Arizona cacti

Heading east from San Diego I descended into the hot Imperial Valley and the Sonoran Desert. Ocotillos appeared signaling entry into this desert province. After a relatively cool spring in northern California, I wasn’t quite ready to be thrust into summer temperatures. My first major destination was Organ Pipe Cactus National Monument, just north of the Arizona-Mexico border. It lies right in the heart of the Sonoran Desert.

Organ Pipe and other cacti at Alamo Canyon.

Night sky at Organ Pipe Cactus National Monument.

I first visited Organ Pipe 20 years ago, when with a class of UC Santa Cruz students we descended on northern Mexico to study the ecology of the Gulf of California. The road to Sonoyta, Mexico passes bisects the monument. I arrived after dark, and turned up a dirt road to a small campsite at the head of Alamo Canyon. It was a warm evening. I stayed up for some time photographing landscapes dominated by Saguaro cacti. A half moon shone brightly in the sky above but was gone by 4 AM when I awoke again to photograph stars and the Milky Way.

With the new day, the diversity of cacti was evident from the start. Saguaros were very common, and organ pipe cacti became more common as I made my way up the valley for a morning hike. There were menacing cholla cacti, with their beautiful golden spines covering plants like a blanket. I also recognized the prickly pear cactus, Opuntia.

The organ pipe cactus (Stenocereus thurberi) is a collection of rather large stems, emerging from a common base. The namesake of the monument, the species is more common in Mexico than in the US and has a limited presence here due to its intolerance of frost. Flowers apparently only open during the night and are primary pollinated by bats.

Saguaros (Carnegiea gigantea) were actually very common at Organ Pipe Cactus NM, growing in most regions of the monument that I observed. However, there is a whole national park named in honor of this species, and that was my next destination. Bisected by the city of Tuscon, Saguaro National Park comprises two units east and west of the city. I briefly visited some of both, catching the sunset at the west unit and doing some morning hiking at the east unit.

Saguaro cacti. Left: spines (which are actually modified leaves on cacti) on a plant at Saguaro NP. Right: An open flower on a plant at Organ Pipe Cactus NM.

The morning of my hike started off warm. I began at a trailhead at Loma Verde and hiked about 2 miles into the Saguaro Wilderness along the Squeeze Pen and Carillo trails. Saguaro, barrel cacti, and Opuntia were common. Mesquite, palo verde (literally, “green stick”), and other trees and shrubs were also abundant in the area. At Wildhorse Canyon I found pools of water remaining in a wash.

Flowering cholla cacti at Saguaro NP. At left is the pencil cholla, Cylindropuntia arbuscula.

Though not a cactus, ocotillos were common at both Organ Pipe Cactus and Saguaro NP and were frequently in bloom, their bright red flowers adding color to the landscape. At Saguaro I saw many ocotillos with leaves (some green, some senescing), a sight that may not be particularly common since these plants readily shed their leaves during times when water is scarce. It is a true deciduous desert species.

Ocotillos. Left: With leaves at Saguaro NP. Right: Hummingbird visiting flowers at Organ Pipe Cactus NM.

Wildlife was fairly common at both of these desert parks. I saw coyotes (Organ Pipe), a Gila Monster (Saguaro), a hummingbird (Organ Pipe) and other birds, a deer (Saguaro), Desert pupfish (Organ Pipe), and lots of lizards (both parks).

Barrel cactus, Ferrocactus, at Organ Pipe Cactus NM.

Chainfruit cholla cactus, Cylindropuntia fulgida, at Organ Pipe Cactus NM.

An organ pipe cactus near the US/Mexico border that is half skeleton at this point.

Prickly pear, barrel cactus, and saguaros at Saguaro NP.

Sunset with Saguaro cacti and ocotillo at Saguaro NP.

Adaptations of the Cactaceae

Cacti are iconic plants of desert environments. These succulent plants occur in the family Cactaceae. There are about 1800 species worldwide in the family, grouped into 125 genera. The family is endemic to North and South America, distributed from southern Argentina to Canada. One additional species occurs in western Africa and some of the islands of the Indian Ocean.

As a group, the Cactaceae are believed to have evolved in South America about 65 million years ago (around the same time as the extinction of dinosaurs), though molecular data suggest a more recent origin of about 30 million years. 65 mya, South America was warm and dry, but disconnected from North America. Cacti spread north, using Cuba and other islands as dispersal stepping stones, arriving in Mexico about 36 million years ago. In North America, the western deserts expanded about 2-5 million years ago with further radiation of cactus species.

The beavertail cactus, Opuntia basilaris. Left: Plant with flower buds from Death Valley National Park, March 2016.
Right: Plant from Joshua Tree National Park, February 2012.

Cacti are excellent plant examples of structural adaptations to meet unique environmental circumstances. The fleshy part of a cactus is actually the stem. In many species the stem is succulent, allowing for water storage in the dry habitats where they live. Water is stored in the stem's parenchyma cells. Stems have additional adaptations to minimize water loss such as the presence of a thick waxy outer cuticle and stomata that are sunken. Individuals can withstand water loss of up to 70-95% because the tissues have so much water storage potential. The longitudinal ribs present in some species are capable of expanding and contracting like an accordion without damaging the protective cuticle as water content in the plant varies.

Mammillaria tetracistra from Panamint Valley in
Death Valley National Park, March 2016. The
black spines on this species are hooked.

The spines of cacti are actually modified leaves. They are non-photosynthetic but have several other functions. The most obvious is defense against herbivores. Spines can also help a cactus acquire water when dew condenses on them, or help shade the plant. The spines may assist with dispersal of segments of the plant when they are caught in the fur of animals and transported around. Some "gland spines" produce nectar which help the plant attract pollinators.

Cacti also couple physiological adaptations with anatomical and structural modifications to deal with arid growing conditions. Plants acquire carbon dioxide (needed to build sugars and other organic molecules) from the air through their stomata, but having stomata open in a hot dry environment makes plants susceptible to water loss. One way to acquire CO₂ but minimize water loss is through CAM photosynthesis, a variation of C3 photosynthesis present in most other plants. In CAM species the stomata remain closed during the day, but then open at night to acquire CO₂ when the potential for water loss is reduced. Carbon dioxide captured at night is temporarily stored as an acid in the cells and later used to build sugars.

According to the recent Jepson manual, California has 37 native species of cacti. On my trip to Death Valley last month, I observed at least five of those species. Two species were particularly common: the beavertail cactus and the clustered barrel cactus. The beaver tail belongs to the genus Opuntia, plants which are also known as prickly pears. Prickly pear stems grow as segments, appearing as upside down tear-shaped fleshy leaves. In early March flower buds were forming on the beaver tails (O. basilaris) but I was too early to see open flowers.

Examples of Echinocactus polycephalus from Death Valley National Park. Incidentally, the Greek
etymology of this species name is fun: 'spiny cactus of many heads'.

Another relatively common cactus at Death Valley was the cholla, a group of cacti classified in the genus Cylindropuntia. I was first introduced to chollas as an undergraduate student when my marine ecology field class traveled from Santa Cruz to northern Mexico through the Arizona desert. We were warned to avoid touching these plants because of their menacing barbed spines which can be difficult to remove from the skin. Chollas have cylindrical stems that also occur in segments like the prickly pears. The small stem segments break off and lay on the desert floor. California has 10 species of cholla and I've also observed specimens of this genus in Joshua Tree National Park, northern Arizona, and Sonora Mexico.

Cholla from Joshua Tree National Park, Feb 2012. These plants are so densley covered  in light-colored spines
that one can hardly see the green stem. It is possible that the dense covering of spines helps reduce water
 loss from the plant.

References

Baldwin BG, Goldman DH, Keil DJ, Patterson R, Rosatti TJ, Wilken DH. 2012. The Jepson Manual. Vascular Plants of California. 2nd ed., University of California Press, Berkeley, CA.

Ingram S. 2008. Cacti, agaves, and yuccas of California and Nevada. Cachuma Press, Los Olivos, CA.

Zomlefer WB 1999. Guide to Flowering Plant Families. The University of North Carolina Press, Chapel Hill, NC.