Cryptogamic Soil Crusts in the Deserts of North America

Desert systems of the world are generally character- ized by scant ground cover and extensive open areas between plants. In these open areas, soils are often ex- posed to the erosive forces of nature, particularly wind and water. Soil erosion is often extensive in deserts, as CryptogamicSoil evidenced by desert pavement, arroyos, and soil- hummocking beneath plants. However, not all areas in deserts are eroded. Of primary importance in protecting Crusts in the desert soils in many areas are small, inconspicuous plants known as cryptogams which form well-defined communities often known as cryptogam crusts, pop- Deserts of corn soil, or rain crusts (figs. 1 and 2). Cryptogams are plants such as mosses, lichens, algae, and fungi which produce spores. Cryptogams usually North America inhabit more mesic environments including forests or even streams. Only during recent years has it become Downloaded from http://online.ucpress.edu/abt/article-pdf/44/8/472/39997/4447572.pdf by guest on 02 October 2021 known that cryptogams are also important in desert re- gions. When well established and undisturbed in deserts, such plants form distinct crusts which play an important role in soil stabilization (Anderson, Harper, and Holmgren in press). Cryptogamic crusts vary according to dominant species (table 1). Algae are usually the primary com- Samuel R. Rushforth ponents of these crusts, but they are often accompanied Jack D. Brotherson by lichens and mosses (Anderson and Rushforth 1977). Studies indicate that algae are the most effective in bind- ing the soil particles and thus protecting against erosion (Anantani and Marathe 1974). Where cryptogamic crusts are well developed, the soil surface is almost al- ways highly stable. When cryptogam crusts are highly disturbed, such as under conditions of overgrazing, the soil is usually unstable and massive erosion often occurs. Other research has been done on several aspects of the biology of soil crusts and cryptogamic communities. Ecological relationships of the crusts were studied by Anderson, Harper, and Holmgren (in press). Taxonomy of the crusts has been studied by Anderson and Rush- forth (1977). The roles of crusts in nitrogen fixation has been studied by Rychert and Skujins (1974). The effects of crusts on water infiltration and sedimentation have been studied by Loope and Gifford (1972) and by Bro- .l~' therson and Rushforth (in review). Cryptogam crusts appear to be distributed in desert systems throughout the world. Preliminary studies indicate that crusts can be found in India, Pakistan, Israel and the Near East, and Australia as well as in the western United States and Mexico. We are presently investi- gating such crusts from several areas of the world and The authors teach in the Department of Botany and Range Science hope to expand these studies in the near future. at Brigham Young University, Provo, Utah 84602. Both hold the Ph.D. degree. Rushforth (left) currently teaches classes in general botany and algology while Brotherson teaches in human ecology and range management. They have been doing research together Crusts and in the Great Basin of North America for the past decade. They are Soil Fertility currently studying hot spring systems of western North American Most desert soils are lacking in organic matter and ni- deserts. Both are members of several scientific societies and have been active in programs intended to disseminate the results of scien- trogen. In addition, soil fertility is often low due to ex- tific studies to the public. cessive erosion which strips the topsoil and prevents the 472 THE AMERICAN BIOLOGY TEACHER, VOLUME 44, NO. 8, NOVEMBER 1982 TABLE1. Alphabeticallist of cryptogamspecies common in TABLE2. Factors affected by the presence of cryptogamic crustsof the desertsof westernNorth America. crustson desertsoils. Data were takenfrom a study performedby the authorsin NavajoNational Mon- ALGAE: Microcoleus vaginatus ument,Arizona. Nostoc muscorum Oscillatoria tenuis PercentChange Scytonema myochrous Factor Due to CrustPresence FUNGI: Aspergillus phoenicis Waterpenetration + 70% Chaetomiurmspecies Down slope spread -40% Penicillium jenseni Areaof spread -30% Trichoderma saturnisporum Soil movement -100% Evaporation -1,000% LICHENS: Collemaspecies Soil moisturebeneath crust + 75% Dermatocarpon hepaticum Lecanora calcarea Lecidea decipiens swelling tightly binds the soil surface particles and thus reduces erosion and evaporative water losses. MOSSES: Aloina pilifera Pterygoneuron ovatum Downloaded from http://online.ucpress.edu/abt/article-pdf/44/8/472/39997/4447572.pdf by guest on 02 October 2021 Tortula intermedia Tortula ruralis Crusts and Desert Erosion Cryptogamic crusts have a protective influence on development of a stable soil profile. Studies have shown desert soils in four major ways. First, they bind the soil that when cryptogamic crusts are extensive, soil fertility surface particles with the intertwining growth of algal is enhanced in two major ways. First, soil erosion is de- and fungal filaments. Second, the moss and lichen con- creased, thus allowing organic components to accumu- stituents of the crusts aid in stabilizing the soil by cover- late and particles of silt and clay to remain in the soil. ing the surface with vegetative bodies and penetrating This allows for the establishment of a more diverse vas- the soil surface with root-like rhizoids. Third, the irreg- cular plant community. ularities of a well-developed cryptogamic crust surface The second way in which cryptogams enhance fertil- tend to break up micro-wind patterns and thus reduce ity is by the process of nitrogen fixation. This process windborne soil movement. And fourth, with less water changes atmospheric nitrogen, which is unavailable for movement as discussed above, there is also less soil plant growth, to a form which is soluble in the soil solu- movement. tion and usable by plants. This process occurs in several We have been studying cryptogamic communities in of the blue-green algae and lichen species which com- the deserts of western North America for the past dec- prise the cryptogamic crust communities. In some cryp- ade. During this time, we have come to believe that togam communities the amount of nitrogen made avail- these communities are among the most important in our able is quite large and significant. In addition, as the western deserts. Wholesale loss of desert habitat is pos- cryptogam communities grow and die, the remains of sible with the destruction of cryptogamic crusts. One of these plants decay and aid in the building of soil fertili- the best examples we have seen of this is in northeastern ty. Arizona in the vicinity of Navajo National Monument. Three conditions of the cryptogamic crust communities are apparent in this area. In the Betatakin Canyon seg- Crusts and Desert Water Relations ment of the Monument, the crusts have been protected from grazing for several decades and are in excellent Well-developed cryptogamic crusts influence the condition. Outside the Monument boundary, two levels water relations of desert soils in several ways. First, they of impact from grazing are apparent on the cryptogams. decrease water runoff from the soil surface (table 2). Medium-to-light grazing levels have impacted the lichen This occurs due to irregularities on the surface of the and moss components of the crusts most significantly, cryptogamic crusts which causes water to pond, allow- but have not destroyed the algal components. In such ing more time for water penetration and thus decreasing areas, the algae are still somewhat effective in protecting runoff (fig. 3). This is also responsible for the second ob- against erosion. However, in areas of severe grazing, all served influence of crusts on water relations-that the components of the crust communities have been de- depth of water penetration is increased significantly stroyed. Where this has happened, catastrophic erosion (table 2). Third, our studies have shown that the crusts has occurred within a matter of a few years. Such ero- seal the surface of the soil by wetting the sheaths of the sion has occurred both from wind, to form extensive algal filaments. This causes the filaments to swell and dunes and blowout areas (fig. 4) and from water, to form a mucilaginous sheath at the soil surface. This form large and extensive gulley networks (figs. 5 and 6). CRYPTOGAMIC SOIL 473 Downloaded from http://online.ucpress.edu/abt/article-pdf/44/8/472/39997/4447572.pdf by guest on 02 October 2021 474 THE AMERICAN BIOLOGY TEACHER, VOLUME 44, NO. 8, NOVEMBER 1982 Figures 1-6. 1) Overview of a desert soil with a well-established cryptogamic crust community. The crust in this figure is composed largely of algae, mosses, and a few lichens. 2) Closeup of a cryptogam crust showing the three-dimensional structure of the soil when crusted. This is primarily an algal, moss, and lichen crust. 3) This crust shows ponding of water in the areas between the crusted soil hummocks. Such ponding allows the water to stay in place long enough to run into the soil rather than off the soil. This in turn increases water available to other desert plants and protects against water-caused soil erosion. 4) Area of soil blowout due to severe overgrazing. This photograph was taken near Inscription House, Navajo National Monument, Arizona, in an area of heavy sheep grazing. The cryptogam cover was completely removed from this area which allowed severe wind erosion to occur. 5) Arroyo near Inscription House, Navajo National Monument, Arizona. This example of severe water-caused soil erosion occurred in an area of cryptogam removal near that in figure 4. This arroyo was formed in about four years and is enlarging each year. 6) Closeup of the eroded gulley system shown in figure 5. Such extensive erosion may occur when the cryptogam crusts are removed due to overgrazing or other disturbance. The entire topsoil of an area may be removed by wind and water once cryptogamic cover is removed.

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