This is the third in a series of posts on carbon fixation in Mohave desert plants. In this post we will focus on plants that use CAM carbon fixation which includes cactus, yucca and agave. The most important benefit of CAM to plants is the ability to leave most leaf stomata closed during the day. Plants employing CAM are most common in arid environments, where water comes at a premium. Being able to keep stomata closed during the hottest and driest part of the day reduces the loss of water through evaporation and transpiration, allowing such plants to grow in environments that would otherwise be far too dry. Plants using only C3 carbon fixation, for example, lose 97% of the water they take up through the roots to transpiration – a high cost avoided by plants able to employ CAM. The Mojave Desert is the northernmost “hot desert” in North America and essentially a transition land between the Great Basin and Sonoran. It’s the smallest of the Big Four, covering some 54,000 square miles of southeastern California, southern Nevada, and itty-bitty strips of southwestern Utah and northwestern Arizona. Roughly speaking, the Great Basin Desert yields to the Mojave at the northern range limit of creosote bush, the defining shrub of North America’s hot deserts; its distribution essentially outlines them. You can rightly think of it as the hot-desert equivalent of big sagebrush. But the trademark plant of the Mojave, the one whose geography basically maps out this desert, is the Joshua-Tree. This outsized yucca actually flourishes best on the Mojave margins, reaching peak development on middle slopes of foothills and bajadas. Interestingly, the Joshua-Tree uses C3 carbon fixation while most of the remaining yucca and agave use CAM carbon fixation, along with all of the cactus species.
The garden at Relais de la Reine is concentrated on succulents and cactus and is one of the more remarkable collections I have seen. They have a collection of the Malagasy succulents Didierea and Alluauda from the famous “Spiny Forest”, a beautiful collection of indigenous Aloe and Kalanchoe, a collection of rare and unusual euphorbia from the Americas and Africa and an equally amazing collection of unusual cactus with the very rare Brasiliopuntia cactus. You will be forgiven if all these names just went over your head, this post will focus on pictures of these unusual plants with a minimal description. Even if you don’t know their names, these plants are something you will not want to miss. The Spiny Forest in the south of Madagascar is a world of spiky octopus trees and swollen baobabs, and almost all its species exist only in Madagascar.
Most people who do not live in a desert environment consider cactus to be an unattractive species. Nothing could be farther from the truth, cactus are a beautiful species, similar to euphorbia in Africa. Euphorbia can be found all over the world. The forms range from annual plants laying on the ground, to well developed tall trees. In deserts in Madagascar and southern Africa, convergent evolution has led to cactus-like forms where the plants occupy the same ecological niche as cacti do in deserts of North America and South America. The genus is primarily found in the tropical and subtropical regions of Africa and the Americas, but also in temperate zones worldwide. The 1,500 to 1,800 species of cacti mostly fall into one of two groups of “core cacti”: opuntias (subfamily Opuntioideae) and “cactoids” (subfamily Cactoideae). Most members of these two groups are easily recognizable as cacti. They have fleshy succulent stems that are major organs of photosynthesis. They have absent, small, or transient leaves. They have flowers with ovaries that lie below the sepals and petals, often deeply sunken into a fleshy receptacle (the part of the stem from which the flower parts grow). All cacti have areoles highly specialized short shoots with extremely short internodes that produce spines, normal shoots, and flowers. In Las Vegas we have one of the best cactus gardens in the world at the Ethel M Botanic Garden.
At the Larco museum in Peru, they had a collection of the most unusual cacti I have ever seen. Earlier in cactus taxonomy, Cereus was a name that had been applied to nearly all known cactus species that were ribbed, columnar plants. Many of these plants have since been moved out into separate genera. Consequently, the 30 or so plants that remain in the Cereus group are largely plants that have not been moved out of the genus rather than plants that have been included because they fit the description of Cereus. This inclusion-by-lack-of-exclusion makes for a very messy and unsatisfactory grouping. The name Cereus peruvianus has been applied to both C. hildmannianus and C. repandus which are both recognized as legitimate species today. The trouble is, neither of them resemble the many plants that we see labeled as Cereus peruvianus.