|Joshua Tree: photo by Cliff Hutson|
Yucca brevifolia, the Joshua tree, is a plant native to California; indeed it is the signature plant of the Mojave Desert. It is also found in western Arizona, southern Nevada, and southwestern Utah.
A member of the “century plant” family, Agavaceae, it is not truly a tree. But, its size - between 15-40 feet in height,with a diameter of 1-3 feet - makes it seem like one.They grow 2 to 3 inches a year, take 50 to 60 years to mature and they can live up to 150 years. One interesting characteristic is that the evergreen leaves live for about a year, then hang on to form a brownish thatch around the branches. Flowers may appear from February to late April.
Each species of yucca has a specific moth that pollinates it. In this case it is the Joshua tree yucca month (Tegeticula synthetica). The moth is the tree’s only pollinator. The tree is the only food the moth’s larvae eat. An example of mutualism; the tree and the month share the task of making more trees and more moths.
However, these plants face a problem in that the primary disperser of their seeds has not been around for about 11,000 years. The Shasta ground sloth is believed to have played this important role. Clearly the trees are still with us, but it has been proposed that the lack of these sloths helping to spread the seeds to more favorable climates is causing the trees to decline. It has been further suggested that, if climate change continues on its present course, the Joshua tree will eventually no longer grow in its namesake national park.