Yucca brevifolia
Joshua Tree
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Yucca brevifolia, Red Mountain, Highway 395, Western Mojave Desert
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The magnificent Joshua Tree, Yucca brevifolia, is endemic to the Mojave Desert region of the southwestern United States. The earliest known record of Joshua Tree is by J. C. Fremont in 1844, an army lieutenant who became one of the important explorers in the Kern and Tulare Counties regions of California. The Fremont Cottonwood, Populus fremontii, is named in honor of him.
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Fremont's Cottonwood, Populus fremontii,
Kern River Audubon Preserve, Hwy 178, Weldon
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One version for the origin of the common name "Joshua" is that it was given by Mormons who crossed the Mojave Desert in the 19th century. The shape of the tree and arching branches reminded them of the Biblical character, Joshua, who raised his hands in prayer.
For other versions, see:
How Did the Joshua Tree Get its Name?
The botanical name "Yucca" comes from the Carribean "yuca" which refers to a different plant. The word was adopted by Linneaus as the Yucca genus.
The species "brevifolia" describes the short (brevi-) leaves (folia).
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In 1935 Susan McKelvey, botanist at the Arnold Arboretum of Harvard University, described Yucca brevifolia var. jaegeriana which she observed in the Eastern region of the Mojave Desert. This variety is named in honor of Edmund C. Jaeger, naturalist and educator in Riverside, California. It usually branches lower on the trunk than Y. brevifolia and has shorter leaves.
Dr. Jaeger penned this description in 1957 in his The North American Deserts:
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Near Cima and again in close-by Lanfair Valley is found the largest concentration of yuccas in the United States. It is a giant forest of tree yuccas of the subspecies jaegeriana (Yucca brevifolia jaegeriana), with trees in places so thickly set that it is difficult to see far among them. This yucca differs from the tree yuccas of the western and northern Mohave Desert in having an average shorter main trunk, shorter leaves, a greater tendency to freely branch, and a different chromosome constitution.
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Note Dr. Jaeger's use of "tree yucca." Later he would refer to "Joshua Tree."
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Yucca brevifolia var. jaegeriana,
Living Desert Zoo and Gardens, Palm Desert
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In 1959, Philip Munz, botanist, taxonomist and educator at the Rancho Santa Ana Botanic Garden, recognized Yucca brevifolia var. herbertii, which is found mainly in Kern County in the Western Mojave Desert. This variety is named for Herbert John Webber, American plant physiologist and first director of the University of California Citrus Experiment Station. One of its characteristics is a robust spreading root stock (rhizomes) which produce offshoots, observed especially following fires.
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Yucca brevifolia var. herbertii, Highway 178, Kern County, Western Mojave Desert.
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Taxonomists have argued for other varieties; for raising from variety to sub-species; and even for creating a separate species, eg, Yucca jaegeriana. However, Jepson recognizes just one species with two varieties.
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Pollination
There are two species of Yucca Moths, genus Tegeticula, that pollinate Y. brevifolia. Western Y. brevifolia are primarily pollinated byT. synthetica, while the eastern Y. brevifolia var. jaegeriana are primarily pollinated by T. antithetica.
Trees pollinated by the largerT. synthetica have significantly longer stylar canals. This moth has a longer ovipositor, whereas T. antithetica has a shorter ovipositor for Y. brevifolia var. jaegeriana
to accommodate its shorter stylar canal.
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Some biologists are concerned about the future of the Joshua Tree. In the Joshua Tree National Park, recent droughts and excessive hot summers have put an undue stress on the trees.
In 2015 the group, WildEarth Guardians, petitioned to list the Joshua Tree as an endangered species:
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"Joshua trees exist in a precarious equilibrium. They have evolved a mutualistic relationship with
yucca moths, on which they depend for pollination. They depend for seed dispersal on the
vagaries of rodent caching. They require a narrow climate window to thrive. Lastly, Joshua trees
are slow-growing, slow-reproducing plants and therefore respond very slowly to changes in their
environment.
"The delicate balance allowing Joshua trees to survive is being disrupted by several human caused
threats. Climate change is first among them; climate models indicate that by 2100, as
much as 90% of Joshua tree habitat may disappear. Secondary and interacting threats include
drought, pollution, invasive plants, and changing fire regimes.
"These iconic trees are an irreplaceable part of the Mojave Desert and the American landscape.
Because of their nature, efforts to save them must look to the future on a timescale of decades, if
not centuries. We therefore urge the Service to take the long view and proactively conserve these
incredible plants before they become endangered."
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"Joshua trees depend on a web of ecological relationships with other species for their continued existence. That web of relationships allows Joshua trees to survive in the present-day Mojave. If you make the Mojave less hospitable or threaten these relationships that enable the trees to survive, the trees may suffer greatly. If you do both, it's even worse."
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