Blackfoot Daisy (Melampodium leucantham)
The Silver Creek area of Sevilleta National Wildlife Refuge
So, what makes a desert? Lack of water, right? But why exactly do deserts lack water? There are a few possible explanations. Firstly, the simple fact of being far from the ocean, which is where most of the world’s atmospheric moisture originates, can cause aridity. As humid air moves inland, precipitation occurs, and the further inland that air goes, and the more precipitation events that stem from it, the less available moisture there is for those inland, landlocked areas.
Apache Plume (Fallugia paradoxa), with its feathery styles swaying like lavender tinsel in the breeze
Another cause of desertification (which can result in deserts actually being on the coast, right next to the ocean) can be the presence of cold water just offshore. This is explained by the phenomena of condensation and evaporation, and can be attributed simplistically to the notion that warm air can hold more moisture than cold air (holding isn’t actually the correct way to describe this, though it is commonly stated as such; it really has to do with temperature affecting the speed of the water molecules and influencing their tendency toward a gaseous or liquid state; see http://www.usatoday.com/weather/tg/wevapcon/wevapcon.htm). Water evaporates in the middle of the ocean, raising the moisture content of the air above it. As this humid, warmair moves toward land, it passes over the relatively cold coastal seas, where much of it condenses as rain or fog. By the time this air mass makes land, it has lost much of its original moisture, and as it warms over land and its relativehumidity decreases even further (because warm air “holds” more water), the chance of precipitation is slim, and voila, you’ve got an arid coastline.
The Sierra Ponce cliffs rising 1,500 feet above the Rio Grande in Big Bend National Park
Another “desert cause” involving condensation is the rain shadow effect, which you’ve probably heard of (of course, you could have very well heard of, and understood better than I, all of these mechanisms I’ve so long-windedly described to you). As warm, humid air moves inland from the ocean, it hits a mountain range, forcing it to rise, cool, and ultimately dispend much of its moisture as precipitation along the range’s sea-side slopes. Once our air mass clears the peaks and begins to descend, it warms, lowering its relative humidity (as in the previous example) and making for little chance of rain in the forecast for whatever unfortunate souls reside inland. It is this mechanism which causes the aridity of the Chihuahuan Desert, which is sandwiched between the Sierra Madre Oriental range to the east (blocking air from the Gulf of Mexico) and the Sierra Madre Occidental range to the west (blocking air from the Pacific).
Evapotranspiration rates are also very important in determining desert aridity, but I won’t go into that. No, really, please, you don’t have to thank me. Basically, it gauges the water lost from a landscape based on a combination of surface evaporation and plant transpiration. As you could probably guess, these rates are VERY HIGH IN DESERTS.
In mid-September we traveled to the Sevilleta National Wildlife Refuge, about three hours north of Las Cruces, where we were put up in very posh, new research housing. Compared to our usual accommodations of rocky desert ground, with lechuguilla, prickly pear and dog cholla threatening impalement on all sides, this was a real treat, indeed! (To be completely honest, I do sleep better outside, but having a kitchen and shower was extremely convenient.)
Greeting us when we first arrived at Sevilleta, this Blister Beetle (Genus Cysteodemus) is a true desert jewel
The Juniper grasslands of Sevilleta underneath the dramatic cover of a building thunderstorm
One Sevilleta spring emerged in this spectacular limestone canyon
The Refuge system was started by Theodore Roosevelt in 1903 and is tasked with protecting the nation’s fish, wildlife and plants. As told to me by a Refuge employee: whereas the National Parks system is, “people and recreation first, natural resources second,” a Refuge’s mission is, “natural resources first, people second.” Of course, with the ever needling fingers of politics and bureaucracy this goal is not always realized, but the principle of the idea is a good one and, I like to believe, often realized. Many Refuges are open to limited recreation such as hiking, hunting and fishing, but Sevilleta is maintained mainly as a research and conservation reserve, with little public access. Thus, it was especially exciting to visit!
Bordered Plant Bug nymphs (Largus sp.) feeding on Horsetail Milkweed (Asclepias subverticillata)
A Giant Whipscorpion (Mastigoproctus giganteus), about 7 inches in length and incredibly creepy
While scoping out a spring at Sevilleta, I crouched down to pass under a dead tree limb and almost stepped on the rattle of my first rattlesnake!
I froze (well, I did have to get a picture…), he (she?) turned to look at me for what seemed like about 60 seconds but was probably 5, and then slithered under an evergreen sumac. Later I identified it as a Mojave Rattlesnake (Crotalus scutulatus), which has the distinction of being one the most venomous snakes in the New World. While most rattlesnakes carry a hemotoxic (tissue-destroying) venom, one variety of Mojave possesses a strong neurotoxic venom that can cause severe paralysis and nervous system damage. I’m not sure if this individual happened to be of that variety, and I’m glad I didn’t find out. I carefully sidestepped the sumac bush and continued along the brook, and promptly came across my second rattlesnake 10 feet downstream!
This one was completely unperturbed by our sampling and did not move a centimeter for the entire duration of our workday.
The next tour took me once again to Big Bend National Park, where I got my first taste of the Chisos Mountains.
Sun rising over the Chisos
Scarlet Bouvardia (Bouvardia ternifolia) paints brilliant splashes of red along the paths of the Chisos, and is a hummingbird favorite
Trans-Pecos Spiderwort (Tradescantia brevifolia). I did not place the feather there.
A Christmas Tree cholla (Cylindropuntia sp.) in bloom
Cacti such as the cholla above possess a unique metabolic pathway called Crassulacean acid metabolism (CAM) that is specially adapted for arid climates. Here’s how it works: plants have small openings / pores in their leaves called stomata (singular: stoma), which they open and close as needed to assimilate and release gases like carbon dioxide. However, when a plant opens its stomata to suck in some CO2, it loses water through transpiration. This is, of course, a problem in the desert, but CAM plants have a workaround. When water is scarce, they open their stomata only at night, when transpiration rates are much lower. But obviously, this means that no sunlight is available for photosynthesis. So what CAM plants do is to take the assimilated carbon dioxide and combine it with an enzyme called PEP carboxylase, forming malic acid. These four-carbon molecules are then stored in the plant’s vacuoles (plant storage closets), which in succulent plants like cacti are especially large and able to dilute the potentially damaging accumulation of acids. The next day, with the stomata safely closed, the plant “disassembles” the four-carbon molecules back into their constituent parts and uses the resulting carbon dioxide to proceed with photosynthesis.
Chocolate Scented Daisy (Berlandiera lyrata)
A Metallic Borer Beetle (Acmaeodera sp.). The “Jewel Beetles” of this family (Buprestidae) have been used historically in jewelry making and Victorian art. Thanks to the folks over at whatsthatbug.com for their help in identifying the insect species in this post.
Creosote Bush (Larrea tridentata) is a characteristic shurb of the Chihuahuan Desert and one of its most remarkably well-adapted species. Most plants cannot survive dehydration to less than 75 percent water content; Larrea can go below 50 percent. Like many other desert species, it has very small leaves that allow the plant to stay cooler (due to a smaller boundary layer) and reduce transpiration rates. The leaves are also covered with a waxy cuticle that protects against water loss, and are partially drought-deciduous, meaning that as summer approaches Larrea can drop some of its leaves if water resources are too scarce (less total leaf area = less water loss) . As you can see in the pictures above and below, it covers large expanses of desert, with the individual plants spaced apart so evenly it almost looks like they were planted. There are a few theories to explain this, one being that Larrea’s incredible efficiency in water uptake simply prevents other seeds from germinating in the soil around it. The regular spacing could also be caused by allelopathy, or the plant’s ability to restrict the growth of other organisms in the micro-environment immediately surrounding it through chemical means.
Next post will be mountainous — up in the Chisos of Big Bend and the Mogollon Mountains of the Gila Wilderness.