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The soils of arid lands, worldwide, possess fragile living crusts composed of cyanobacteria (blue-green algae), true algae, lichens, fungi and bacteria. In damper areas, mosses and liverworts also contribute to these crusts. Because they are composed mostly of lower plants, or cryptogams, these crusts are usually termed cryptogamic crusts. The nature of cryptogamic crusts varies with habitat. On dry desert soils, they are often inconspicuous and tend to be formed by cyanobacteria and lichens. On more moist soils, such as those beneath shrubs, a rich and conspicuous crust with liverworts and mosses may develop. Although inconspicuous, crusts may have a considerable living biomass, often exceeding 200-300 kilograms per hectare. Cryptogamic crusts play several major roles in dryland soils. First, they protect the soil surface against erosion. Cyanobacterial filaments, together with fungal filaments and moss rhizoids (root-like structures), form an intricate network that binds mineral particles of the soil. Cyanobacteria also produce a thick gelatinous sheath around their filaments. These gelatinous materials glue mineral particles of the soil together. Experiments show that crusts greatly reduce the erosion caused by raindrop impacts. The thick surface mat and anchoring rhizoids of mosses and liverworts give even greater protection, and also promote infiltration of water. Furthermore, mature crusts often develop a rough texture that slows the surface flow of water, puddling it and further encouraging filtration. Second, cyanobacteria fix nitrogen from the air and increase nitrogen input to the soil. Over the year, fixation stores up to 100 or more kilograms per hectare. Since the annual input of nitrogen in rain is usually less than 4 kilograms per hectare in desert ecosystems, crustal fixation is the major source of new soil nitrogen. Third, the crustal organisms add organic matter to the soil, increasing its capacity to hold mineral cations (positively charged ions). Organic compounds include the gelatinous materials secreted by cyanobactena and the remains of tissues of other organisms. Improved nutrient retention increases fertility of the soil for higher plants. Cryptogamic crusts are very sensitive to disruption by hoofed animals, motor vehicles and human trampling. Even light disturbance markedly reduces nitrogen fixation. In Utah, Arches National Park scientist Jane Belnap found that a single footprint from a deep-tread hiking boot reduced fixation by 32-92 percent. Single passages of off-road vehicles or mountain bikes had similar effects. Crvptogamic crusts have been damaged over vast portions of the West by livestock grazing, off-road vehicle activity and trampling by humans. Much of this damage occurred before ecologists were aware of the significance, or even presence, of these crusts. The massive erosion that accompanied early ranching in the West was probably in part due to the destruction of soil crusts. Damage by livestock continues in many areas. The enormous increases in ORV and mountain bike activity, along with foot traffic, are more recent causes of damage to soil crusts on public lands. Other threats exist as well. Fire, for example, is now more frequent in many Western Deserts because of the invasion of Eurasian annuals, which create a continuous ground layer of fuel. Fire kills many of the crustal lichens, mosses and liverworts. Once disturbed, crvptogamic crusts recover very slowly. In Arches National Park, Belnap found that full recovery of the crusts takes decades to centuries. Your help is needed to protect these fragile and valuable crusts. When you visit our sensitive arid lands, park your vehicles and walk only on established trails. Don’t be a crust-buster!
George Cox, author of textbooks on ecology and conservation biology, is a Professor Emeritus of Biology at San Diego State University. He now lives in Santa Fe, serves as a research consultant and volunteers for the SFBG. Visit www.santafebotanicalgarden.org to learn more about how Santa Fe Botanical Garden |
