Definition and Floristic Classification of
The Borrego Desert

The Borrego Desert is the shorthand term I am using for the northern part of Anza-Borrego Desert State Park Region below an elevation of 3000 feet, which includes the town of Borrego Springs, Borrego Valley, Borrego Badlands, Borrego Mountain, and the surrounding areas below an elevation of 3000 feet.

The term Borrego Desert is an integral part of the name of the state park, the Anza-Borrego Desert State Park, and has been in use off and on in the past for this region. In fact, Borrego Desert was the first name proposed for what became the Borrego Palms Desert State Park. It was only after the park was extended to include the Vallecito and Carrizo deserts in 1938, that this park was renamed as Anza Desert State Park, with the Borrego Desert now the name for the northernmost unit of the Park.

In 1953, the Borrego Desert portion, defined as everything north of SR78, was given the name Borrego State Park as a management district within the State Park. This lasted until 1957, when all units were combined again into Anza-Borrego Desert State Park.

Reference: Lindsay, Anza-Borrego A to Z, pp. 35, 75-76.

A map of the area, which also plots locations where I have conducted floristic surveys so far, is shown below:

In the future, I'll add more detailed maps showing the different floristic regions of this area.

This area is actually an ecotone, a transition between the California Floristic Province to the west and the Sonoran Desert to the east, and contains a fascinating suite of species that appear together only here. Those species include significant number of species found only in the California Floristic Province, a significant number of species found only in the Sonoran Desert, and the special species that live only in this Transition Zone between those two floristic areas.

Since this area is an ecotone, one cannot draw precise boundaries for the floristic provinces here. Many people carry the Jepson Desert Manual with them when they hike the Borrego Palm Canyon Trail, but a significant fraction of the species on that Trail are not even present in that book! (I'll quantify that fraction in the future.) The Jepson Desert Manual explicitly states on page 1:

One of the important decisions faced .. was whether to include or exclude taxa that occur along the western edge of the California deserts. ... plants characteristic of the California Floristic Province often spill out onto the western edge of the deserts, especially in areas where the transition between Mediterranean and desert conditions is relatively gradual. Strict adherence to a policy of inclusion of all taxa recorded from at least one site inside a sharply delimited western boundary for the deserts would have defeated our goal of producing a small, simple manual for desert botanists. We trust that the values we chose to emphasize offset the limitations of the Jepson Desert Manual for identifying all vascular plants in transitional areas at the western desert edge.

The lower elevation areas in this region are often considered to be part of the Lower Colorado Desert, which in turn is a region of the Sonoran Desert. However, botanists familiar only with species lists for other areas of the Sonoran Desert would not recognize many of the species found here. Similarly, botanists familiar only with this area would not recognize many of the species found in those other areas.

A good example is given in the Introduction to the book Ecology of Sonoran Desert Plants and Plant Communities, 1999, ed. R.H. Robichaux:

The Sonoran Desert region of the southwestern United States and northwestern Mexico harbors a rich diversity of plant life. Though best known for its large columnar cacti, such as the saguaro (Carnegiea gigantea), cardon (Pachycereus pringlei), and organ pipe cactus (Stenocereus thurberi)...

Among the more unusual plants are the drought-deciduous trees with large, swollen trunks, such as the elephant trees (Bursera microphylla and Pachycormus discolor) and boojum tree (Fouquieria columnaris). Other perennial elements ... such as yuccas (e.g., Yucca valida) and palms (e.g., Sabal uresana).

Of those eight Sonoran desert species, only a single one is found in the Borrego Desert, Bursera microphylla! And even that species is extremely rare here.

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Copyright © 2009 by Tom Chester.
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Updated 18 January 2009.