Joshua Tree National Monument: As Lush As the Mojave Gets
While Death Valley represents the most extreme environment in the Mojave Desert, Joshua Tree National Monument in Southern California presents the Mojave at its balmy best. Of course, this is relative, since this high-elevation desert with its giants' playground of granite boulders and an octopus's garden of bizarre yucca swelters in summer, freezes in winter, and qualifies in every way as a desert.
Still, it seems a biologically rich wonderland, thanks largely to the exuberant presence of that trademark plant of the Mojave Desert, the Joshua tree, which seems like a mutant yucca on steroids. Joshua trees remain the most distinctive visual clue to the Mojave. Few grow in the Sonoran Desert, where the saguaro remains the distinctive plant. And few grow in the Great Basin Desert, which is too cold and dry. So the presence of the Joshua trees marks the Mojave as a transitional desert between the low, warm Sonoran and the high, cold Great Basin. Early travelers called them dagger trees, because of the furious cluster of sharp-tipped spines serving as leaves. But Mormon settlers who in the 1800s spread throughout much of the Southwest looking for their own promised land where they could escape religious persecution called them Joshua trees after their great prophet.
The desert-dwelling Kawaiisu Indians account for the Joshua tree with a story. They say that Coyote and his brother were being chased by enemies, when they made their escape by turning their many angry foes into Joshua trees. When Coyote and his brother reached the end of the world, they in turn were turned into two stones. These stones sometimes bang together, causing the earthquakes that plague this area. Interestingly enough, the nearby San Andreas Fault runs within 100 miles (161 km) of this high desert and heads toward the ocean, just as the myth suggests.
Joshua trees are considered monocotyledons, which means that they don't branch in regular patterns like roses or ivy. Instead, Joshua trees put out arms haphazardly, which makes each plant distinct. They usually grow on a straight fibrous trunk for about 10 feet (3 m) before putting out their first branch, after which they branch in weird and creative profusion. A trip through a Joshua tree forest seems like a fantasy world journey, with each tangled, twisted, bristling tree harboring some individual, strange tale.
In the spring, the branches sport creamy white flowers. Each flower opens for only a single night, hoping to lure a species of yucca moth that specializes in fertilizing Joshua trees in a striking example of one of the most intricate and puzzling relationships in nature. The female yucca moth visits a sequence of yucca flowers, gathering up pollen and rolling it into a ball under her head. At the next flower, she lays an egg at the base and then crawls up the flower and deliberately brushes the tip or stigma of the flower with her head, fertilizing the blossom with the pollen she brought from other flowers. The pollinated Joshua tree then produces seed pods, which contain the moth's eggs. Her larva eventually hatch, safe in the seed pod. When they hatch, they find the table already set with a banquet of Joshua tree seeds. They feast on the seeds in the pod, but somehow always leave some seeds behind.
The specialized relationship between the yucca and the yucca moth is one of the most remarkable examples of a symbiotic relationship, in which each partner helps the other. The mother moth that gathers the pollen does not eat either the pollen or the nectar. Many insects visit flowers to eat the nectar and in the process get dusted with the pollen. When they visit the next flower, they inadvertently fertilize the next plant while grubbing about for the nectar. But the adult yucca moths get no direct benefit from visiting the flowers and gathering up a ball of pollen and nectar.
Instead, this whole complex sequence of actions directly benefits the Joshua tree, which then benefits her offspring. Pollinating the Joshua tree stimulates the development of the seed pods that feed the moth's larvae. Just to finish the strangely deliberate element of this mutually beneficial relationship, the larvae do not eat all the seeds, which enables the Joshua tree to reproduce. Of course, biologists believe that the yucca moth has developed this elaborate behavior through a long process of natural selection, so that the behavior is hardwired into the moth's DNA rather than the result of conscious thought. But the interaction is so intimate that each species of yucca now has its own species of yucca moth. Neither the yucca nor the moth could survive without the other.
These remarkable plants dominate the landscape of Joshua Tree National Monument, one of the most biologically diverse areas in the entire Mojave Desert.