Friday, March 31, 2017

Plant Life of the Colorado Desert

The Colorado Desert, the portion of the Sonoran Desert that spills over into California, is a wonderfully diverse ecosystem, botanically speaking.  The Sonoran Desert extends across southern Arizona and south into Mexico and Baja California on both sides of the Gulf of California.  In the state of California, we are lucky to claim a little corner of the Sonoran Desert as our own.  The assemblage of plant life in California's Colorado Desert is markedly different from our more northerly Mojave Desert.  The Colorado Desert, like other regions of the Sonoran Desert, experiences two rainy seasons, one in winter and a second at the end of summer.  This allows for a greater diversity of species here than in the adjacent Mojave Desert.

Chollas, with blooming Desert Senna (Cassia armata)

A drive through Joshua Tree National Park allows visitors to experience both desert ecosystems.  The northern part of the park, where its namesake Joshua Trees are found, lies within the Mojave Desert.  This desert is slightly higher in overall elevation, with cooler average temperatures.  The Mojave Desert can receive a small amount of winter snow, while it very seldom freezes in the Colorado Desert.  The southern part of Joshua Tree National Park lies within the Colorado Desert.  An excellent place to see an assemblage of typical Colorado Desert plants is in the vicinity of the Cottonwood Visitor Center, along the Bajada Nature Trail and on the hike to Lost Palms Oasis.

Trail to Lost Palms Oasis

Sonoran Desert plant communities show greater species diversity as well as more variation in plant form than other deserts.  For example, while the Mojave Desert is dominated by acres and acres of evenly spaced Creosote Bush (Larrea tridentata) and Joshua trees (Yucca brevifolia) - both fascinating plants in their own right - the Sonoran Desert supports more layers of desert vegetation, with towering Saguaro cacti (though just a few are within the borders of California), sub-trees such as Palo Verde and Desert Ironwood (Olneya tesota), and a variety of small-leaved shrubs.  (Creosote Bush also occurs in the Colorado Desert, though mixed with other species; its presence is not quite as obvious as it is in the Mojave Desert.)  The Colorado Desert also supports more species of cacti than the Mojave, giving it more of the "typical" desert look.

California Barrel Cactus (Ferocactus cylindraceus)

In addition to cacti, shrubs and sub-trees, the Colorado Desert supports an abundance of wildflowers.  Common species include Desert Sand Verbena (Abronia villosa), Desert Sunflower (Geraea canescens), several species of phacelia, plus numerous species of desert primroses. 

Engelmann's Hedgehog Cactus (Echinocereus engelmannii) with yellow desert primrose (Oenothera sp.)
and purple phacelia

Two rainy seasons allow more species of wildflowers to flourish in the Colorado Desert.  Annual wildflowers that are adapted to respond to several months of light winter rains (like those of the Mojave Desert) as well as those that respond to short periods of intense summer rain (like those of the Chihuahuan Desert) overlap here, both types able to grow and bloom in the Sonoran Desert.

Sand Blazing Star (Mentzelia involucrate)

Desert Chicory (Rafinesquia neomexicana)

If you've just botanized in Death Valley and the Mojave Desert, then traveled to Joshua Tree National Park eager to explore the flora of the Colorado Desert, expect to recognize some familiar Mojave species, like my favorite, Apricot Globe Mallow (Sphaeralcea ambigua).

Apricot Globe Mallow (Sphaeralcea ambigua)

Apricot Globe Mallow (Sphaeralcea ambigua) growing in a desert wash.

Other common (or noteworthy) plants of the Colorado Desert include Ocotillo (Fouquieria splendens), Indigo Bush (Psorothamnus schottii), Chuparosa (Justicia californica), yuccas, agaves, and a number of species of cacti, including the chollas (which may look huggable, but don't get too close!)

Teddy Bear Cholla (Cylindropuntia bigelovii)

A unique habitat in the Colorado and Mojave Deserts is the palm oasis, where western North America's only native palm is found.  Desert Fan Palms (Washingtonia filifera) indicate the presence of water in the desert.  Oases occur along fault zones, where the movement of tectonic plates has pulverized underlying rocks, grinding some into clay.  The crushed rocks and clay act like an underground dam (or impermeable barrier) to force groundwater toward the surface.  Southern California is the best place to visit a fan palm oasis.  There are only 158 desert fan palm oases in North America, and few exist outside of California. 

Lost Palms Oasis, a California Fan Palm oasis in Joshua Tree National Park

As you might have already guessed, fan palm oases provide a luxurious patch of critical habitat for a great number of desert dwellers, humans included.  Desert Bighorn Sheep (Ovis canadensis nelsoni), Coyotes (Canis latrans) and Gambel's Quail (Callipepla gambelii) visit oases, feeding on palm fruit and subsequently dispersing the seeds.  Orioles (Icterus sp.) nest in the "skirts" of the palms, and a species of bat roosts exclusively in these palm oases.

Trail through the Ocotillos

I highly recommend the 7.2 mile round-trip hike to Lost Palms Oasis in the southern portion of Joshua Tree National Park (hike only in the winter or spring).  The trail passes through beautiful Colorado Desert vegetation (including a neat stand of Ocotillos) and leads to a secluded fan palm oasis, tucked at the bottom of a canyon.  A more accessible (though less impressive, in my opinion) oasis can be seen at the Oasis of Mara visitor center, near the northern entrance to Joshua Tree National Park.

Colorado Desert hillside with chollas and blooming Brittlebush (Encelia farinosa)

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