"The Wise Gardener!"
The Sensual Pulse of the Tropics!

Consider: Xeriscaping for A MORE "Sensible"
Dry-Climate Landscape!


Carnegiea_gigantea.jpg (7003 bytes)The word desert paints a picture of desolation in most people's mind. A place of burning sand and barren rocks. A place where life is scarce and death is close at hand in the form of a sun-bleached skull of a cow. And some deserts are close to devoid of life, but never completely. There are always a few tough plants and animals that persist, that even thrive, where conditions are the harshest. The plants of the desert have a unique set of adaptations that allow them make a home of a hot, dry land.

Bloated Hoarders of Water

The most obvious of these desert adaptations can be seen in the many kinds of cacti (plural for cactus) that inhabit deserts of the New World. No desert is entirely without rain, and cacti are adapted to soak up large amounts of water, when the rains come, and store it in their stems. Cacti can survive months or even years between rains. They may gradually shrink in size as they use the water up, but they will swell up again later. One of the oddest things about cacti is that they seem to be all stems with no leaves. In reality, their spines, for which some cacti like the cholla or prickly-pear (Opuntia) are famous, are modified leaves. Their great reduction in size and their toughness prevent water loss that is tremendous in normal leaves, and their large numbers help shade the cactus stem. They, of course, provide protection against would be predators too.

What is not immediately apparent about cacti is an important biochemical adaptation to desert life. All plants must do photosynthesis in order to survive. Using carbon dioxide and water, with sunlight as an energy source, plants are able to manufacture all the energy molecules they need to make sugars and all the other molecules they need to survive. Because photosynthesis requires light, it must be done during the day, which presents a great risk for desert plants. Carbon dioxide can only enter the plant through little pores called stomata. These little pores, in addition to bringing in carbon dioxide, also release oxygen and result in a considerable amount of water loss.


Prosopis_juliflora_var_torreyana_Brousseau.jpg (15885 bytes)In the desert, water loss can be life-threatening and during the hot daytime hours water loss would be at its worst. Cacti have a method for circumventing this problem. They open their stomata only at night, when air temperatures are much lower and water loss is at a minimum. The carbon dioxide taken in during the night is trapped by a special biochemical pathway and is stored in the form of organic acids. During the daytime cacti keep their stomata tightly closed, but they can still get the carbon dioxide they need from the organic acids produced during the night. The carbon dioxide is released from these organic acids during the day and photosynthesis is thus able to take place. This type of photosynthesis is called crassulacean acid metabolism (CAM) and was first discovered in members of the stonecrop family (Crassulaceae), a family of succulent plants popular in rock gardens. Although this method of photosynthesis is much slower than normal photosynthesis, the water savings more than offset the disadvantages, but cacti pay a price in that they are extremely slow growers.

Cacti are not the only water hoarders. there are a number of other succulent plants in the desert, including agaves (Agavaceae). Agaves have a rosette of large leaves that are often sharply serrated with extremely sharp leaf tips. Many agaves do not flower until they have grown for a number of years and one is even called the century plant, and legend says it only flowers once every 100 years. Africa has its own brand of succulents, the most famous of which is the stone plant (Lithops). Stone plants are often very small and appear much like
pebbled stuck into the ground. Some even show cryptic coloration that blends in with the surrounding rocks and many are popular among succulent enthusiasts.


The Leaf Droppers & Deep Diggers

Another group of desert plants simply drop their leaves during the driest parts of the desert year. Most of the water lost by plants is lost through the leaves, so by losing the leaves and going dormant, such plants can survive extremely dry conditions. They may look like they are completely dead, but they are only "asleep" and will awaken again when the rains return. At that time they will quickly put out new leaves, and often flowers as well, and the desert will briefly look more like a sparse forest than a wasteland. A striking example from Central Asia is the black saxaul (Arthrophytum aphyllum), which is also capable of withstanding a moderate amount of soil salinity. A common leafdropper in the deserts of the American Southwest is ocatillo (Fouquieria).   Fouquieria is the only genus in the Fouquieriaceae and only 11 species are known.  Ocatillos look superficially like cacti, because they have spines, but they also have true leaves, although they are very small. Along with developing leaves when water is available, they can also produce brilliant red flowers. The stems have been used for fences and huts by native peoples.

Some desert trees and shrubs will solve their water problems by digging deeply into the dry soil, some even reaching permanent water from an aquifer. Mesquite (Prosopis) puts out deep roots that are estimate to reach 75 feet or more beneath the surface. In addition, mesquite trees send out long lateral roots in all directions so that they can quickly soak up the intermittent rains. Many of the acacias (Acacia) also have long roots reaching downward 50 feet or more.

 

Flower Today, Seed Tomorrow

Another significant group of plants are the so-called ephemerals. Their name is based on the word ephemera, which is often used by antique collectors to refer to items that were originally intended to have a short useable life-span, like newspapers. And just like newspapers, which are meant to be read today and thrown away tomorrow, ephemerals germinate, grow, flower and set seed in a matter of just a few weeks. These plants are annuals that are finely tuned to the whims of desert rainfall. When a significant rain occurs, the seeds germinate rapidly and within a few days the desert explodes with color. Some ephemerals have seeds that have a very tough seed coat that must be softened by tumbling over the rocks in a stream bed during a flash flood, a process known as scarification.

Of course, when these many ephemerals appear after the rain, they also draw other organisms. Many pollinators will either come out of dormancy or they will fly in from other areas to feast on the bounty. Other herbivorous insects and rodents will have a heyday as well. And on the heels of the insects will be the many insect eating birds, in addition to nectar feeders (e.g. hummingbirds) and seed eaters. For a brief period the desert doesn't seem much like a desert. After the blooming subsides, the plants quickly set seed and die. The seeds will
rest on the ground, if not eaten, awaiting the next good, soaking rain.


Plants With Wet Feet

A final common adaptation in many deserts is to grow along (or even in) a watercourse. Assemblages of plants like this are often called riparian vegetation. The obvious advantage to growing near a stream is that, as long as the stream flows, the plants will have abundant and easily accessible water. Along the margins of permanent desert streams willows (Salix) are one of the most common trees, along with a whole collection of smaller perennials, including a number of different kinds of grasses (Poaceae).

Even in cases where the stream is only intermittent, there will be some amount of riparian vegetation. Even after a stream has dried up, there is often a large amount of water just beneath the surface. Anyone who has learned desert survival techniques knows that the quickest way to find water in the desert is to dig in the bed of a dried up stream. Seen from the air these riparian zones look like rich green ribbons galloping across the landscape.


Xeriscaping With Desert Plants

Because desert plants typically require so little water, they have become popular in what is called xeriscaping. Xeriscaping is literally landscaping using xerophytic (drought adapted) plants. A wonderful combination of desert cacti and succulents, along with selected drought adapted shrubs and other flowering perennials can be used to produce a wonderfully beautiful and unique landscape. Unfortunately, the popularity of xeriscaping has posed a threat to some cactus species. Cacti grow extremely slowly, so there is a lot of incentive to simply collect a mature cactus from the wild and transplant it to the home garden. In Arizona, and somewhat in other parts of the American Southwest, there are actually cactus smugglers or plant poachers. They will target rare species of cacti as well as large cacti like saguaro (Carnegiea gigantea) and remove entire plants to sell to homeowners and landscapers. It is always best to check your sources carefully to be sure you aren't buying illegally obtained plants.

 

Paul, The Wise Gardener!