No more Joshua trees? Climate change may wipe out the signature tree of the Mojave.
Predictions of the Joshua tree's retreat from the Mojave were contained in a 2005 report whose lead author, Dr. Kenneth Cole, is a climate scientist for the federal government's Colorado Plateau Research Station in Flagstaff, Arizona. Dr. Cole and his colleagues examined climatic conditions within the present range of the Joshua tree; they studied the Joshua tree's climate tolerances and its potential to shift its range; and then they applied predictions of future climate change to determine the likely effects of global warming on our beloved yucca grande. To quote from the report:
"The future potential range of [the] Joshua tree (Yucca brevifolia) is not only reduced and shifted northward by climate change, but the plant's lack of dispersal mechanisms should reduce its actual extent by at least 90%."
Cole et al released some of their data last year in poster form at a meeting of the US Climate Change Science Program in Arlington, Virginia. The maps below are from that poster. First, Cole and colleagues took what they knew about current Joshua tree distribution and, using that information, they drew the map on the left.
That map has received some quiet criticism from scientists and land managers. It's a markedly optimistic estimate of the Joshua trees' range, showing large populations well away from known stands of the trees, especially in the extreme south part of the species' range in California and Arizona. But the errors, where they exist, only serve to make Cole et al's conclusions somewhat optimistic.
With the data in the first map, optimistic as it may be, the researchers applied projections of future Mojave desert climate in a world with twice the atmospheric carbon dioxide we had in the 20th century, calculated the likelihood of Joshua tree survival based strictly on climatic factors (as opposed to urbanization, fire, non-climatic disease, and so on), calculated the rate at which the trees disperse new seedlings to potentially more welcoming habitat, and mapped the species' projected range in the mid-late 21st century.
Or they tried to. But the projections didn't include any Joshua tree stands large enough to show up on the map. So they assumed that the trees would somehow disperse their seeds 10 times more efficiently than they have been found to, and they got the map on the right.
Even in this extremely optimistic scenario, the Joshua tree will become extinct in Arizona and Utah. No Joshua trees will survive in Joshua Tree National Park, nor in the Mojave National Preserve.
The problem is seed dispersal. Joshua trees don't do it well. Oh, ladder-backed woodpeckers will hammer at fallen fruit to get the worms that live inside, thereby spilling seeds all over, and pack rats will pick up seeds and fruit and carry them 50 feet from the tree or so, and there are a few other birds and rodents that play a role in moving Joshua tree seeds around. Every once in a while, a coyote might eat a fruit and excrete it 10 miles away. But unless the tree is on a long, steep hill, its golf-ball-sized fruit tend to stay within a few meters of the parent tree. The seeds are large, and seem to have no adaptations allowing birds to carry them long distances by accident: no sticky burrs, no sweet pulp surrounding a gut-proof seed coat, nothing but a delicate little black flake. Most of the time when something eats a Joshua tree seed, it kills the seed.
I started wondering a few years back, as I learned more and more about Joshua trees, whether there might not be a piece missing in this puzzle. Joshua trees are tall, heavily defended plants with fleshy fruit growing at the top, sometimes 30 feet off the ground. There are quite a few plants in the Americas that, like the Joshua tree, seem ill-adapted to seed dispersal because their fruit isn't designed for any living animal to disperse it efficiently: the Osage orange, the pawpaw, the avocado. Many of these plants have been linked to the giant extinct animals of the Pleistocene, big beasts who could denude a pawpaw or avocado tree of all its fruit, walk a distance away and excrete the tree seeds, seasoned only slightly by digestive juices. Could Joshua trees be another such plant, dependent for seed dispersal on an animal species that will never come back?
It turns out at least some scientists share my suspicion.
Nothrotheriops shastense, the Shasta giant ground sloth, lived throughout the range of the Joshua tree in the Pleistocene, up until about 12,000 years ago. That's recently enough that you can still find its mummified dung in caves in the southwest. The said dung has often been found to contain the remains of Joshua tree fruit, with seeds that likely would have been viable when excreted.
The Joshua tree's historic range, in other words, may already be a fossil of sorts, an artifact of a relationship with a species that no longer exists. The big problem with extinction is not so much that those species go extinct --though each is a huge loss--but in the extinction of the relationships that species had with every other species it encountered. No species exists in an ecological void, and each provides either sustenance for or competition with many other species. Each species lost is a node untied in that mesh of relationships.
There is nothing to keep Joshua trees from growing well outside their current range. A grove of them has been growing at the Tonopah Airport, one valley north of their present limit, since World War II. There are mature Joshua trees growing in Ione and Fallon, Nevada, more than a hundred miles north of their current range. The trees would likely find the climate of the hills around the San Joaquin Valley salubrious and, if they dispersed as fast as oaks do, they'd be there in 50 years. There are several distinct populations within an hour's blue jay flight. But unless the scientists' deliberate optimism pans out (an unlikely prospect), global warming may very well kill off the last of the wild Joshua trees. And that's not accounting for increased fire danger, or suburban sprawl, or the fact that small mammals eat Joshua tree "bark" in dry years, a measure of desperation that may have killed thousands of trees in Joshua Tree National Park alone. It may be that 100 years from now, the signature tree of the Mojave will survive only in botanic gardens, and the wild species that depend on it gone forever.
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|Publication:||Earth Island Journal|
|Date:||Mar 22, 2007|
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