However, palms are neither invulnerable nor immortal. We have estimated that the average life span is slightly longer than that of a human, about eighty or ninety years. An occasional individual probably lives 150, though the oldest documented palm has survived (or 136 years. The latter tree grows in the Oasis of Mara near the headquarters of Joshua Tree National Monument (Joshua Tree National Monument, 1984). An accumulation of factors usually causes the death of a palm. Each fire removes a little more wood and destroys a few more vascular bundles. Erosion may expose roots and in canyons flood-hurdled boulders often batter trunks. Many palms ultimately topple during flash floods. However, as is so often true in nature, it is the least obvious events that seem to have the greatest impact.
Most, perhaps all mature palms, are the home of a rather
grotesque creature known as the giant palm boring beetle, Dinapate
wrightii. The stocky, pale yellow larva spend up to six years eating their
way through the trunk. Once they have grown to nearly two inches in length,
they metamorphose into the adult beetle and chew their way out. Their exit
holes can be found on nearly every mature tree and we have found numerous palms
whose interior has been completely gutted by beetle larvae. Older trees often
succumb to repeated attacks or at least are made more vulnerable to the ravages
of fire and flood. The cumulative result is that the trees die, but not before
they have produced hundreds of thousands of seeds, some of which will produce
palms better adapted to the present environment than either of the parents.Desert fan palm fruits mature through the summer months and ripen in the fall. Though many birds, including cedar waxwings, western bluebirds and mockingbirds consume the flesh, only two animals are known to regularly eat the seeds; coyotes and man. Nearly ever palm oasis shows signs of early human occupation and in many, bedrock mortars mark the spot where Indian women pounded palm seeds into flour. Of course the seeds were destroyed in this process and though they provided an important food source, were lost as recruits for the next generation of palms.
Coyotes eat the entire fruit; both flesh (pericarp) and seed. The pericarp is assimilated but each seed passes through the coyote's digestive tract unharmed. Studies by Stephen Bullock (1980) and ourselves show that these seeds readily germinate and in our study were twice as likely to sprout as seeds which had not been eaten. In the fall, coyote droppings are filled with palm seeds, most of which are ready to germinate and produce new palms.
Since coyotes are known to travel at least eleven miles in one day (Andelt and Gipson, 1979) and up 1055 miles in one direction over extended periods (Berg and Chesness, 1978), we're confident this mammal is not only the most important, but probably the only long distance dispersal agent of palm seeds today. The Present distribution of Washingtonia filifera, with just three exceptions, can easily be attributed to coyotes. Only the palm oases at Mopah Spring in the Turtle Mountains, Palm Canyon in the Kofa Mountains and a newly discovered oasis near Wickenburg. Arizona, require an alternate explanation. These oases lie outside the boundaries of the Colorado Desert and are far removed from the main populations. Either these remote groves are relics from a time when the species had a much broader distribution than it does today, or Indians are responsible for transporting seeds to these locations. We hope our continued research into the ecology of desert fan palm oases may shed light on this and other interesting questions.
