August 27, 2009

Joshuas in peril

Are Joshua trees facing extinction?

Special to the Victor Valley Daily Press

SUNSET: The setting sun shines on the Joshua tree forest on Cima Dome in the Mojave National Preserve. (CHRIS CLARKE Special to the Daily Press)

The emblematic tree of the Mojave Desert is in big trouble. At a February 2009 climate-change symposium in Joshua Tree sponsored by the National Parks Conservation Association, scientists spoke with gallows humor about the eventual need for Joshua Tree National Park to change its name. The way things are going, it looks as if there will be no Joshua trees left in the park before too many more decades go by.

In a recent study, U.S. Geological Survey scientists Kenneth Cole and his colleagues used climate change models to predict the effects of global warming on the Joshua tree. Cole’s team calculated the likelihood of tree survival and the rate at which the trees reproduce and spread, and mapped the species’ likely range late in this century. The resulting map was empty. Only when the team tweaked their models to assume that the trees disperse their seeds 10 times as efficiently as they actually do, did the model predict Joshua trees would still exist in the wild in the late 21st century.

Even in Cole’s optimistic scenario, the Joshua tree would become extinct in Arizona and Utah. No Joshua trees would survive in JTNP, nor in the Mojave National Preserve. The species would be forced into the northernmost part of its range in eastern California and central Nevada. And all the web of life that depends on the trees elsewhere would unravel.

Take the yucca moth, for instance. The welfare of the Joshua tree depends utterly on the welfare of this inconspicuous insect. The moths and the Joshuas have evolved a remarkable partnership without which neither could reproduce. The moths visit male yucca flowers and gather pollen. They then pollinate and lay eggs in the female blossoms. The eggs hatch and the moth larvae eat some of the developing seeds. Adult moths emerge from the ground when the trees bloom, the timing due to unknown triggers. If changes in spring temperatures prompt moths to emerge when Joshua trees have no open blooms, then both species could suffer catastrophic reproductive failure.

Other desert animals play a role in the Joshua trees’ fate as well. In 2001, rangers observed large missing patches of the bark-like layer on trees throughout JTNP. The next year less than an inch of rain fell in the park. Researcher Todd Esque and his colleagues found that jackrabbits, ground squirrels and pocket gophers, struggling to obtain food in a year when many desert plants failed to germinate or put out new growth, were stripping bark off the trees. Of those trees that had been stripped of even a little bark, very few survived. By 2003 there were no areas of JTNP in which Joshua trees did not exhibit damage. Thousands of the park’s trees died.

Drought interferes with the production of new trees as well. In order to bloom, Joshua trees require significant precipitation in mid-winter, preferably in February. Drought inhibits bloom. Fewer flowers mean fewer seeds and thus fewer baby Joshua trees.

The recent increase in desert wildfires is another climate-related change. Torrential storms have prompted flushes of plant growth throughout the desert, which then provide fuel for dry-season fires. Invasive plants such as red brome grass and Sahara mustard have spread throughout the deserts, increasing fuel loads and filling in bare soil between shrubs that had been a natural firebreak. Before the invasives, lightning could strike a Joshua tree and that tree could burn without the fire spreading. Now fires threaten whole forests of Joshua trees, especially in JTNP.

Joshua trees are damaged directly by fire, with even minor damage often resulting in mortality. They remain standing for years afterward, hellish forests of white-trunked trees with black leaves, like a photonegative of their live counterparts. But if the direct damage done to Joshua trees by fire is devastating, the indirect damage fires do is even worse. Wildfires destroy slow-growing shrubs and facilitate their replacement with grasses. Blackbrush, an important understory plant in most Joshua tree forests, seems not to re-establish at all after wildfires. This is a serious long-term problem for Joshua trees. Joshua seeds that germinate under blackbrush are more likely to escape being eaten by rabbits until they are older and less palatable. Wiping out the blackbrush understory of a Joshua tree forest nearly guarantees that no new trees grow there. And invasive plants fill in where the blackbrush doesn’t, ensuring more frequent future fires.

Migrating trees

Many other trees face threats from climate change, but for most of them the outlook is not nearly as dire. Though individual trees may succumb, if their seeds get dispersed to more habitable locations — as a Clark’s nutcracker might do with whitebark pine seeds, for instance — the species has a chance to survive. But Joshua trees aren’t very good at seed dispersal. Pack rats will carry seeds up to 50 feet from the parent tree, and a few other birds and rodents play a role in moving Joshua tree seeds around. Unless the tree is on a long, steep hill, its seeds don’t move more than a few meters from the parent tree. The seeds have none of the adaptations found in other desert plants to enable wind or animals to carry them long distances: no sticky burrs, no sweet pulp surrounding a gut-proof seed coat, nothing but a delicate little black flake. Usually when something eats a Joshua tree fruit, it kills the seed. It’s very likely that the trees evolved to be dispersed by the Shasta giant ground sloth, which lived throughout the region until about 12,000 years ago, wandering long distances after gorging itself on Joshua tree fruit. Now that the ground sloth is extinct, Joshua trees disperse their seed much less effectively.

There are mature Joshua trees a hundred miles north of their current range. But little is known of what conditions Joshua tree moths can withstand. Some researchers suggest planting Joshua trees in habitats more likely to be hospitable to the trees’ survival. But until we learn more about the needs of the moths, we have no way to know whether those new forests would be able to sustain themselves.

Chris Clarke is a California environmental journalist and natural history writer, whose work can be seen at He has been studying Joshua trees since the mid-1990. This piece was adapted from an Educational Bulletin of the Desert Protective Council in San Diego. The full report is available at