each year to encourage the growth of acorn-bearing black oaks. Fire kept forests open, reducing the risk of ambush, and the open areas helped to expand and maintain meadows. Early park guardians drained swamps, which reduced the number and extent of meadows. In the 1860s there were over of meadows in the valley compared to by the end of the 20th century. The remaining meadows are maintained by manually clearing trees and shrubs. The Park Service has prohibited driving and camping in meadows, a common practice in the 1910s to 1930s and cattle and horses are no longer allowed to roam freely in the park. Fire suppression encouraged the growth of young coniferous trees, such as ponderosa pine and incense cedar; adult conifers create enough shade to inhibit the growth of young black oak trees. By the 20th century, fire suppression and the lowering of water tables by draining swamps led to the establishment of dense conifer forests where mixed and open conifer-oak woodlands had previously grown. Fire suppression policies have been replaced by a fire management program which includes the annual use of prescribed fires. Fire is especially important to the giant sequoia groves, whose seeds cannot germinate without fire-touched soil. Logging used to be carried out in the area. Over one-half-billion board feet of timber were felled between World War I and 1930, when John D. Rockefeller, Jr. and the federal government bought out the Yosemite Lumber Company. Increases in visitation Muir and the Sierra Club initially encouraged efforts to increase visitation to the park. Muir wrote that even the "frivolous and inappreciative" visitors were on the whole "a most hopeful sign of the times, indicating at least the beginning of our return to nature – for going to the mountains is going home." The first automobile entered Yosemite Valley in 1900, but growth in car traffic did not increase significantly until 1913, when they were first officially allowed to enter; the next year, 127 cars entered the park. Park visitation increased from 15,154 in 1914, to 35,527 in 1918, and to 461,000 in 1929. Two-thirds of a million visited in 1946, 1 million in 1954, 2 million by 1966, 3 million in the 1980s, and 4 million in the 1990s. Recreational activities Half Dome is a prominent and iconic granite dome that rises above the floor of Yosemite Valley. It was first climbed on October 12, 1875,