Joshua Tree

Description

Joshua trees aren’t actually trees—they’re succulents, a type of plant that stores water. In their dry ecosystems, however, they are considered trees of the desert. Joshua trees were named for the biblical figure Joshua by 19th-century Mormon settlers who felt the outstretched tree limbs guided them along their westward journey. Joshua trees usually have a single trunk and grow three to nine feet (0.9 to 2.7 meters) tall before branching. Branches end in clusters of spiky leaves and white, rounded flowers. The trunk of the Joshua tree is usually one to three feet (0.3 to 0.9 meters) in diameter. Joshua trees can grow to between 20 and 70 feet (6 and 21 meters) in height, although they rarely exceed 40 feet (12 meters).

Range

Joshua trees are desert plants and they are most commonly found in the Mojave Desert in the southwestern United States. These trees are such a striking feature of the desert landscape that California even has a national park named for them.

Life History

Joshua trees need to undergo a dormant period of cold weather before flowering, but once they flower, they are dependent upon one tiny insect for pollination. Yucca moths (genus Tegeticula) transfer pollen between flowers to ensure seeds will form, and then they lay their eggs inside the pollinated flower. When the larvae hatch, they feed on some of the seeds and the rest are able to disperse and grow into new Joshua trees. This type of interaction, where two organisms are dependent upon each other for mutual benefits, is called a mutualistic symbiotic relationship. A number of other animals are also served by Joshua trees. For example, 25 bird species nest in Joshua trees. Lizards and invertebrates use various parts of the tree for cover, and a number of mammals rely on Joshua trees for food. Humans have used the trees for food and to make baskets and sandals.

Joshua trees are slow-growing, but because of this, they live for a long time. Joshua trees don’t have annual growth rings like actual trees, so accurately determining their age is quite difficult. Instead scientists measure the height of a Joshua tree and divide it by an estimate of growth per year. One Joshua tree in California is thought to be over 1,000 years old. A more common lifespan is about 150 years.

Conservation

Because they require a cold period to flower, Joshua trees are vulnerable to climate change. The U.S. Fish & Wildlife Service is currently reviewing the Joshua tree for listing under the Endangered Species Act.

Fun Fact

Giant ground sloths that went extinct at the end of the Ice Age may have been the original dispersers of Joshua tree seeds. Today the seeds are dispersed by wind and small mammals.

Sources

Defenders of Wildlife

National Park Service

NatureServe Explorer

U.S. Fish & Wildlife Service

U.S. Forest Service