A Joshua Tree stand in the high desert.

This is a distribution map of two varieties of Joshua Tree. The circles are Yucca brevifolia. The Xs are the variety Jaegerianna (x). (McKelvey 1938) 

Joshua Trees

I spent the weekend in Joshua Tree National Park. Although most of the park does not, in fact, have Joshua Trees growing, I happened to camp in a region where there were many. I was fairly captivated by them and thought I'd try and spread the love. They are a fascinating example of the adaptivity of life on earth. They are also one of the many desert plants that look like something straight out of a Dr. Suess book, which makes them fun. Apparently the Joshua Tree is named so because delusional water-starved Mormons making their way west thought a certain gnarled Joshua Tree looked like a person praying to St. Joshua. Interesting factoid. For those that are scientifically inclined, their scientific name is Yucca brevifolia and they are in the Liliaceae family. I did a quick search on google and found many research papers written on the tree for those of you who are interested in learning more about them.

[The] Joshua tree is a much and irregularly branched tree growing to some 30' tall. Young trees are ± covered with spreading or reflexed leaves and usually do not begin to branch until they are from 3' to 9' high, and in older specimens the leaves are clustered near the ends of branches. The rigid leaves are from 12" to 15" long, up to a couple of inches wide, with minutely serrate non-fibrous margins and an apical spine to 1/2" long. The flower clusters extend above the leaf clusters and are up to 20" long. The perianth is cream to greenish-white and generally waxen in appearance. The fruit is a capsule to 4" long that releases its seeds after being blown around on the ground. Joshua tree is an unlikely-appearing member of the lily family and grows on dry stony mesas, flats and slopes from 2000' to 6000' in the Mojave Desert, and is in fact an indicator species of that desert and the principal member of the plant community which is named for it, joshua tree woodland. Joshua trees do not branch until the plant has bloomed, and tall unbranched trunks of joshua trees as high as 10'-15' may be found, indicating that no blooming has taken place. Thereafter, branching takes place wherever there has been a bloom. The flowers are pollinated by the Yucca moth with which the joshua tree has a symbiotic relationship. They typically bloom from March to May.
Source: http://www.calflora.net/bloomingplants/joshuatree.html