Looking like one of Dr. Seuss' creations, the Joshua tree stretches its spiky-crowned arms toward the desert skies above the American Southwest.
Early Mormon settlers gave the plant its name, because it reminded them of the Biblical Joshua, pointing the way to the Promised Land. In fact, however, the Joshua "tree" isn't a tree at all; it's the world's largest yucca plant, yucca brevifolia, a member of the sturdy desert-dwelling branch of the lily family.
Ranging across parts of California, Utah, Arizona, and Nevada, the Joshua tree is a dry-land specialist. Most live in the Mojave Desert, though some can also be found in the Sonoran. Joshua trees grow best on flat or slightly sloping ground, at altitudes of 2,000 to 6,000 feet.
Joshua trees grow very slowly in their harsh environment, less than one inch per year after their sapling stage. The plants can live for hundreds of years, though, so the tallest are more than 40 feet high! Joshua trees generally have one main trunk covered with shaggy "bark," supporting a number of arms topped with dagger-shaped leaves. Unlike true trees, Joshua tree trunks are comprised of bundled fibers, rather than concentric annual growth rings. The giant yucca have surprisingly shallow root systems, which spread out near the surface rather than reaching down into the ground. This adaptation allows the Joshua tree to take advantage of any rain that hits the desert floor, but it leaves the plant vulnerable to blowing over during storms.
When spring rains are plentiful, mature Joshua trees put on a magnificent show. They burst into bloom, sprouting huge pom-pom clusters of waxy yellow-green flowers. These flowers have what could charitably be described as an odd smell, designed to attract the plant's sole pollinator, the yucca moth. The female moths move from tree to tree, laying their eggs inside the blossoms. In the process, they also transfer pollen from one tree to the next, allowing the plants to set fruit. These fruit are spongy, green, and egg-shaped; each one contains between 30 and 50 seeds. When the moth eggs hatch, the larvae feed on the seeds. Each fruit overproduces so that there will be sufficient seeds to sprout into new plants, even after the moth larvae have eaten their fill. This generosity keeps the symbiotic relationship going, generation after generation.
New Joshua trees can spring from rhizomes (rootstock) as well as seeds. The seedlings are soft and vulnerable at first; deer, cattle, and even rodents can graze them down to the ground. For the first five years, the seedlings shelter under neighboring shrubs, and put on an impressive (in desert conditions) 4-5 inches of new height each year. Once they are tall and spiky enough to fend off browsers, the trees emerge from the ground cover and begin to reach toward the sky at a more leisurely pace.
Over the centuries, humans have found the Joshua tree to be useful. The Cahuilla tribe of Native Americans used to roast and eat the seeds and flower buds, and wove the fibrous leaves into baskets and sandals. Early ranchers used the trunks for fencing, and miners burned the trees to fuel steam-driven ore stamping machines. Little did they realize that the trees had spent hundreds of years growing to that size, just to be burned up in a day! Today, Joshua trees are primarily used as a tourist attraction. Each year, more than one million people visit Joshua Tree National Park, in southern California. The park is a great place for rock climbing, as well as admiring the weird and wonderful plant from which it takes its name.
The 19th century western explorer John Fremont once commented that the Joshua tree was "the most repulsive tree in the vegetable kingdom." Since then, however, we have come to better appreciate the strange and exotic beauty of that most tenacious lily, the Joshua tree.
Mojave National Preserve website