What’s cool about the desert?

Desert Biome description, plant and animal adaptations

by Eleanor Burke, originally posted July 2011

Desert biome covers about one-fifth of the Earth’s land surface. Deserts range from very hot and dry to very cold and dry (Antarctica can be considered a desert) with dryness being the defining characteristic. Most sources define the average annual rainfall at 15-20cm or less; if more, the rain occurs seasonally or unpredictably. The largest of the world’s hot/dry deserts occur fairly symmetrically above and below the equator, at or near the subtropical latitudes, approximately 30-35˚ N or S of the equator. Both are high-pressure zones of dry descending air masses. Seasonality varies with local geography, such as the leeward “rain shadow” of a mountain range, or its specific continental placement. Seasonal rains create different vegetation schedules, as seen in the Mojave (winter, Pacific rains), the Chihuahua (summer, Gulf rains), and the Sonora (both winter and summer rains) with a much richer and more complex flora. The Sonora is considered to have the highest diversity of all the world’s deserts, with 560 plant species, 58 species of reptiles, and 261 species of birds during some parts of the year. In comparison, the Sahara has 70 species of mammal, 90 species of resident birds, and 100 species of reptiles (WWF 2001). Deserts have the lowest net primary productivity (NPP) of all biomes. Cramer et al. (1999) show the Saharan Desert NPP at less than 100gC m-2, and the Australian and N. American deserts at about 200-300gC m-2 as compared to tropical rainforests above 1200gC m-2. Townsend, et al. list deserts last, as only .0088 of earth’s total NPP.

Soils

Desert soil conditions are often saline due to high evaporation but little to no drainage in flats and basins; soils are coarse-gravelly to sandy—containing almost no organic matter, thus holding little water, and low in Nitrogen, which favors plants with N-fixing bacteria in root nodules. Many desert soils (as in the Sonora) are covered with a biotic crust, or “hardpan” of symbiotic cyanobacteria, algae, microfungi, and lichens. These protect the loosely aggregated soils from erosion by “gluing” them together with microbial mucilage, or as lichens do with anchoring structures, they contribute nutrients and organic matter to the soils. Cyanobacteria and cyanolichens fix nitrogen, often the limiting nutrient for plant growth; and the crust organisms and mucilage absorb some ten times their volume in water, releasing it slowly back into the soil after rains. The fragile crust is concentrated in the top 1/8 inch of soil, and is easily destroyed by foot treads, bike, and ORV tracks.

Plant adaptations

Plants have evolved adaptations to survive drought and to make the most of water when it appears. Some plants do an end-run around drought as seeds that sprout quickly when it rains, then reproduce and die within weeks. These seeds are often covered in chemicals to prevent germination until rain washes off the coating.  Other plants are drought-endurers or resisters: some have deep taproots to underground springs (mesquite), or root systems that spread out widely just below the surface, to capture water rapidly when it rains (creosote, bursage). The endurers continue activity even in dry periods with leaf adaptations such as waxy coatings, trichomes (hairy coverings) and spines (all three slow evaporation and discourage herbivory by plant-eating desert denizens.) Leaf orientation may self-shade or minimize sun exposure, or may cup water and funnel it to root crowns (mulga tree, yucca plant). Cacti and succulents store water in leaves, fruits, stems or roots, and swell and shrink as water becomes available and then is slowly used. Many (cactus, ironwood, e.g.) photosynthesize in stem and branches, as leaf surface area is held to a minimum. The price paid for these adaptations is slow growth. CAM or C4 photosynthesis allow plants to store CO2 in the heat of day and use it at night for sugar production, thus avoiding opening stomata in the heat of day when evaporation/transpiration would be detrimental. Plants are often regularly dispersed, for better water and nutrient sharing; some plants emit allelopathic root chemicals to deter other plants from germinating or growing too close.

Animal adaptations

Many desert animals are nocturnal, fossorial (burrowing), or rock crevice dwellers (rock hyrax.) Many have deep-set eyes (sun protection) and slender bodies, with long limbs or ears and fat storage away from their core, to aid heat loss. Some get their water from their food (kangaroo rat.) Some estivate underground until rains awake them and they surface and reproduce.

References and sources

Arizona-Sonora Desert Museum Center for Sonoran Desert Studies  http://www.desertmuseum.org/desert/sonora.php

California Academy of Sciences website http://www.calacademy.org/exhibits/africa/exhibit/sahara/

Chihuahuan Desert Blog http://aneyefortexas.wordpress.com/2011/01/31/the-amazing-creosote-bush/

Cramer, W., Kicklighter, D.W., Bondeau, A. Moore, B., Churkina, G., Nemry, B. Ruimy, A. Schloss, A.L., and the participants of the Potsdam NPP model intercomparison. (1999) Comparing global models of terrestrial net primary productivity: overview and key results. Global Change Biology 5 (suppl.1) 1-15 PDF accessed July 21, 2011 at Wiley online library http://onlinelibrary.wiley.com/doi/10.1046/j.1365-2486.1999.00009.x/full

Digital-Desert.com, Mojave Desert Gazette Website (photographs by Walter Feller) http://mojavedesert.net/plants/biological-soil-crusts/

Australian Desert website http://www.gondwananet.com/australian-desert-plants.html

Marietta College Department of Biology and Environmental Science, Biomes of the World Website. http://www.marietta.edu/~biol/biomes/desert.htm

NASA-Earth observatory website  http://earthobservatory.nasa.gov/Experiments/Biome/biodesert.php

Townsend, Colin R., Begon, M., and Harper, J.L. (2008)  Essentials of Ecology. Malden, MA: Blackwell Publishing. (NPP chart p. 360)

University of Western Ontario Biology Department website  http://instruct.uwo.ca/biology/320y/climveg.html

World Wildlife Fund © 2001  http://www.worldwildlife.org/wildworld/profiles/terrestrial/na/na1310_full.html

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