Botanical Profile of the Joshua Tree

by Richard Katz

The Joshua tree is an unusual tree-like species of Yucca, a member of the Lily Family (Liliaceae), and sometimes considered in the Agave Family (Agavaceae).

Habitat and range

It grows uniquely in the desert southwest of the United States, principally in California's Mohave Desert, Antelope Valley and the surrounding area, and also in parts of Arizona, Nevada, and the southwest corner of Utah. It is found typically in the high desert, around 4000 feet (1200 m) elevation where there are freezing temperatures during winter nights, and very hot, dry summers. The small annual rainfall occurs primarily in the winter, with occasional dustings of snow. Thus, while this is a severe climate, it is not quite as hot or dry as the lower elevation Sonoran deserts.

Growth pattern

The Joshua tree is not a true "tree," in that it does not produce a trunk with annual rings. For the first several decades of its life, the Joshua Tree grows as a vertical stem with no branches. It grows very slowly, only 1/2 to 3 inches (1 to 7 cm) per year, typically reaching 5-10 feet (1.5 to 3 meters) before the first blossoms appear. At this point, the Joshua tree literally takes a new turn in its development. After flowering, the blossoms drop off, leaving a length of dried stalk. New leaves grow beneath this dead portion, and a new branch begins its growth in another direction.

Unlike a typical tree branch, this new stem grows rigidly in a totally different direction, at an angle, horizontally, or even down towards the ground. Each branching stem also abruptly ends its growth after blossoming, and further branches veer off in new directions. As well as ending in blossoming, branching may occur where a stem has been damaged by insects.

After many years, some Joshua trees develop a complex system of twisted branches growing in many directions. Others develop a more harmonious tree shape, while still others remain mostly vertical. The amazing variety of shapes and growth patterns imparts an unusual individuality to each tree.

The largest Joshua tree on record was 80 feet (24 meters) tall and was estimated to be about 1000 years old. Joshua trees typically grow more than 20 feet tall (6 meters). They may take 60 years to come to maturity, and can live more than 500 years.

Leaf development

Young Joshua Trees have soft, tender leaves which make them vulnerable to desert animals. Therefore, they do best when they are seeded next to a “nurse plant” which protects them when they are first growing.

After they are about 10-12 inches (25-30 cm) tall they develop their characteristic sharp, pointed leaves, sword-like in their intensity. These not only afford protection, and because of their form, they minimize the loss of water through evaporation. During infrequent rain showers, the concave shape of the leaves captures the water and directs it to the trunk and down to the roots. As well, the mass of foliage tends to create a still-air zone around the plant, further reducing the evaporative effects of the drying desert winds.

Leaves are generally 5-12 inches (12-30 cm) long. The younger ones remain green, but as they age the leaves fade to gray and become a fibrous residue which droops and finally covers the branch or trunk in a protective coating. As mentioned above, the leaves are somewhat concave, which allows rain to collect and be drawn into the plant, where it can make its way down the trunk to the roots. Thus the leaves embody this plant's ability to survive in the harsh, dry desert climate.

Flowers and pollination

The flowers begin as greenish buds clustered near the end of the branches. The six tepals (3 petals and 3 similar sepals) then open up, to reveal a pod-like ovary, surrounded by six stamens, with one pistil. The flowers are waxy, white-to-greenish-white, and 2-3 inches long (5-7 cm). The flowers emit a sweet fragrance reminiscent of coconut.

The Joshua Tree pollen is so sticky that it cannot be carried by the wind. Pollination is accomplished only with the help of the tiny Yucca moth, with which this species has evolved a symbiotic relationship. This moth (Pronuba yuccasella) collects the pollen from a number of flowers and then chooses one flower as a home for its offspring. It deposits the pollen on the flower’s stigma in the process of laying eggs in the flower’s ovary. The newly hatched larvae will feed on the seeds (consuming only about 20 out of 200 in each ovary).

After the Joshua Tree flower is pollinated, the ovary swells with seeds and the flower falls away. The ripened fruit then falls to the ground where the seeds are scattered, often taken up by the wind. Very few will be able to germinate in the harsh desert conditions, but it is by this process of seed propagation that the new plant colonies are established.

The Joshua Tree can also propagate by shoots from its roots or crown, and this is particularly important when the upper part of the tree is destroyed by fire.

Seasonal and Diurnal Cycles

The blooming cycle of the Joshua Tree is totally dependant on climatic conditions. Depending on the timing and intensity of winter rains, blossoming can occur any time from March to May, and can very from very sparse to a rare abundance of blossoms in relative wet years.

Joshua Tree has a special relationship with the night. The leaves actually remain dormant during the day, and save their respiration cycle (exchange of carbon dioxide and oxygen) for the night-time hours when the air is cooler, and moisture can be conserved. It is also at night, after dusk, when the Pronuba moth pollinates the flowers. A number of observers have noted that the Joshua Tree flowers, which never appear fully open, reveal themselves more to the night, as well as releasing their fragrance into the night air. Joshua Tree is in its full glory in a moonlit desert night, as its white flowers glow in the eerie luminescence of reflected light.

History and Lore

Archaeological evidence indicates that the Joshua Tree dates back to the Pliocene Epoch, some two million years ago, when the climate was more moist and warm, and its habitat more extensive.

Native Americans knew the Joshua tree as a source of food and fiber. The flowers were considered a sweet delicacy, roasted over a fire. The leaves were used for baskets and cordage.

Early European settlers often saw the Joshua Tree as grotesque and misshapen. However, the Mormons, trying to find their way back from California to Utah, used the Joshua Tree as a guide. They gave the plant its name in the early 19th century, because it reminded them of the biblical prophet Joshua, with his arms upraised in prayer. They saw this plant as a spiritual sign of welcome into the Promised Land.

Botanical relationships

The Joshua Tree (Yucca brevifolia) is the largest of the Yucca species. Often considered part of the Agave family (Agavaceae), or grouped with Agave (Century Plant) in the Lily family (Liliaceae), the Joshua Tree has the same linear, sharply pointed leaves as Agaves and other Yuccas. The species name brevifolia describes the shortness of the leaves. Joshua Tree differs from other Yucca not only in its size and longevity (well over 500 years), but also in their irregular growth patterns (described previously).

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