This is the final post in my series on photosynthesis in Mohave desert plants. While green leaves are commonly considered as the primary sources of photosynthesis, higher plants can potentially use almost all vegetative and reproductive structures to perform carbon fixation. Chlorophyll-containing bark and wood tissue, most fruit, root and fertile flower organs typically perform an effective internal CO2 recycling using the respiratory released CO2. Photosynthetic stems have positive effects on the carbon economy of plants through two main mechanisms. Photosynthetic stems can either assimilate atmospheric CO2 and contribute to the net carbon gain of the plant through the process of stem net photosynthesis, or decrease respiratory losses by recycling CO2 previously respired by roots and stems through the process of stem recycling photosynthesis.Plants with green stems are categorized by one of three types of stem photosynthesis syndromes. Retamoids include leafless or almost leafless woody plants that have stomata in the stem epidermis allowing for gas exchange with the atmosphere (Schaedle 1975). Another type of photosynthetic stems are the succulent pachycauls or the sarcocaulescent group, which has large-sized stems with translucent exfoliating bark, a large amount of parenchymatous tissue that serves as a water reservoir, and non-succulent, drought-deciduous leaves (Franco-Vizcaino et al.1990). An example of this would be the Boojum Tree (Fouquieria columnaris) and the related Ocotillo (Fouquieria splendens) as both lack stomata and recycle respiratory released CO2 to survive during drought. Another type of photosynthetic stem is found in cactus, which take up carbon dioxide at night with stoma using CAM photosynthesis. The inadequacy of the current state of knowledge for describing or understanding the diversity of structure, function, and ecological significance of photosynthetic stems suggest areas for further research. The diversity of taxonomic types, and classes of photosynthetic stems, results in a large diversity of structural characteristics. In this post, I have simply chosen interesting examples without resolving these difficulties. I have also decided to throw caution to the wind and use lots of technical jargon.
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.
Desert plants tend to look very different from plants native to other regions. They are often swollen, spiny, and have tiny leaves that are rarely bright green. A desert always has a limitation of water but the temperature may be hot or cold, high altitude and cloudy like parts of Costa Rica or low altitude and windy like the Cape Preserve in South Africa. The strange appearance of these plants is a result of their remarkable adaptations to the challenges of the desert climate. Desert plants have developed three main adaptive strategies with diverse implementations often in different species with convergent evolution to the same form: succulence, drought tolerance and drought avoidance in annual plants. Each of these is a different but effective suite of adaptations for prospering under conditions that would kill plants from other regions. These differences often extend to the cellular level with the development of special structures to store water in leaves and stems, the periodic shedding of leaves, and special adaptations to even the basic photosynthesis process. Chlorophyll (the green pigment in plants) is the only known substance in the universe that can capture volatile light energy and convert it into a stable form usable for biological processes (chemical energy) through the Calvin Cycle and the enzyme RuBisCO. Green plants use blue and red light energy to combine low-energy molecules (carbon dioxide and water) into high-energy molecules (carbohydrates or starch), which they accumulate and store as energy reserves. There are at least three variations of photosynthesis, all of which use the same basic mechanism, C3 carbon fixation used by most plants, C4 carbon fixation used in about 3% of plants and the CAM (crassulacean acid metabolism) carbon fixation pathway that evolved in plants like cactus as an adaptation to arid conditions.
Sometimes, looking for plants and flowers in winter can be interesting, particularly near a source of fresh water in the desert. In November, I visited Rogers and Blue Point Springs on the north shore of Lake Mead in the Lake Mead National Recreation Area. Rogers Spring and other springs in the “North Shore Complex” comprise one of the terminal discharge areas for the regional carbonate-rock aquifer system of eastern Nevada and western Utah. The source of the water to this spring and other regional carbonate-rock aquifer springs is uncertain. The prevailing theory suggests that much of the recharge water that enters the carbonate-rock aquifer occurs in the high mountain ranges around Ely, Nevada, located 250 miles north of Lake Mead. As this ground water flows south through the carbonate rocks, it encounters several faults along the way, including the Rogers Spring Fault, which has caused the older carbonate rocks (primarily limestone and dolomite) to be displaced against younger evaporite deposits of the Muddy Creek and Horse Spring formations. Here, the lower permeability of these evaporite deposits, along with high subsurface water pressure, forces the ground water in the carbonate rocks to flow upward along the fault and emerge at the surface as Rogers Spring.
I thought I would add a post on the trees of Costa Rica since there are so many beautiful and unusual specimens. This trip, I visited during the winter or dry season, so many of the plants were without leaves or flowers. Nonetheless, there were many fascinating examples of trees that we in North America rarely get to see. I have roughly divided them into fruit trees, large trees and palm and palm-like trees. Cassia grandis, seen above, is one of several species called pink shower tree, and known as carao in Spanish. It is a flowering plant in the family Fabaceae, native to the neotropics, that grows up to 98 feet (30 m). The species is distributed from southern México, to Venezuela and Ecuador. It grows in forests and open fields at lower elevations, and is known to be planted as an ornamental. In at least Costa Rica, its pods are stewed into a molasses-like syrup, taken as a sweetener and for its nutritional and medicinal effects, called Jarabe (or Miel) de Carao.
Most of Costa Rica’s forests can be primarily classified into three groups; rainforests, cloud forests and topical dry forests. And while rainforests are the most common habitat, the cloud forests of Costa Rica are a magnificent sight to behold. Rainforests can be found in the southwest of the country as well as in the Atlantic lowlands, with towering trees and looping vines that create a magical wispy environment. Receiving a high annual rainfall, these dense forests are characterized by a wealth of plant and animal life. Rainforests are located at lower elevations, and as a result, they tend to be much warmer, especially during the dry season. Cloud forests, on the other hand, are usually located at much higher elevations, and are much cooler. This difference in temperature contributes to the mist and fog that is often visible in cloud forests, as the milder temperatures slow the evaporation process. However, despite being a little cooler than rainforests, cloud forests are very humid. Cloud forests generate water by capturing water from fog (surface clouds). Water condenses on the leaves and branches of cloud forest trees, epiphytes and lichen, drips to the forest floor, and enters streams. The tropical evergreen cloud forests on the slopes of the Cordillera de Talamanca in Costa Rica’s southern highlands is of vital importance both as a source of drinking and irrigation water to the main cities in the Valle Central and as a bastion of many endemic species. This is not meant to be a comprehensive survey of the plants in the cloud forest, concentrating instead on important and noteworthy plants in this ecosystem.
When we visited Vancouver, we came across the most amazing little conservatory. The Bloedel Conservatory is essentially a large bird cage located at the peak of Vancouver in Queen Elizabeth Park. There are 120 exotic mainly tiny birds and over 500 kinds of tropical plants inside the dome. This elaborate cage for the birds and plants reveals something very ancient and primal to humans, the desire to bring the outdoors inside. In this case, these are exotic birds and plants that would not survive in the relatively harsh climate of Vancouver but even in antiquity local birds were kept by the wealthy, in particular for harems and by mariners to find land in the open sea. In medieval Europe, bird keeping was mostly for the wealthy. Kings, Queens and the Clergy would often keep parrots. The Sumerians, the oldest civilization known to have kept written records, had a word, subura, for birdcage. Do we bring birds inside our homes because we are unable to enter theirs? Do we try to tame wild nature because we fear we can never tame our own? These bits of philosophy are thanks to a beautiful essay on caged birds by Jerry Dennis, found below. For the purpose of this post, I thought we would just enjoy these birds, in one of the best settings for an aviary I have yet to see.