"A cactus is a desert’s rose." — Matshona Dhliwayo

By DR. CAMERON BARROWS

When I think of desert plants, those plants that are superbly adapted to the harsh aridity and temperature fluctuations of deserts, I think of cacti. 

Except for one species, all of the other nearly 2000 species of cacti are restricted to the Americas, both North and South America and the Caribbean islands, collectively the Western Hemisphere. The exception is Rhipsalis baccifera which is found in west Africa and Madagascar. No one has a clue how it got there, and it is very non-cactus like in appearance. I have seen it in Madagascar; it grows up on tree branches as an epiphyte, a dense cluster of thin, spineless stems, almost mistletoe-like (though not a parasite). Only its flower structure and its genetics reveal it as a member of the cactus family. Other more typical cactus species (Opuntia spp.) have been introduced to Africa, Madagascar, and Australia where they have become invasive weeds, but otherwise native cacti are only found in the Western Hemisphere.

North American deserts are relatively young. The first hints of true desert, arid-adapted vegetation doesn’t show up in the fossil record here until the very late Miocene and Pliocene Epochs, maybe 5-6 million years ago. The deserts as we know them today did not form until the late Pleistocene Epoch, 10-12,000 years ago, about the same time creosote bushes reached what is now the Mojave Desert. That doesn’t leave much time for so many cactus species to evolve. The explanation, in part, is that cacti did not evolve in deserts, rather their origins are in the American tropics. That seems at odds with the dry desert landscapes where we typically see cacti, and at odds with the sure way to kill a cactus growing in your yard – water it too much. However, visiting tropical islands in the Caribbean, or on the mainland in Costa Rica or Panama, you can find cacti growing, but you need to look up into the trees. Tropical cacti are typically epiphytes just like that one species I saw in Madagascar. While it obviously rains a lot in the tropics, up in the forest canopy on tree limbs there is no soil, nothing to hold onto the abundant rain, and so it is effectively very desert-like. Living on a tropical forest branch requires adaptations for holding onto and storing water in stems. They evolved thick spongy stems, green stems that photosynthesize sunlight into plant foods, that in turn attract arboreal animals who would chew on those stems to access that precious water. One way to both discourage being chewed on and to reduce water loss is to develop specialized leaves that become sharp spines.

Perhaps that Malagasy Rhipsalis is the last relative of what was the earliest cactus occurring on the Gondwana supercontinent when all the Earth’s land masses were connected some 500-600 million years ago. Then, when Gondwana fragmented into the continents we know today, those left in Africa and Madagascar hung on in the dark branches of those tropical landscapes, while those in the Central and South American tropics radiated into forms that would eventually populate our deserts. Thick spongy stems and sharp spines then enabled these early cacti to move north out of tropics and to exploit the increasing aridity of the Pliocene and Pleistocene Epochs. Their water-storing adaptations allowed them to first move into the seasonally dry, tropical deciduous forests of central America and Mexico and eventually reach the Chihuahuan, Sonoran, Colorado, Mojave, and Great Basin Deserts. Today there are about 120 cactus species in Baja California, 70 species of cacti occupying the Sonoran Desert in Arizona, 28 species in the Colorado Desert, 27 species in the Mojave Desert, and just 16 in the California portion of the Great Basin Desert.  These northern, desert adapted cacti have eschewed their tropical, epiphytic life and have radiated into several forms all firmly rooted to the ground.

Some have adopted a tall columnar form. The largest columnar cactus is the Cardón found in Baja California Mexico, where they can reach over 60 feet tall and weigh over 10 tons. Other columnar species include Organ Pipe cacti, Senita, and Saguaros. Saguaros are the iconic species of Arizona’s Sonoran Desert, with just a handful found on the California side of the Colorado River. There is a curious occurrence of just two Senita cacti in the Corn Springs area of the Chuckwalla Mountains of California, otherwise these tall columnar cacti have not found suitable habitats in the rest of the California deserts. One possible reason for not being able to exploit the California Deserts is that Cardón, Saguaros, and Organ Pipe cacti are all pollinated by bats, and those nectar-feeding, pollinating bats are not found in California. Another reason might be related to the much lower amount of summer rainfall in California.

Another cacti group are cholla, in the genus Cylindropuntia. They are intricately branched cacti, that can reach two to six feet tall (one, Munz’s cholla, can be up to 10’ tall), and have cylindric stems that are usually densely covered in spines. In California’s Colorado Desert and the surrounding mountains, you can find pencil cholla, Gander’s cholla, silver cholla, Chuckwalla cholla, teddy bear cholla, Munz’s cholla, and California cane cholla. In the Mojave Desert pencil, silver, and buckhorn cholla are the primary species. Wherever they occur, chollas are favored nesting sites for black-throated sparrows and cactus wrens where their hatchlings are well protected from hungry squirrels, coyotes, and snakes. Closely related are prostrate, ground hugging chollas; one called matted cholla, Grusonia parishii, in the Mojave Desert, and the other, sagebrush cholla G. pulchella found in the Great Basin Desert.

Also related to the cholla are prickly pear – beavertail cacti, of the genus Opuntia. Rather than cylindrical stems, this group has broad, flattened stems often more than six inches across. Also, rather than the somewhat subdued flowers of the cholla, Opuntia flowers are big and brightly colored, beckoning native bees to come and visit. Prickly pears often have large, succulent, tasty fruits, offering them to coyotes and foxes in exchange for distributing their seeds across the desert landscape. The five species of desert Opuntias are broader in their distributions, found in both the Mojave and Colorado Deserts, with one, the plains or grizzly bear prickly pear also reaching well into the Great Basin Desert.

Often hidden among rocks are the diminutive fishhook cacti (Mammillaria) and foxtail cacti (Escobaria/Coryphantha alversonii). The common fishhook cactus, Mammillaria tetrancistra, is, like the prickly pears, equally distributed between the Colorado and Mojave Desert. The Baja fishhook, M. dioica, reaches its northernmost range in the Santa Rosa Mountains in the Colorado Desert, otherwise extending down the length of the Baja California Peninsula.

Hedgehog cacti (Echinocereus) grow in multi-stem clusters and produce arguably the most beautiful flowers of any desert plant, perhaps any North American plant. One, the Mojave hedgehog, or Mojave mound cactus, is restricted to the Mojave Desert. Its flowers are a deep red. The other, California desert-wide species is Engelmann’s hedgehog cactus which has bright purple flowers.

Finally, there are the barrel cacti, (Ferocactus and Echinocactus). Both species, F. cylindraceus, (California barrel cactus) and E. polycephalus, (cottontop cactus) occur broadly across the Colorado and Mojave Deserts. Mostly less than three feet tall, I have seen California barrels over six feet tall. These along with most California desert cacti were important foods for the indigenous people who first occupied these lands. The flower buds were collected and boiled to remove much of the bitter flavor, with a result reported to be like brussels sprouts. Barrel cacti are also an important water source for bighorn sheep when other water is unavailable.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.