October 9, 2014 The Joshua Tree: Staff of Life
Often used by Hollywood, this bizarre looking member of the mostly subtropical agave family provides a striking silhouette standing tall with rigid arms extending in every direction against the orange Mojave Desert sky.
Named by 18th century Mormons after the biblical prophet Joshua, the plant also known as a yucca had special meaning for those wandering across the parched high desert in search of the Promised Lands. Its arms pointing towards heaven, as the story goes, confirmed that they were on course.
Welcome to the weird world of perhaps one of the strangest tall forms of life on our planet: This tall, short-leafed, southern Californian yucca can attain heights of 45 feet, living for more than seven centuries, tolerating ice and snow of the high desert.
Each needle-sharp leaf is tipped in brown, as if dipped in an ink well and edged with tiny sharp teeth. Dead leaves hang like a shaggy gray, gnarled beard below its green tops.
The tallest of these giant yuccas lives in Joshua Tree National Park among two billion year old rocks. The desert climate is hot during the summer and freezing in winter. In order to obtain 35 millimeters of moisture these magnificent specimens live at elevations between one and two thousand metres above sea level.
The Joshua plant has derived ingenious methods of survival. It tolerates boring beetles, wildfire and has co-evolved with a moth depending upon it, rather than the wind, to spread its pollen.
If a Joshua seed can germinate and survive its first two winters then it’s likely the tree will live to a ripe old age. In order for the seed to successfully germinate it needs a helping hand and it relies upon the thorny protection of a nearby shrub. Ecologists call this a nurse plant. The barbarous nurse plant protects the nutritious seed from being eaten by mule deers, black-tailed Jack rabbits, ground squirrels and wood rats.
Seedlings stand erect like soldiers and will not begin branching until the bud at the tip has either produced a flower or been attacked by the yucca-boring beetle. If it has been attacked it will seal off the injury by laying down silica, which inadvertently petrifies the damaged wood. The tip of the branch will fork, after flowering or beetle attack, and produce two healthy buds. This process will be repeated many times throughout the life of the plant.
Mature Joshua plants will send out bamboo-like runner roots that can also sprout new seedlings called runner plants with tough, unpalatable leaves. It’s these runner plants with their vast reserves of carbohydrates or food, from the well-developed Joshua runner roots, that the Navaho yucca beetle bores into, lays its eggs and supports its larvae with copious food. How this beetle is able to determine between seedling plants and runner plants is remarkable – they look identical yet seedling plants cannot support the voracious Navaho larvae.
In March a cluster of creamy white, musty-smelling flowers that look like a cross between a cauliflower and artichoke occur. By May, light green seed pods resembling big pecan shells form. In order for flowers to fertilize they rely upon the yucca moth to cross-pollinate them. In so doing she lays her eggs in the soon-to-develop yucca plant ovary. These growing seeds guarantee a food source for the young larvae. But the moth never lays too many eggs so all the yucca seed will be devoured. For without new seedlings the yucca moth would ultimately perish.
A dark brown wasp patrols the forest and conducts a search of each Joshua seed, inserting one egg carefully into the tough green skin of the yucca seed. The wasp only deposits an egg into seeds infested by yucca moth larvae. Moth larvae serve as a food source for wasp larvae. How this little wasp detects the deeply buried moth larvae is a desert mystery.
Occasionally, lightning-induced fires ignite Joshua forests. These wonderful plants have adapted to fire by sprouting new plants from the base of their trunks.
Though these giant yuccas are tall enough to be considered a tree, in the true botanical sense they are not. They do not produce growth rings like maples or pines. Rather their wood is soft and spongy and often decayed in the central core. It is very difficult if not impossible to age the Joshua from the standard core sampling technique which provides annual growth rings. So how do tree scientists know how old these plants are? Based on the average growth rate of 10.5 millimeters a year, some of the trees at Joshua Tree National Park are at least 700 years old.
Joshua’s are the tree of life in Mojave Desert because 25 different bird species use it for nesting. Ladder-backed and northern flicker woodpeckers bore into the limbs and create dens used by other birds. Scott’s orioles weave nests from dead Joshua leaves that look like suspended hammocks.
Loggerhead shrikes, known as butcher-birds, use the dagger-like leaves to skewer lizards, and hang the meat in Joshua trees, like a butcher, before returning later in the day.
Wood rats trim razor-sharp Joshua leaves and use them as palisades around their nests.
Termites help recycle fallen limbs returning basic nutrients back into the soil. The desert night lizard lives underneath fallen branches eating termites and other insects. And the spotted night snake slithers quietly searching for its favored prey – night lizards.
The complex ecology of Joshua trees is fascinating and highlights the importance of a healthy, interconnected web of life in California’s high desert.
Join Earth Dr Reese Halter is his crusade to protect our planet by watching Earth Calling SOS.
Text © by Dr Reese Halter 2014. All rights reserved.