Not really a tree, the Joshua tree was named as such because of its tree-like growths. It is a yucca, a perennial shrub, that is distinctively American. It grows only in the southwestern United States, in the arid soils of the Mojave Desert. The Yucca brevifolia, as it is scientifically known, is the largest member of its genus with a height between five and fifteen meters (16.4 feet and 49.2 feet). The Joshua tree has long, straight, dagger-shaped leaves that are about a centimeter (0.39 inches) wide at the base and taper to sharp points at the end. The leaves grow in a spiraling pattern at the ends of the stems; the dead leaves of previous seasons remain on the stem and build up below the new growth. When a winter freeze occurs and the amount of seasonal rainfall has been sufficient, these trees flower between February and April. Its off-white blossoms grow in clusters and give off an unpleasant odor.
The Joshua tree reproduces in an exclusive and mutually beneficial arrangement with the female Yucca moth, whose organs are uniquely capable of gathering and transferring the tree’s pollen. The moth helps the tree reproduce while she completes her own reproductive cycle: she gathers the pollen when she lays eggs inside the ovary of a flower. As her eggs incubate, the tree’s seeds grow, and the moth’s larvae feed on the seeds when they hatch. The larvae usually leave enough seeds to grow more trees, but the Joshua tree has special capabilities to assure that the Yucca moth offspring don’t inadvertently overeat their welcome: if there are too many moth eggs in an ovary, the tree can abort it.
In addition to growing from seeds, the Joshua tree can grow from rhizomes of other trees. This type of growth helps the tree to survive floods and fires that kill the main tree but leave the root system unharmed. The tree grows rather slowly. In it’s first few years, seedlings may gain two centimeters (0.79 inches) a year, but afterwards they generally grow only one centimeter a year (0.39 inches). The tree trunk is made of fibers and therefore doesn’t possess the growth rings that most other trees have. It also has a shallow root system that must support its disproportionately large and heavy size, which makes survival in the desert difficult. Despite this, a Joshua tree often lives to be several hundred years old.
It is believed that Mormon settlers named the plant after the biblical prophet Joshua. The shape of the tree’s outstretched branches reminded them of the Biblical story in which prophet Joshua reaches his hands towards the sky. Joshua Tree National Park gives the tree another important place in American history: Franklin Roosevelt dedicated the park in 1936 to assure that California’s rapid urban sprawl wouldn’t threaten the unique desert ecosystem in which the trees are king.