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Why Joshua Trees Look The Way They Do

It's supposed to look like this. | Photo: Atomische * Tom Giebel/Flickr/Creative Commons License

The first time I awoke in the Mojave, I was cramped uncomfortably into the passenger seat of my girlfriend's Honda in a roadside rest area on Route 58 near Boron. It was not yet light. I had agreed to drive her car from Los Angeles to the Bay Area alone; she would fly back to spend time with her new boyfriend. Our breakup was not far off. I dropped her off at the airport, then drove off to head anywhere but home. A couple hours later I turned east on 58 from Mojave, vaguely remembering a rest stop from a visit five years before, not knowing which way I'd go when I awoke the next morning.

Through the dry, cloudless air, a few spring morning constellations shone brilliantly. The nearby hills on the Edwards Air Force Base, speckled with lights from one installation or another, made it hard to tell where the sky ended and the earth began.

And then I saw a difference between land and air. The black sky to the east at first seemed to turn a deeper black, as if some unseen hand had poured a vat of purple ink into it. The cast was enough to outline a few objects on land: texture crept slowly into the featureless dark. Then a pale red smudge began to outline the horizon. As I watched, rapt, it spread to the whole eastern sky, a brilliant vermilion wash fading to violet overhead. I left the car to stretch. Behind the spot where I'd parked, a twenty-five-foot Joshua tree stood outlined against the red morning sky.

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It was a flat silhouette, no relief discernable in the dim light, and I could not tell which of the curving, intersecting, crazily twisted branches was in front and which behind. It seemed an odd, alien thing, drooping wood and daggers, a painting done by Bosch and Dali with a Edvard Munch sky.

A few minutes later the sun came up, but the tree looked only a bit less odd for being three-dimensional.

Route 58 was once called Route 466, a feeder road for Route 66, the great Mother Road, the route mid-continent refugees took to get to the promised land. Between Dust Bowl and deliverance lay desolation. The migrants poured past my resting spot in a veritable stream, bound for low-paying, abusive jobs in the fields of California. 466 was part of the pipeline from Oklahoma City to Bakersfield. In the most important cinematographical treatment of that migration, "The Grapes of Wrath," director John Ford portrayed the horror, the frightening otherness of the Mojave transit by showing the slow passage of Joshua trees at night.

It's not surprising: the trees are strange. Joshua tree fanciers are fond of quoting the famous statement of John C. Frémont, western explorer and first Republican presidential candidate, on his first glimpse of Joshua trees somewhere in the vicinity of Walker Pass:

"Their stiff and ungraceful form makes them to the traveler the most repulsive tree in the vegetable kingdom."

Oddly enough, few quote the passage in full, which is from his "Report of the Exploring Expedition to the Rocky Mountains in the year 1842, and to Oregon and North California in the Years 1843-44." (The expanded 19th-century style title doesn't seem quite so long when you consider the original byline, which read: "by Brevet Captain John C. Frémont, of the Topographical Engineers, under the orders of Colonel J.J. Abert, Chief of the Topographical Bureau.)

The sentence that precedes his bon mot, dated April 14, 1844:

"Crossing a low sierra, and descending a hollow where a spring gushed out, we were struck by the sudden appearance of yucca trees, which gave a strange and southern character to the country, and suited well with the dry and desert region we were approaching."

Modern desert-loving aesthetic sensibilities notwithstanding, Frémont didn't intend that as a compliment. His bad mood can be forgiven. He had recently spent several long weeks floundering in the deep snow of the Sierra Nevada. He had lost his beloved horse Proveau to fatigue only a month earlier. Several of his favored compatriots had abandoned his expedition, seduced by the wiles of coastal California. Just that week he had confirmed that the fabled Buenaventura River, which allegedly ran from the west slope of the Rockies to the San Francisco Bay, could not possibly exist: the Sierra Nevada was just too big to permit such a river to flow through it. He was therefore about to crush the fond dreams of thousands of boosters and speculators back in the United States. Also, he had not had a decent cup of coffee in weeks, a problem with which every Mojave traveler could commiserate before the advent of certain chain coffee places.

It's no secret that I have a bit of a bias in this matter. But after extended spans of time spent in Joshua tree country, I drive through the well-watered landscapes west of the San Gabriels or the Sierra Nevada and the apricot and almond trees and the live oaks just look wrong. Their growth is too exuberant, their branches twee, a rococo profusion of twigs and dainty leaves getting in the way of the sky. It takes me a day or so to readjust when I leave the Mojave.

Stiff? Ungraceful? Quite the contrary.

Unlike trees such as oaks or pines or redwoods, the stems of Joshua trees are remarkably flexible. Other trees, hardwoods and softwoods alike, have trunks and branches that are reinforced with lignin, a polymer associated with cellulose that makes up as much as a third of the weight of the wood of many trees. Lignin makes trees rigid and that rigidity confers strength, but with such strength comes brittleness. A thick tree trunk bends only a little in a strong wind, and too strong a wind will shatter it.

Further, lignin is heavy, and so horizontal branches pose a special engineering problem for woody plants. Too much weight at the far end, and the branch will break off near its point of attachment. Lignified trees must reinforce all but the lightest horizontal branches by adding structural support, either by thickening the undersides of branches at the trunk (compression wood) or by buttressing the top of the branch to help hold it up (tension wood.)

If Joshua trees look different on the outside from most other trees, they are even more different on the inside. The trunks of hardwoods and softwoods are essentially a thin cylindrical shell of living tissue, the vascular cambium, surrounding a non-living core. The vascular cambium produces two kinds of vascular tissue: xylem on the inside of the cylinder, phloem on the outside. The tree's leaves transpire water, creating a vacuum that siphons water up the xylem from the roots. Leaves turn water and sunlight into sugar, which flows down the phloem to the roots for storage. Each year the cambium produces another layer of xylem, creating those annual tree rings. Xylem is essentially dead matter, a set of static pipelines to the leaves. In the center of a growing tree the oldest xylem becomes a landfill, a waste repository for resins and other byproducts of plant metabolism. The tubes are clogged with plaque, close off, harden. This is heartwood, the densest, darkest wood in the tree. It is structural support and toxic waste repository, and not much more.

When I'm with a friend who's new to the Mojave, I sometimes lift good-sized pieces of fallen Joshua tree under the pretext of looking for a desert night lizard. In truth I'm generally seeking to impress, if only momentarily. The trunks of Joshua trees are nearly as light as balsa. You will find no heartwood there, and precious little lignin. Sawn in cross-section, they show no annual growth rings annual or otherwise, perplexing those who would guess at a tree's age. In the Joshua tree trunk, xylem and phloem are combined into vascular bundles, little two-way conduits wrapped together with a thick insulating cellulose fiber between them. The trees depend for their strength on those long strands of fiber, which hold the tree up the way bridge cables hold up the Golden Gate.

In the early days of settlement of the Mojave, Joshua tree wood was often used to splint broken limbs. Peel off a cylinder of the wood to fit a broken arm, and it would immobilize the injured limb until the bone could be set -- but was pliable enough to bend it into place, and to remove without a cast saw. And unlike a plaster cast, it breathed.

In that flexible strength is the secret to the Joshua tree's appearance.

A young Joshua tree's terminal bud grows toward the sky. Its stem is springy, capable of bearing great weight. If prevailing wind, or sudden loosening of soil, or brighter sun just around a rock outcrop spur the bud to grow less than absolutely vertically, the increasing weight of the stem causes it to sag ever so slightly. The bud responds to the change in direction by putting on more growth, heading for the vertical again. This increases the load on the stem, which causes the bud to again change direction, and the stem increasingly bows.

Auxins, the plant hormones that suppress the growth of buds, move more or less by the dictates of gravity. There is less auxin at the top surface of a horizontal stem. New buds hidden beneath the surface push their way between plates of bark. They emerge and grow skyward, adding more weight to the mother branch, themselves bending and bowing when they reach a certain length.

After years of balance and adjustment, an old branch of a Joshua tree can resemble a draftsman's French curve, broadly sloping at the trunk, bowed gently but firmly back toward the ground but ending a sudden upswept tip. More likely, it will become a fractal nest of such curves, each of its daughter branches -- from tip or adventitious buds -- succumbing to the sway and warp. Should a branch become so heavily laden that it settles, over years, to the ground, roots will often form where it touches the soil. When a tree becomes so lopsided that the roots on one side of its flared trunk can no longer keep their grasp on the subsoil, such branch roots may keep the upended tree alive for dozens more years.

I find this dance between growth and gravity, this yielding, flexible response to the harsh Mojave climate worthy of emulation.

A half century ago I had a toy, a little wooden dog, made of blond beads threaded together on strings. The strings fed through holes in the dog's paws through a little platform, where they were glued to a spring-loaded button in the base. Press up on the button with your thumb, and the thread slackened. The little dog would flop amusingly. Release the button and the spring would tense the threads, bringing the dog to attention, alert and ready to play.

I sometimes remember that toy as I sit beneath the Joshua trees, built from more or less the same blueprints. Taut-stringed branches wave gently in the slightest breeze, support and weight always finding balance somehow. Their slow dance both announces and calms the wind. They distill grace from tension, making them to this traveler the most attractive tree in the vegetable kingdom.

An excerpt from the author's forthcoming book on Joshua trees.

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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