The desert fan palm, Washingtonia filifera, is native to the low hot deserts of southern California where it can live for 80 to 90 years. Towering up to 75 feet, the desert fan palm is among the tallest of North American palms. It is definitely the heaviest; a mature desert fan palm can weigh as much as three tons. Its distinctive leaves are shaped like a fan and folded like an accordion. They measure up to six feet in length and are nearly as wide. Looking much like "petticoats," the fan palm’s dead leaves remain attached to its trunk until removed by fire, wind, or flood. [There are 158 desert fan palm oases in North America. Five of these are located in Joshua Tree NP.
On our first day in the park, the four of us visited the most accessible one: the Oasis of Mara, which lies next to the visitor center and entrance at the northeast entrance to Joshua Tree National Park. (Later that day, my younger and fitter cousins hiked to one of the more spectacular ones: 49 Palms.) The oasis was first settled by the Serrano Indians who called it Mara, meaning “the place of little springs and much grass.” The Serrano planted 29 desert fan palms during their first year there. The palms provided the Serrano with food, clothing, cooking implements, and housing. In addition, the palms are habitat for a wide variety of desert creatures.
On our last day in the park, we visited Cottonwood Spring Oasis, at the south end. The spring, the result of earthquake activity, was used for centuries by the Cahuilla Indians, who left bedrock mortars and clay pots, or ollas, in the area. Cottonwood Spring was an important water stop for prospectors, miners, and teamsters traveling from Mecca to mines in the north. Water was necessary for gold processing, so a number of gold mills were located here. Cottonwood Spring was first mentioned in a gold mine claim filed in 1875, indicating that the [cottonwood] trees are native. Fan palms first appear around 1920, perhaps growing from seeds deposited by a bird or coyote.
Information from nps.gov - JTNP-Desert Fan Palm Oases]