Deserts

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Deserts

Desert ecosystems are characterized by an extremely arid, arid, or semi-arid climate, low relative humidity, high air and soil temperatures, strong winds, high solar radiation, low precipitation levels, extended drought periods, soils low in organic matter, low net primary productivity, and a spatially patchy distribution of vegetation and soil resources. In them, water is the predominant controlling factor for most biological processes; precipitation is highly variable and occurs as infrequent and discrete events throughout the year; and precipitation events are highly unpredictable in both space and time. Desert ecosystems may be classified into three groups based on annual precipitation: extremely arid (less than 60 millimeters), arid (60 to 250 millimeters), and semiarid (250 to 500 millimeters). The plant communities of arid lands expand and contract in accordance with alternating wet and dry periods as well as with anthropogenic activities that contribute to desertification (also known as land degradation). While arid ecosystems occur on all continents in both hot and cold environments, this article will not focus on polar deserts.

Distribution of Deserts Worldwide

Earth's major deserts lie within the tropics of Cancer and Capricorn where stable, high atmospheric pressure creates an arid climate at or near latitudes 30°N and 30°S. Deserts are generally located in the interior of large continents. Continental deserts are separated from ocean moisture by large distances or topographic barriers, such as large mountain ranges, which create a rainshadow. Deserts may also be situated on the west coast of large continents adjacent to cold ocean currents, which draw moisture away from the land. Subtropical deserts, such as the Mojave Desert of California, lie within the latitudes of 30°N and 30°S. Cool coastal deserts, including the Peruvian Atacama Desert, occur where cold offshore currents generate high atmospheric pressure and large masses of dry air, which create arid conditions upon their descent. Rainshadow deserts, including the Great Basin Desert in the United States or the Gobi Desert in Mongolia, occur where a topographical barrier such as a mountain range interrupts the flow of moist oceanic air. As moisture-laden air masses travel inland, they are deflected upward on the windward side of a mountain range, lose their moisture, and descend as dry air masses on the leeward side of the mountains. Continental interior deserts, such as the Great Sandy Desert in Australia, occur far from marine moisture.

Plants in the Desert Environment

In order to understand the ways in which plants have adapted to arid lands, it is essential to consider the physical environment. Of all the abiotic constraints imposed on plant activity—high air temperatures, extremely high soil temperatures, high winds, intense solar radiation, and limited moisture—high temperatures and limited water are the two factors that severely limit plant growth. Summer air temperatures in the Sonoran Desert in Arizona may reach 40°C during the day but drop to 15°C at night. Soil temperatures may reach 80°C or higher. High temperatures generally are accompanied by strong winds in coastal deserts, such as the Atacama in South America and the Namib in Africa, as well as in continental deserts, including the Chihuahuan and Sonoran in the United States. As well as producing spectacular dust storms and dust devils (small whirlwinds containing sand or dust), wind also abrades and desiccates desert plants.

Water is the single-most limiting factor to the growth and productivity of desert vegetation. The highly sporadic nature of desert rainfall creates a pulse-reserve system of water and nutrient availability that influences many biological processes, especially plant productivity. In the Chihuahuan Desert of New Mexico, gentle winter rainfall penetrates deep into the soil profile and provides most of the moisture for the growth of perennial shrubs, such as creosote bush and mesquite. In contrast, the high-intensity, brief summer thunderstorms provide minimal water for plant growth because most of the water runs off of the soil surface. Many plant species take advantage of rainfall immediately and grow rapidly following precipitation events, then slow their growth when soils dry and moisture once again becomes limiting.

Second only to moisture, the availability of soil nutrients, primarily nitrogen and phosphorus, limits plant productivity in deserts. Nitrogen is the key limiting nutrient in North American deserts, phosphorus is most limiting in Australian deserts, while nitrogen, phosphorus, and potassium are limiting in sand dune communities in Africa's Namib Desert. Soil nutrients and organic matter tend to be concentrated in the upper 2 to 5 centimeters of soil with the greatest amounts underneath the canopies of individual desert shrubs in "islands of fertility." These resource islands harbor greater concentrations of water, soil nutrients, and microorganisms than adjacent soils.

Certain plant species, such as creosote bush, are often referred to as nurse plants. Nurse plants effectively reduce high-incident solar radiation and high temperatures under their canopies and create ideal sites for seed germination and seedling growth. The concentration of limiting resources in islands of fertility or under nurse plants generates a spatially patchy distribution of vegetation across the desert. Competition for water maintains this spacing of plants. While this phenomenon has been most studied in U.S. deserts, it occurs in arid lands worldwide.

Desert Soils

Hot deserts exhibit generally similar soil types. Immature and alkaline with weakly developed soil horizons , desert soils are dry most of the year, and poor in soil organic matter, nitrogen, and phosphorus, but are rich in inorganic ions, carbonate, and gypsum. The main soil orders of hot deserts are Entisols, soils without well-defined layers that are formed from recently exposed rock, and Aridisols. Aridisols, exclusive to arid regions, contain two dominant suborders: Orthids and Argids. Orthids are young calcareous and gypsipherous soils with a caliche (or calcium carbonate hardpan) within 1 meter of the soil surface. The thickness of the caliche layer has been correlated with the size of creosote bush shrubs in Arizona's Sonoran Desert: the thicker the layer, the smaller the shrubs. Argids are older soils and lack the carbonate hardpan layer, but are clay-rich and may be good agricultural soils when water is available.

Plant Adaptations to the Desert Environment

Desert plant species show various physical, physiological, and pheno-logical (timing of growth and reproduction) characters that enable them to survive and grow in arid, nutrient-limited environments. Some plants, such as summer and winter desert ephemerals , restrict all growth and flowering to periods when water is available. They are able to withstand droughts and high water stress because their underground rhizomes or bulbs remain dormant during the dry season. In extreme droughts, desert ephemerals may remain completely dormant, eliminate reproduction, or limit growth to the vegetative phase. Other species, such as the California poppy and other desert annuals, complete their entire life cycle during the rainy season. Their long-lived seeds germinate only under suitable environmental conditions. As a result, they respond to the pulse-reserve system of resource availability, showing high rates of primary production in favorable years and minimal, or no production, in drought years. Ephemerals and annuals, while showy, produce minimal biomass .

In deserts worldwide, perennial shrubs and subshrubs, such as the creosote bush and jojoba, produce most of the desert plant biomass. These species limit water loss and reduce heat loads at the leaf surface by limiting the surface area to many small single, dissected, or compound leaves covered with a waxy cuticle or leaf hairs. Most shrubs have canopies with a compact globe or inverted cone shape. This morphology allows water to funnel directly to the plant roots and reduces the amount of surface area that is exposed to sunlight. Perennials have a large root-to-shoot ratio, and most roots are distributed in the soil in one of two ways. The roots may be confined to the upper meter of the soil profile and fan out horizontally from the base of the shrub, enabling shrubs access to even the slightest rainfall. Alternatively, the roots may extend deep into the soil profile—up to 12 meters with mesquite—and allow plants to obtain water that is stored at these depths. As with other desert plants, perennials may also limit or suppress flowering and fruiting in years of extreme drought.

Perennials are able to remain metabolically active at very low soil- and plant-water potentials, high internal water deficits, and high temperatures. They have sensitive regulation of leaf stomata as a function of internal and external conditions, including water stress, temperature, atmospheric humidity, and light intensity. Most shrub species acquire carbon throughout the C₃ photosynthetic pathway, despite the fact that the alternative C₄ pathway is thought to increase the amount of carbon gain per unit of water used (water-use efficiency [WUE]). The only desert perennials that have the C₄ pathway are the halophytic (salt-tolerant) species, such as tamarisk, short-lived summer active perennials, and most grasses.

Cacti, common to deserts, show unique adaptations to the desert environment. They have shallow, horizontally extended root systems, an upright, ribbed trunk that reduces the midday heat and solar radiation load and water storage within their trunks. Saguaro cacti, located near Tucson, Arizona, expand and contract like an accordion depending on the moisture conditions. In wet years the cacti are plump and green, but in dry years they are slim and yellow-green in color. Because cacti lack typical broad leaves, the overall green coloring derives from the photosynthetic trunk. Over evolutionary time, cactus "leaves" have been reduced to hairlike spines that reflect solar radiation or spikelike spines that protect the plant from herbivores . Other noncactus species, such as ocotillo and the boojum trees native to Baja California, produce photosynthetically active leaves only in wet years and limit photosynthesis to the stems when drought prevails. Cacti and other succulent species obtain carbon through the crassulacean acid metabolism (CAM) photosynthetic pathway. CAM photosynthesis allows the cacti to open their stomata only at night in order to reduce water loss.

see also Cacti; Desertification; Photosysthesis, Carbon Fixation and.

Anne Fernald Cross

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