Since the theme of The Joshua Tree is mostly spiritual or religious, The Joshua Tree is often seen as a religious symbol. In religious legends, the joshua tree is said to be the tree that pointed Joshua, the successor of Moses, the way to Jericho, with its outstretched and entwining branches. The joshua tree is also often reffered to as the "Jesus Tree" because its branches seem to appear as if it is praying (early Christians used to pray with their arms outstretched towards the sky).
Niko R. poeticthiefgmail.com (22:th of November 2005)
The name "The Joshua Tree" is actually a reference to more than Jesus praying. "Joshua" is the English version of the name "Jesus", and the cross is colloquially referred to as a "tree". Thus, "The Joshua Tree" is an allusion to the cross on which Jesus was crucified. This is consistent with the theme of the album: one of loss, bitterness, and sacrifice, conjoined to a search for redemption and reinforcement from an outside source.
Tom Ordeman ordemanproaxis.com (2nd of September 2006)