Steve Warren has spent much of his career looking at the ground in arid areas of the world. A disturbance ecologist with the U.S. Forest Service Rocky Mountain Research Station, Steve has devoted over 35 years to closely examining something most people would trample over without ever noticing—small ecosystems on the soil surface in arid areas known as “biological soil crusts.”
Biological soil crusts are found in every desert in the world, including the driest desert on Earth—the Atacama in Chile—where the rainfall can be less than 1 millimeter per year. Although soil crusts are variable in composition, they generally develop when microscopic algae and fungi, lichens, and mosses grow on and in the soil surface, entwining and adhering soil particles to form a matrix that helps to stabilize the soil.
Soil stabilization is especially important in arid areas where there are large expanses lacking plant cover. Dozens of studies have found that soil crusts reduce or completely eliminate erosion of soil by water and wind that would otherwise be widespread in these areas. Biological soil crusts also trap soil moisture, fix nitrogen from the atmosphere, and provide sheltered areas for plants to germinate and grow.
Because of the importance of soil crusts in arid ecosystems, managers need information on how to minimize impacts, the length of recovery time after a disturbance, and ways to speed up recovery through restoration practices. Warren, who is part of the Grassland, Shrubland, and Desert (GSD) Program at RMRS, and collaborators, including Larry St. Clair, a lichenologist at Brigham Young University, have spent several decades addressing these issues.
To the trained eye, biological soil crusts are easy to see. “They have a certain color and a roughness, even in the early stages of colonization after a disturbance,” says Warren. In the Great Basin and Colorado Plateau, one of the earliest soil crust species to colonize an area is the cyanobacterium Microcoleus vaginatus. This species secretes sticky substances that adhere to soil particles and act as a glue to help to stabilize arid soils. “I can take a knife and lift a piece of the crust and see the filaments of this cyanobacteria hanging down with sand particles stuck to them,” Warren explains. After the cyanobacteria colonize the soil surface, other organisms may join them. “As a general rule,” says Warren, “the successional sequence of crust development in the Great Basin would be the cyanobacteria, then lichens, and then the mosses, although the earliest colonizing organisms are there throughout.” In the hotter, drier deserts such as the Mojave, cyanobacteria and algae remain the dominant components of the crusts, giving a relatively smoother appearance. In areas that are a little wetter and exposed to freezing and thawing—like the Great Basin and Colorado Plateau—the crust may take on a rougher appearance due to frost heaving and higher proportions of lichens and mosses that follow the cyanobacteria. Biological soil crusts can cover large areas in arid ecosystems where plant cover is naturally limited by water availability. In some ecosystems, the biomass produced by soil crusts is even greater than that produced by vascular plants. The crusts add stability and erosion-resistance to soils, while performing other important ecological functions.
Cyanobacteria can capture nitrogen from the atmosphere and add it to the soil, so the nitrogen content of crust soils may be several times that of soils lacking crusts. In sandy soils, crusts reduce the rate that water trickles into soil during a rainstorm, which can allow water to accumulate around the base of shrubs and allow them to survive in areas that would otherwise be too dry. Once water has entered the soil, the crusts act as a barrier that reduces evaporation. Soil crusts can also provide places where plant seeds are sheltered from the weather extremes and have a greater chance of germinating.
Cyanobacteria (also called “blue-green algae”) are often the first soil crust organisms to colonize an area after a disturbance. These primitive bacteria are photosynthetic and can capture atmospheric nitrogen into a form that is available to vascular plants. Thin filaments of cyanobacteria secrete sticky substances that bind soil particles together. One of the most common cyanobacterium in the Colorado Plateau and Great Basin Desert (and also worldwide) is Microcoleus vaginatus, although other species may be more common in the hot deserts of the Southwestern United States. Green algae are light green to black photosynthetic organisms occurring as single cells or colonies. In biological soil crusts, they are found on or just below the soil surface.
Green algae dry out and become dormant during dry times, but they “wake up” with even small amount of moisture. Unlike their aquatic counterparts, green algae in crusts are well adapted to living and reproducing in dry desert environments.
Fungi in biological soil crusts usually occur as free-living organisms, but they can also form symbiotic relationships with plant roots. Free-living fungi function as decomposers, feeding on organic material such as leaf litter, and contribute to the cycling of nutrients in the soil crust. Like the cyanobacteria, fungal filaments secrete substances that help bind soil particles together and increase soil stability.
Bryophytes are small, non-vascular plants known as mosses and liverworts, with mosses being more common in soil crust communities.
Lichens are symbiotic systems involving a fungal partner and photosynthetic alga or cyanobacterium. The alga or cyanobacterium provide the fungal partner with food (carbohydrates), while the fungus provides a suitable environment by effectively regulating moisture and sunlight. Lichens come in a wide variety of shapes, sizes, and colors.
The integrity of these fragile microecosystems has been under assault for over a century, beginning with cattle grazing in the 19th century and continuing with mining operations. It is possible that 20 to 30 percent of rangelands have lost most of their soil crust, although, as Warren points out, “Some sites may still have crust, but it may not be in good condition.”
When biological soil crusts are disturbed, they lose the capacity to perform their basic ecological functions. Warren believes that the biggest current threat to soil crusts comes from extensive and widespread mechanical disturbance from livestock and vehicles. Estimates of recovery times range from years to millennia depending on many factors including: the severity, extent, and type of disturbance, the underlying soil type, the time of year of the disturbance, proximity to established crust that can colonize disturbed areas, and the post-disturbance rainfall patterns. In most arid ecosystems, soil crusts evolved without large herds of grazing animals, and, obviously, without vehicle traffic. The damage caused by livestock trampling is proportional to how intense the impact is, which is related to the stocking rate, distance to water sources, season of use, and amount of time on the allotment. Rainfall also matters. In general, biological soil crusts recover slowly after disturbance in the driest deserts, and during the drier season in semi-arid areas.
St. Clair has observed how the timing of precipitation affects the speed of soil crust recovery following grazing. “People predict that it can be 200–400 years before these crusts recover, but I have seen an area that was grazed and heavily impacted for 50–60 years recover in a few years when the timing of grazing removal happened to coincide with a wet period in the early 1980s,” he recalled. Vehicle traffic causes similar impacts as grazing, except that people are potentially more wide-ranging than livestock. Warren stresses the importance of keeping vehicle traffic in discrete areas—not everywhere—to protect biological soil crust communities. “It’s OK to drive in these areas, or to graze them, but only in small proportions of them,” he says.
Concentrating recreational use in arid areas to particular locations can help prevent widespread soil crust damage. It may also be helpful to restrict use of these areas during the dry season when the crusts are more vulnerable to damage.
Wildfire, according to both Warren and St. Clair, is emerging as a new, significant threat to biological soil crusts. “It used to be that a desert wouldn’t carry much of a wildfire, because the space between the shrubs simply didn’t provide enough fuel,” explains St. Clair, “Now with the invasion of non-native annual grasses you have the fuel to carry substantial and intense fire across hundreds and thousands of acres, which decimates both the native plant and biological soil crust communities.”
So what changed? Invasive annual grasses like cheatgrass have spread into well-developed biological crust communities of the Great Basin and Colorado Plateau. Cheatgrass can grow directly on top of the crusts and fill in disturbed areas between patches of crust. Ecosystems invaded by cheatgrass have higher and more continuous fuel loads, setting the stage for larger and more severe wildfires. Some research suggests that higher levels of grazing are related to both lower cover of soil crust and higher cover of annual grasses, but more research is needed to determine the potential relationship between these factors and wildfire.
How about the impact of prescribed burning on slow-growing soil crusts? Research by Warren, St. Clair, and collaborators suggests that low-severity fires pose less of a risk to biological soil crusts than high-severity fires. Low intensity, cool-season fires in sagebrush ecosystem do not cause serious soil crust damage, nor do fires in younger and less-dense juniper stands. Fire usually cannot burn the soil surface in spaces between shrubs, so crusts in unburned areas remain mostly intact and can eventually recolonize the burned areas. However, more severe fires in older and denser juniper stands can burn soil crust and reduce the likelihood of post-fire recovery.
Given the long recovery time after disturbances, researchers have looked into the viability of restoring soil crust artificially. One method attempted by Warren and St. Clair earlier in their careers was a crust “transplant.” The researchers harvested biological soil crust from one place, made it into a slurry, and applied this slurry to damaged sites. According to St. Clair, “We had pretty good success with that, but it’s like ‘robbing Peter to pay Paul’, and not really viable for large-scale applications.”
Other methods have focused on reintroducing the cyanobacterial component of the crust. An approach used by Warren, St. Clair, and others involved laboratory-grown cyanobacteria pellets that could be applied to the soil surface, with the idea that cyanobacteria would establish in place and start to stabilize the soil. Unfortunately such efforts were not successful. Cyanobacteria had poor survivorship in the pellets, and UV radiation hitting the soil surface might have killed cyanobacteria in the field.
Given the high cost and high failure rate of artificial crust restoration projects, Warren and St. Clair have turned their attention to passive restoration. “The idea is that we don’t need to apply crust organisms to the soil because they and their propagules are blowing around in the air and atmosphere,” Warren points out. The premise of passive restoration is that crust organisms land on the soil surface and reestablish themselves naturally when there is enough moisture and time free from disturbance.
Currently, Warren and St. Clair are working on a review of this topic, where they explain that most components of biological soil crusts have in fact been detected in the atmosphere. Dust storms might be one of the primary means that soil crust components hitchhike over large arid areas, while on a smaller scale, “dust devils” can lift dust into the atmosphere and transport it several miles away.
Whether soil crust organisms are applied artificially or colonize from the air, the key to recovery is that the area receives adequate moisture to hasten the process. Warren explains, “If you have a large area that is badly disturbed, you can just lay off of it. It may take 20 years to recover, depending on how much it rains, but the organisms will colonize the area from the air.”
The reality of managing biological soil crusts is that the tools are limited. Protection of existing crust is the most effective method, but whether or not this requires the complete removal of grazing and recreation is unclear given the many variables that factor into how quickly an area recovers.
Warren has some simple recommendations for managers looking to minimize impacts to soil crusts or to enhance their recovery from physical disturbance: “First, limit disturbances to discrete areas, and in areas being impacted, let the area recover by removing the disturbance agent. Then, know that the speed of recovery will depend largely on how much precipitation falls.”
The scientific understanding of biological soil crusts has come a long way in the past few decades, and so has awareness in the management community. According-to St. Clair, “Some time ago, many of federal and state land management agencies in the West were not aware of biological soil crusts at all. They have really come onto the managers’ radar in the past ten or fifteen years.”
St. Clair thinks one of the most important ways to protect soil crusts is awareness and advocacy within land management agencies. He explains, “We need to continue to communicate to the land managers and people with ‘boots on the ground’ that these crusts are there and that they are ecologically significant, and to have them act as advocates for and protectors of these areas.”
Warren, S.D.; St. Clair, L.L.; Johansen, J.R.; [et al.]. 2015. Biological soil crust response to late season prescribed fire in a Great Basin juniper woodland. Rangeland Ecology and Management. 68(3): 241–247
Warren, S.D. 2014. Role of biological soil crusts in desert hydrology and geomorphology: Implications for military training operations. Reviews in Engineering Geology. 22: 177–186.
Belnap, J.; Warren, S.D. 2002. Patton’s tank tracks in the Mojave Desert, USA: An ecological legacy. Arid Land Research and Management. 16: 245–258.
Warren, S.D.; Eldridge, D.J. 2001. Biological soil crusts and livestock in arid ecosystems: Are they compatible? In: Belnap, J.; O.L. Lange, O.L., eds. Biological soil crusts: Structure, function, and management. Berlin, Germany: Springer-Verlag: 403–417.
Belnap, J.; Kaltenecker, J.H.; Rosentreter, R.; [et al.]. 2001. Biological soil crusts: Ecology and management. Technical Reference 1730-2. Denver, CO: U. S. Department of the Interior, Bureau of Land Management. 118 p.
STEVE WARREN is a Disturbance Ecologist with the Shrub Sciences Laboratory of the USDA Forest Service Rocky Mountain Research Station in Provo, Utah. He earned his MSc in Wildlife and Range Resources from Brigham Young University and his PhD in Watershed Management from Texas A&M University. Steve’s main research interests include the ecology and management of biological soil crusts, disturbance-dependent species, the role of disturbance in ecosystem functioning and biodiversity, and remote sensing applications to biodiversity.
LARRY ST. CLAIR is a Professor (recently retired) in the College of Life Sciences at Brigham Young University and the Director and Curator of the Herbarium of Non-Vascular Cryptogams in the M.L. Bean Life Science Museum. Larry earned his MSc in Botany from Brigham Young University and his PhD in Botany from the University of Colorado. His main research interests are biomonitoring of air quality using lichens and characterization and restoration of biological soil crusts.