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?
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.