Joshua Tree (Yucca brevifolia) and its sister species (Yucca jaegeriana)
are vegetative icons of the arid, continental climate region of the southwesterly
Mojave Desert floristic region. By contrast, as iconic are oak woodlands of
cismontane inverse – winter wet, maritime climate California. The two are exactly inverse.
Lenz (Aliso 24:97-104. 2007) admirably shows that there are clearly two
species of Joshua Tree – at this juncture neither taxon has been investigated
with molecular methods. The attached map is an approximation drawn
from Little (1976) – red is the distribution of Yucca brevifolia Engelmann and
green the distribution of Y. jaegeriana (McKelevy) L.W. Lenz.
The photograph is a site in Oak Creek Canyon, Kern County, in the Techachapi
Mountains (15 April 2003, ca. 35.03218 -118.39499) where Yucca brevifolia reaches its most mesic
incursion into, barely, cismontane California, thus barely within the
California Floristic Province sensu stricto.
Might these extreme western-most Joshua trees have something going for them in the genome
department?
Reference:
Little, E.L., Jr., 1976, Atlas of United States trees, volume 3, minor Western hardwoods: U.S. Department of Agriculture Miscellaneous Publication 1314, 13 p., 290 maps.
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