the higher elevations. Cultural impacts In addition to their natural diversity, the parks also host a unique cultural and historical record. Archaeological sites date back at least five thousand years, indicating the presence of Native American peoples throughout the Sierra Nevada. Past herding of sheep (1850-1900) and cattle (1850-1940) has resulted in significant changes in the species composition and extent of upland and foothill vegetation. Wildfire suppression during the 20th century has interrupted successional patterns. Air pollution from outside the biosphere reserve affects several prominent tree species. Recreational impact is locally acute. The two parks have been jointly managed by the National Park Service as the Sequoia and Kings Canyon National Parks since 1943, and Sequoia National Park contains a number of visitor centers, campsites and supply facilities. In Sequoia and Kings Canyon National Park, recent times have brought new public safety issues to its park. A big threat to its environment is forest fires. Park administration conducts a number of controlled burns each year to prevent and reduce future forest fires. In addition, illegal marijuana grow sites have been found throughout the park. The drug trafficking organizations have had adverse affects on the environment. The growers clear-cut vegetation, reroute waterways, utilize illegal fertilizer and pesticides, and alter the natural landscape. See also National parks in California Protected areas of the Sierra Nevada References Kings Canyon National Park Sequoia National Park History of the Sierra Nevada (United States) 1943 establishments in California Biosphere reserves of the United States Protected areas of California