Sunday, February 25, 2018

Joshua Tree Woodlands: A Tale of Sloths, Moths and the Trees that Need Them

In 1844, early California explorer John C. Fremont gave a somewhat scathing review of Joshua trees when he wrote, "...their stiff and ungraceful form makes them to the traveler the most repulsive tree in the vegetable kingdom." 

Rather unkind, don't you think? 

But when Joshua trees were formally described by scientist William Trelease in the 1890's, he noted that Joshua Trees are "the most attractive of all the Yuccas."  I am inclined to agree with this sentiment, and must disagree most heartily with Lieutenant Fremont!

I, for one, adore Joshua Trees, with their whimsical shapes and great ecological significance!  If you struggle with plant identification, the iconic Joshua tree, hallmark of the Mojave Desert, is a good place to start.  With densely spiked tufts protruding from the ends of gangly arms stretched towards the heavens, this looks like a plant straight from a Dr. Seuss book.  Native people called these trees “humwichawa,” and used their leaves for making baskets and sandals, and ate the flower buds and seeds.  As the story goes, the Joshua tree received its common name from Mormon settlers that were reminded of the Old Testament prophet Joshua, his arms either outstretched to Heaven in prayer, or beckoning weary pioneers on to the promised land (take your pick).  In fact, it's Latin name, Yucca brevifolia, tells us more useful information (as Latin names are wont to do).

The Joshua tree is a member of the Yucca genus, which is in the lily family (Liliaceae).  It's not actually a tree at all!  The species name, brevifolia, translates to "short-leaved," as its dagger-like leaves are shorter than other types of yuccas.  The Joshua tree is the largest of the yuccas and its range corresponds quite nicely with the boundary of California's mid-elevation desert, the Mojave.  But the range of the Joshua tree is shrinking.

As it turns out, the range of the Joshua tree has been shrinking for quite some time - since the end of the Pleistocene Ice Age, roughly 12,000 years ago.  To fully understand what has caused the decrease in Joshua trees, we have to examine the ecology of the past and remember that even species of long ago lived in community with one another, comprising complete ecosystems.  Every living thing is interconnected and always has been.  After the last ice age, it wasn't simply that Joshua trees couldn't survive in the changing, warming climate of most of their range.  The answer is a little more surprising and way more interesting.

In the early 1900's, botanist Susan McKelvey wrote of the Joshua Tree, "One would not be surprised to see a huge prehistoric monster standing by and feeding upon the fruit."  Here we have a clue.

Rather than the trees themselves, it was the animals responsible for the dispersal of Joshua tree seeds that didn't survive: giant Shasta ground sloths (Nothrotheriops shastensis).

"Sketches of the skeleton and the restoration of the Aden Crater shasta ground sloth. Note, in the restoration, how the sloth walked on the knuckles of its front feet and the heel and outer edges of its hind feet. The sketches appeared in a report "Aden Crater: A Late-Quaternary Volcano, Southcentral New Mexico," by Jerry M. Hoffer and associates."  From DesertUSA.  Read more:

In the 1930's, scientists discovered the preserved remains of giant ground sloth dung (along with skeletal remains) in a dry cave in Nevada; there have been numerous other similar discoveries across the southwest.  The preserved dung contained leaves, fruit and seeds of the Joshua tree, providing evidence of the vital role these creatures once played in their ecosystem.  Ground sloths of the southwest became extinct between 12,000 and 13,000 years ago, and with them went the Joshua tree's major method of seed dispersal.

Sloths wandered upwards of 10 miles or more before depositing dung, moving trees across great distances.  The seeds of Joshua trees are otherwise poorly adapted for dispersal: they are not sticky or otherwise equipped for hitchhiking, unable to be born by the wind, and are not encased in fruit likely to be attractive to many animals; in fact, it seems that the seeds are usually killed by the stomach acids of the animals that do still consume them.  In the absence of sloths, the seeds are collected by woodrats, sedentary little creatures that may live entire lifetimes without straying more than a few hundred feet from home.  Other desert animals also probably play small roles in dispersing seeds short distances, but the trees were really adapted for dispersal by the sloth.

Another example of an evolutionary anachronism, as it is called when the extant member of a coevolutionary pair is left with an extinct partner, such as the Joshua trees and ground sloths, is the wild avocado tree.  Truly anachronisms, they are chronologically out of place.  Avocado trees, with their large, fleshy fruit and enormous seeds, were designed to be eaten and dispersed by sloths and other large mammals.  With the megafaunal extinction at the end of the last ice age, avocados lost their seed dispersers, just like Joshua trees.  Humans of course stepped in at some point because we too enjoy the fruit of the avocado, but today, wild avocado trees are quite rare.  (If you're interested, there is an interesting article about anachronistic trees on the American Forests website.)

Originally, the proposed name for Joshua Tree National Monument (before its promotion to national park status) was "Desert Plants National Monument," due to its incredible diversity of plants and plant communities.  Driving through Joshua Tree National Park from northwest to southeast, you will drop in elevation and pass from the Mojave desert, characterized by Joshua trees, to the very different and distinct lower elevation Colorado desert (less than 1,000 feet) which supports its own unique assemblage of plants - and no Joshua trees.  Joshua trees favor slightly higher elevations, from 1,300 to 6,000 feet.  In fact, 1,500-foot Cima Dome, in Mojave National Preserve, supports the world's most extensive Joshua tree woodland.  Without ground sloths to disperse seeds upwards, and ever-warming lower elevation deserts preventing a downward shift, the range of Joshua trees is squeezed in the middle.

While creosote bush scrub is the dominant plant community across the Mojave (and indeed it is thought to be the most widespread plant in all of California), the Joshua tree woodlands of higher elevations really are the eye-catching centerpieces of the desert landscape.  More than that, Joshua trees may be considered a keystone species, a plant upon which numerous species of wildlife depend for food, moisture and shelter.  In an ecosystem as fragile as a desert, they are crucial.  The Mojave Yucca (Yucca schidigera) is a second yucca that is also abundant in Joshua tree woodlands; associated plants include paperbag bush (Salazaria mexicana), Mormon tea (Ephedra spp.), sages (Salvia spp.), horsebrushes (Tetradymia spp.) and chollas (Cylindropuntia spp.), along with abundant annual wildflowers during years of favorable conditions.

Joshua trees provide shelter for a number of species, including a couple dozen species of birds, as well as lizards and mammals.  Ladder-backed woodpeckers (Picoides scalaris) and Scott's Orioles (Icterus parisorum) are both strongly associated with Joshua trees.  (In the Central Valley's riparian woodlands, we have Nuttall's woodpecker and Bullock's oriole, closely related species.)  A few mammals associated with Joshua tree woodlands include woodrats, kangaroo rats, antelope ground squirrels, desert cottontail rabbits and coyotes.

Scott's Oriole nest of woven yucca fibers, suspended in a Joshua tree.

Desert night lizards are inextricably linked with Joshua trees, and may spend their entire life cycle living on the ground beneath fallen yucca branches.  These tiny lizards (less than 3 inches long) remain under the shelter of plant debris, feeding on small invertebrates, such as ants, termites and beetles which are attracted to decaying plant material.

Illustration of "The small world of a Joshua Tree," as pictured in Allan A. Schoenherr's
A Natural History of California (Chapter 9, California's Deserts)

And now we come to the Joshua tree's most important living ally: the yucca moth (moths of the family Prodoxidae).  We've talked a lot about seed dispersal, but what about the process of pollination required to produce those seeds?  It turns out that the story of the Joshua tree and its symbiotic moth pollinizer is really fascinating!  (But of course I would say that.  Read on and I will explain.) 

Joshua tree flowers produce very little pollen, and no nectar.  They rely entirely on yucca moths for pollination, and the yucca moths depend on the Joshua tree flowers as a safe place to lay their eggs, and the seeds as a source of food for developing larvae.  Because yucca moths lay their eggs exclusively on yucca plants, the moths deliberately pollinize the flowers. 

Botanist Susan McKelvey described the Joshua Tree's flowers as “curious rather than beautiful.”
Can't they be both?

First, the moth uses specialized mouthparts to collect balls of pollen from an inflorescence on one plant.  Then she chooses an unfertilized flower - on a different plant to ensure cross-pollination - in which to lay her eggs.  Making a small hole in the flower, she lays her eggs directly on the unfertilized ovules - again, deliberately.  She then pollinates the flower by applying the pollen directly to the stigma at the top of the pistil (the female reproductive part of a flower).  This ensures that the seeds of that flower will develop into a source of food for the newly-hatched caterpillars.

Photo of yucca moth (Tegeticula sp.) borrowed from the Forest Service. Photo by M.J. Hatfield,

The methods of the yucca moth are so deliberate and so fascinating, Charles Darwin described the relationship between yucca and moth as, "The most remarkable fertilization system ever described."  The trees do lose a portion of their seeds in the exchange, but without the moths, they would not be able to set seed and reproduce at all.

You might be surprised to learn that a young Joshua tree spends its first few decades of life in a columnar, unbranched form, looking nothing like the sculptural plants we know and love.  The branching structure begins to form only after one of two things happens.  After the tree flowers for the first time, which always occurs at the growing tip, the blooms drop off and that stem ceases to grow.  This causes the stem to branch.  In addition to flower-drop, new branching growth is also caused when the larvae of an insect, the yucca weevil, nibbles off the end of the growing stem; the branch then splits or changes direction, over time forming the unique branching structure of the Joshua tree.

Some quick Googling turns up a number of articles on the somewhat bleak future outlook of Joshua trees (one 10-year-old piece from NPR, a few more recent from National Geographic and plenty of others).  I even found some crazy interesting ideas floating around out there about bringing ground sloths back from extinction to combat the loss of Joshua trees.  (*Cue Jurassic Park music now*) 

While it seems odd that a desert plant could be threatened by warming temperatures, remember that desert plants are already surviving on the boundaries of environmental extremes.  Moreover, Joshua trees require a period of cold, even freezing temperatures in order to reproduce (hence their exclusion from the hotter Colorado desert).  With warming temperatures, it seems they may not be getting the cold snaps they need.  And without the giant sloths around to move Joshua tree seeds into higher, cooler areas, the trees are in decline.  

The Joshua tree is just one of many, many species out there that are acting as the proverbial "canary in the coal mine" in terms of alerting us to a rapidly changing environment.  And unlike some charismatic creatures (polar bears on melting sea ice) or worldwide phenomena (global decline in amphibian populations), the plight of the Joshua trees is something that most people just aren't aware of.

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