organism to exhibit a dramatic range of sizes. On our trips in 2000 and 2002, we observed the calliope hummingbird (Stellula calliope), the smallest bird in North America, during our visits to Beaver Creek. Calaveras Big Tree State Park is also the stop where we have had the best luck seeing the pileated woodpecker (Dryocopus pileatus), usually during the early evening in the North Sequoia Grove, perhaps on their return from foraging for insects elsewhere in their likely 40-hectare (100-acre) territory. There are 9 species of woodpeckers in the Sierras, but, at a length of over 40 cm (16 inches), pileated woodpeckers are about twice the size of any of the other 8 species in these forests.

After a day among the sequoias, we travel the narrow, winding highway over Ebbetts Pass (2,661 m = 8,730 feet), which allows us to compare the forest communities at different elevations on the western and eastern slopes of the Sierra Nevada. As we leave the lower montane community associated with the sequoia groves on the western slope, lodgepole pine (Pinus contorta) and red fir (Abies magnifica) become more numerous until we reach a subalpine forest of western white pine (Pinus monticola) and mountain hemlock (Tsuga mertensiana). Jeffrey pine (Pinus jeffreyi) and white fir are common on the drier eastern slope.

Our last night in the Sierra Nevada is spent at Lake Tahoe, a good jumping-off point for the drive across the Basin and Range topography of Nevada. On some trips, we camp atop Eagle Point, which provides us with great views of Lake Tahoe and Emerald Bay, as well as a "bird's eye view" of ospreys (Pandion haliaetus; there are 2 ospreys in photograph below, right).

A short hike along the Rainbow Trail at the Taylor Creek Visitor Center (operated by the US Forest Service on the south shore of Lake Tahoe) provides us with an overview of the ecology of the lake and the associated streams and wetlands. The wetlands help to filter water before it enters the 10^th deepest lake in the world (486 m = 1,595 feet deep), which contributes to the amazing clarity of the water (in some areas, objects are visible to a depth of 23 m = 75 feet). However, the clarity of the water is threatened by continued, heavy development of the basin, which has eliminated wetlands and increased the rate at which sediments and nutrients enter the lake. The nutrients support plankton blooms that impair the clarity of the water. A cut-away view of Taylor Creek provides us with the opportunity to see Lahontan redsides (Richardsonius egregius) or Tahoe suckers (Catostomus tahoensis). Water from Taylor Creek and 62 other streams enters Lake Tahoe, but only one stream (Truckee River) flows out to the Great Basin into Pyramid Lake, a remnant of Lake Lahontan, which covered much of northwestern Nevada during the Pleistocene. Thus, the water of Lake Tahoe does not flow to the ocean. We spend our next day traversing this great internal basin.

Literature

• Johnston, V. R. 1994. California Forests and Woodlands: A Natural History. University of California Press, Berkeley.
• Whitney, S. 1979. Sierra Club Naturalist's Guide: The Sierra Nevada. Sierra Club Books, San Francisco, California.

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