Question of the Day - 29 December 2020

The taxonomic classification of the Joshua tree (yucca brevifolia) is monocotyledonous, defined in part as having no annual rings (similar to the palm). Rather than trees, they're succulents; that is, a type of plant that stores water. In their dry scrubby ecosystem, however, they're the tallest plant -- and therefore considered "trees." The Joshua tree is the largest yucca.

They aren't marked by annual rings (similar to the palm), which makes it difficult to determine their age. It's estimated that they grow about a half-inch per year. The tallest Joshua tree in Joshua Tree National Park is 40 feet tall, which would make it almost 1,000 years old. The tallest recorded Joshua tree grew to be 56 feet high, meaning it would be more than 1,300 years old. Both ages are unlikely. Most don't live that long, because they're somewhat precarious, having top-heavy branches and shallow roots. More likely is that growing conditions (heavy rainfall, soil conducive to an extensive root system, lack of insect infestation, etc.) prompted it to shoot up. 

That's the case with the most famous tree in the best place to see these ungainly trees, Joshua Tree National Park, a million-acre preserve northwest of Palm Springs in southeastern California: The "Barber Pole" tree, with 32 feet of trunk before it branches, is estimated to be 40 years old.

The trees' maximum lifespan is about 150 years. Wind ultimately causes their demise. Since they're so asymmetrical, when a high wind sheers off a limb, they get out of balance; the next high wind can knock down the whole plant.

The Joshua tree is found mostly in the Mojave Desert, with stray groves in perfect environments in the Great Basin and Sonoran deserts, but nowhere else in the world. That's because they require only six to ten inches or rainfall a year, but also freezing temperatures in the winter. 

Its trunk, though dry and brittle on the outside, is soaking wet on the inside and is said to contain a "column of water." It's often the only place desert animals can get access to water, even during the driest years and in severe droughts; they gnaw through the bark to get at the moist tissue within. These trees are very efficient at storing water in their roots and trunks and they have very small leaves (hence, the Latin name brevifolia).

The Joshua tree was named by Mormon missionaries traveling from Salt Lake City to southern California in the mid-19th century. The limb-like branches of the plant reminded them of the Biblical story in which Joshua reaches his hands to the sky in order to welcome pilgrims to the Promised Land.

Native Americans used the tough leaves of the tree to weave baskets and sandals; they also ate the flower buds raw or roasted.

They bloom between February and April, though not necessarily every year; it depends on sufficient rainfall at the critical moment in the annual life cycle. The flowers consist of six petals, from a creamy-white to a silvery-green color. When they bloom, the flowers are pollinated by the yucca moth, which also lays its eggs inside the flower. The tree produces a fruit about the size of an apricot; they're edible, but extremely bitter. Joshua trees can grow from seed; they can also sprout from the rhizome (an underground shoot or root) of another tree.