Special areas in many national forests have been set aside for research in the natural sciences. These areas are called experimental forests and are extremely valuable, especially for long-term research. In California, the Pacific Southwest Research Station has seven experimental forests that span elevational ranges from near sea level to about 9,000 feet on the west side of the Cascade, Sierra Nevada, and San Gabriel mountains, and over the crest to the 5,000- to 6,000-foot range on the east side. In places, they form a continuum with every major forest cover type included. This increases the worth of the experimental forests because the progression of vegetation and forest development over a large elevational range can be followed without interruption.