All about Joshua trees, a keystone species

While desert fan palms, Washingtonia filifera, are confined to canyons and fault zones where water reaches the desert’s surface, they broadly define the extent of the Colorado Desert; they occur naturally nowhere else.

Similarly, the range of Joshua trees is synonymous with the boundaries and core of the Mojave Desert; they occur naturally nowhere else. The Mojave Desert is mostly a higher elevation landscape with colder winters with an expectation of occasional snow, an event that is exceedingly rare below the higher mountain elevations within Colorado Desert.

Joshua trees were so named by early Mormon immigrants while crossing the desert, and perhaps in dehydration-induced delirium, thought the branches of this tree-like yucca reminded them of the up-turned arms of their prophet Joshua, beseeching the heavens. 

A grove of Joshua Trees against a dramatic sky with swoosh clouds


Naturally occurring species of Yucca (the genus of Joshua tress) are restricted to the arid regions of North and Central America, including some Caribbean islands, and include perhaps 50 different species. While most species of Yucca lack branches, there are several species like Joshua trees that are tree-like, including the Baja California tree yucca (Y. valida), the Cape tree yucca (Y. capensis), also endemic to Baja California, and the Mexican tree yucca (Y. filifera), found in portions of the Chihuahua Desert. Joshua trees (Y. brevifolia) are the most northern tree Yucca and the only one to live where it snows. 

Yucca flowers are visited by many species of birds and insects, but their primary and most efficient pollinator is a moth; each species of yucca has its own species of moth pollinator. Unlike bees and birds which are drawn to flowers by the lure of a meal of pollen or nectar, yucca moths do not partake in either. Yucca moths do collect yucca pollen but they nor their young eat it. Nevertheless, they scrape the pollen up into a tight ball and then move to a new flower. If the moth does not detect (presumably by odor) that another moth has already visited that flower, she then will drill into the flower’s ovary and lay her eggs. If she does detect that another moth got their first, she will move to another flower. After depositing her eggs, she will then take her ball of pollen and pat it down into the flower’s style where the pollen grains will then transfer their DNA into the flower’s ovary to fertilize the seeds there. 

This is where this story gets interesting. The moth lays her eggs in the Yucca flower ovary so that her hatching caterpillars will have a ready meal of Yucca seeds. She makes sure that the seeds will grow and mature and so provide food for her babies by making sure that the flower is pollinated. The caterpillars grow and mature within theYucca fruit, and so have protection from birds and parasites. But what does the yucca get from this relationship?? Yucca species are of course monocots, and so their ovaries (and then fruits) are partitioned in 3 or 6 chambers. The moth lays her eggs in no more than half the chambers. The growing caterpillars cannot move between chambers, so only half (or less) of the seeds are eaten by the caterpillars and the other half grow into mature seeds capable of growing a new Yucca plant. Wow. But what keeps the moth from cheating, laying her eggs in all the chambers? Perhaps a future naturalist will answer that question.

Like desert fan palms, Joshua trees are keystone species, meaning that when present both species host a variety of additional organisms that otherwise would be rare or absent altogether, and so add to the species richness of a region beyond the presence of just the palm or the yucca. For desert fan palms, that includes several species of owls, orioles, and reptiles. Joshua trees can be the only tree with any height and stature for hundreds of miles in any direction and so they provide essential nesting platforms for red-tailed hawks, which are later re-used by great horned owls. Scott’s orioles are particularly fond of weaving their basket-like nests in Joshua trees, as are cactus wrens. Ladder-backed woodpeckers excavate nesting cavities in the soft trunks of Joshua trees, holes that are later used by western screech owls, and western fence and desert spiny lizards. Joshua trees regularly shed branches that fall to the ground and there provide habitat for desert night lizards and night snakes and darkling beetles.

As monocots, like desert fan palms, their trunks and branches filled with fibers that transmit water and nutrients between the leaves and roots. However, whereas those fibers in the thick palm trunks provide a mechanism for surviving wildfires in palm oases, the much smaller trunks and branches of Joshua trees are insufficient for protecting these plants from wildfire, unless the fire cannot find fuel against the Joshua tree’s trunk. Years after a Mojave Desert wildfire one can still detect the patterns of fire intensities as those few Joshua trees that survived the fire are testimonies to patches where fuels were sparser and so the fire burned just a bit cooler. Of course, wildfire in deserts should be a rare or never event. There should not be enough fuel to carry fires between the widely spaced shrubs or trees that characterize deserts, but because of inadvertent nitrogen fertilization from smog blowing from the Los Angeles Basin into the Mojave Desert, invasive, non-native grasses have proliferated there. Those grasses fuel wildfires. Those wildfires endanger Joshua trees along with most other desert shrubs and trees.

Then there is climate change and the increasing aridity and heat it is generating across our deserts. Joshua trees do not bloom every year, but when they do and when the moths do their jobs, there will be thousands of viable Joshua tree seeds produced by every tree. Those seeds then become food for kangaroo rats, pocket mice, cactus and pinyon mice, ground squirrels, and sparrows, and insects. Some seeds will survive that gauntlet, but will only germinate if there is enough rain, and enough soil moisture. Those that sprout out in the open will succumb to jack rabbits or will shrivel in the summer heat. Those few seeds that germinate below a small shrub might not be detected by a rabbit and the shrub may provide enough shade to keep the soil moisture higher, if there were ample winter rains and snow. Such small shrubs are referred to as “nurse plants” and are believed to be critical for Joshua tree seedling survival. But climate change is making it hotter and drier, effecting the survivorship of the nurse plants, and effecting the ability of Joshua tree seedlings to find sufficient soil moisture to survive their first years of life. At the lower, drier end of the range of Joshua trees there is no visible recruitment, no young plants to replace the elder trees as they pass on.

At the higher elevations or on cooler, north-facing slopes, there are still seedling Joshua trees to be found and their populations are doing well. Such areas are climate refugia. Still, it is not clear how well those refugia will function as such as our deserts become yet hotter and more arid. 

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.