Window on Nature
By Lowell and Kaye Christie, F47246
January 2002
Joshua trees really aren’t trees at all. But how were Mormon pioneers supposed to know this when they named them? The upturned, uplifted branches reminded them of the Biblical Joshua praying. The name stuck, whether it was accurate or not.
John C. Fremont, the intrepid explorer of the American West, called the Joshua tree “the most repulsive tree in the vegetable kingdom.” Hey, John, calm down! In a hostile desert environment, few “regular” trees thrive. Yes, we’ve seen ugly, black skeletons of Joshua trees that have fallen victim to summer lightning strikes or, more likely, the carelessness of man. We agree that they’re no longer attractive, but repulsive they’re not.
Joshua trees don’t meet the requirements to be considered trees — they do, however, fit nicely into the yucca genus. There are important differences between a tree and a yucca. To begin with, yuccas (including Joshua trees) possess no bark on the outside, have no wood inside, and thus can’t produce annual tree rings.
Although they look woody, the stems of these giant “trees” are actually fibrous. These fibers form the core of the stem, which is augmented each autumn by another coat of leaves. As with palm trees, when fall comes, Joshua tree leaves don’t drop; instead they form a skirt around the stem or branch. The process sounds messy, but the results are aesthetically pleasing.
Given hospitable surroundings, Joshua trees (Yucca brevifolia) will form a treelike forest. One area where such forests thrive is Joshua Tree National Park, near Desert Hot Springs, California. The 800,000-acre park is the meeting place of two deserts: the “low” Colorado Desert and the “high” Mojave Desert. Joshua trees can cover slopes, fill valleys, and hide among the rocks in the Mojave Desert where the elevation rises above 3,000 to 5,000 feet.
The park claims a 32-foot specimen that is, so far, the tallest we have read about. Joshua trees are notoriously slow-growing, so how long does it take one to reach 32 feet tall? Obviously, tree rings can’t be counted, but biologists have found other methods to determine age, and it’s their educated estimate that the 32-footer has been around for 900 years.
During its 900-year lifespan, that Joshua must have produced an enormous amount of seeds. The trees’ propagation method is fascinating. It’s a good illustration of a mutual dependence relationship between yucca moths and Joshua trees. Each species of yucca can be pollinated only by its designated species of yucca moth. And each contributes to the other’s existence.
After mating, a female yucca moth gathers pollen from a flower of the Joshua tree; she then flies to a different Joshua tree and lays her eggs in the ovary of one of its flowers — Joshua serves moth. Next, she inserts the ball of pollen she gathered from the first Joshua tree into a special depression in this flower. The second flower is fertilized with pollen from the first — moth serves Joshua. This pollination system is so strict that yuccas grown in regions where the yucca moth is absent can’t produce seeds unless the plants are hand-pollinated.
After fertilization, seeds develop within the flower’s ovaries. At about the same time, the eggs of the moth hatch and the larvae begin feeding on the seeds. There are always plenty of seeds, so the insects can eat and future Joshua trees can grow.
Whether or not the uneaten seeds sprout depends upon the amount of winter precipitation — more is better. Adequate rainfall transforms the desert into a colorful wildflower display and provides the moisture needed to sprout Joshua tree seeds.
A good many Joshua sprouts are required to sate the appetites of grazing cows, bighorn sheep, deer, coyotes, and foxes. Of course, it’s the little guys — the mice, antelope squirrels, and chipmunks — that get to the sprouting seeds most easily. Yucca and cacti that sprout within the protective canopy of a bush (called a nurse plant) live longer. Only after several years of growth do Joshua trees have enough dagger-like leaves to provide their own protection.
A good Joshua tree bloom is one of the delights of the Mojave Desert, when the mature Joshua trees produce spectacular clusters of white flowers. The optimum time for seeing these blooms is March and April, and the prime elevation is between 3,000 and 5,000 feet.
In addition to Joshua Tree National Park, southern California offers other good places to see Joshua trees: in the Yucca Valley, north and east of the city of Palmdale; around Saddleback Butte State Park; and the Mojave National Preserve.
Obviously, the flowering period is our favorite time to visit. But one year, on the day after Christmas, we awoke to the sight of these fantastic “trees” laden with freshly fallen snow. It certainly was a peak experience.