GEOGRAPHY 368

Human Impact on Biological Soil Crusts

Desert Southwest, USA

Simenson, David Lloyd

10/2/2012

Abstract

Tourists visiting national parks in the southwest region of the United States will find an increasing number of signs warning about stepping off the path. Rightfully so: each step causes damage to that environment which takes decades, even centuries to repair. Not well known by visiting tourists, biological soil crusts are an important life source in arid regions. This paper will provide a brief analysis of the major functions of these organisms, and focus on how our interactions with them will shape the landscapes of tomorrow.

Introduction

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While visiting the southwest you may look out into a sun beaten, hot desert landscape and

notice nothing but a mirage against the horizon. Focus out and you may be delighted to find the

seemingly unlivable desert is teeming with life. The desert’s floors of the United States are home

to a biotic blanket of organisms often referred to as biological soil crusts. Associated national

parks are decorated with signs warning you of these crusts and effects of stepping off the trails. It

is important to respect the limitations of the trails. Although not immediately obvious, these

organisms have many duties pertaining to the wellbeing and succession of the desert ecosystem.

Since this layers discovery in the 1950s researchers have used words such as cryptobiotic,

cryptogamic, microfloral, and organogenic, to describe it. Whichever term referred to, it is

describing a mutual interaction between cyanobacteria, green alga, microfungi, lichens and

mosses (Belnap 2003). Together they create a delicate layer on top of the desert soils. Though

research into their biology is relatively recent, its organisms (cyanobacteria), have been found in

rocks 1.2 billion years old (Horodyski and Knauth 1994).

Just as algae may be found all throughout history, biologic soil crust can be found all

over the world. They prefer dry climates and are especially abundant in hot, arid regions, such as

Death Valley. Varied with the given dryness in a climate, cryptogamic soils are forced to grow in

an extremely short growing season. Not only does this make growth difficult, but the dormant

stage they assume without moisture also leaves them in a very fragile state.

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The physical structure they adopt is

influenced greatly by the climate,

disturbance regimes and ratio of organism.

Generally, younger or recently disturbed

crusts will blend in with soil color as

cyanobacteria start to grow. Assuming the

climate is moist enough for such, mosses

and lichens will additionally grow and start

to give the crust color and texture as depicted in Figure 1. If a climate receives little to no rain,

lichen and mosses will grow slowly or may not develop at all. This leaves crusts in dryer

climates smoother and less visible to human eyes.

As alluded to earlier, the soils are extremely fragile. This is due to their thin and fibrous

structure, as well as moisture deficiency in their climates (Belnap 2003). Mosses and lichens

develop to have 75% of their photosynthetic biomass on the top .3mm of crust (Belnap and

Gillette 2005). Though optimizing light input, the feature makes them susceptible to process

harming inflictions.

Primary Functions of Cryptogamic Soils

Soil crusts play an enormous role in arid ecosystems. Infrequent rainfall and intense solar

radiation can rob unprotected soils of nutrients and water, which is in short supply to begin with.

Wind generated off the Southwestern desert plains in United States further increase the erosion

damage and lead to the degradation of the ecosystem. In three primary ways, biotic soil crusts

help to prevent desertification and the destruction of land.

Figure 1: Dark lumpy looking soils indicate moss and lichen development. Light colors indicate here indicate disturbance

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The most prominent way in which cryptosoils benefit desert landscapes, is by protecting

against erosion. Organisms such as the cyanobacteria and micro fungi found in biological soil

crusts secrete mucilage around their cells (USGS 2001). When moist, the mucilage will move

down into the underlying soils and form a bond, securing the organism to the soils and loose soil

particles together. Biological soil crusts also keep structure on soils through its other biotic

components. Lichens and mosses tend to have shallow root systems that form a web and anchor

into the layers of soil below (USGS 2001). The longer these systems go undisturbed and the

maturity they are allowed to reach, determine how strong their connections can be, thereby

increasing their ability to perform described functions.

As well as increasing soil stability, these webs do a lot in terms of soil health. The

photosynthetic nature of the mosses and lichens help to contribute carbon and phosphogen to the

soil below. Lichens and moss also have the abilities to “fix” atmospheric nitrogen levels by

absorbing it from the air and making it readily available via soil for other plants. (Rosentreter et

al, 2007). Though they can only metabolize and release the nutrients when moist, the xerophytic

developments of the crusts allow them to maintain function down to levels of 5% moisture

(Belnap 2003).

When rainfall occurs in arid regions, soils crusts regulate the amount of water the soils

below receive. This also is dependent on the morphology of the crust and dominant organism.

Roughened and pinnacled surfaces such as shown in Figure 2, slow water runoff and allow water

to infiltrate the crusts surfaces. Smoother crusts contribute using water stores that allow them to

Figure 2: Different morphologies of BSCs are dependant on climate.

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take in high amounts of rainfall. They then release it at slower rate, benefiting surrounding

vascular plants (USGS 2001).

Negative Human Impacts

Now with a brief understanding of major functions of cryptobiotic soils, we can see the

importance they have within an arid ecosystem. Unfortunately, their resilience against intense

solar radiation, severe drought, and extreme temperatures, means little when faced against

human disturbance.

Human impact is the number one cause of crust destruction (Belnap 2003). With the

growth of the U.S. economy and population, land managers look wide-eyed out to the untapped

desert. However, what could result from the continued intrusion onto arid landscapes?

Currently, the most common encroachment comes from the humble tourists who may

occasionally stray from the path. The impact of the common step was studied in 2006 with an

experiment conducted by J. Belnap and D. E. Gillette. They sought to find the friction threshold

velocities of biological crusts in different stages of development before and after disturbance.

Wind tunnels were set up on a selected research site 16km south of Moab, Utah. Soils of similar

sandy composition where split into classes depending on developmental stage as represented in

text above Figure 3. The wind tunnels would measure the wind speed generated before any carry

of sediment.

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Results showed that an undisturbed Class 3 crust had 1283 times the resistance to wind

than that of bare sand, class 2 with 81 times resistance, and class 1 with 30 times resistance

respectively. This attested to the superiority of developed soils compared to undeveloped soils.

When a footstep with lugged hiking boots disturbed a well-developed Class 3 layer, following

results would show a reduction of friction threshold velocity by a staggering 73%. Low FTVs are

directly associated with sediment movement (Belnap and Gillette). A drop as such as that would

Figure 3 Shows the classes and results. (a) indicates FTVs reached undisturbed. (b) indicates FTVs reached after walked over with lugged boots. (c)

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allow average wind speeds of the area to pick up sediment off the disturbed soil and ultimately,

initiate erosion processes. This may come about from just a footstep.

According to USGS Research Ecologist and leading biological crust authority, Jayne

Belnap, the reduction of crust cover leads to reduced fertility of soil, reduced carbon/nitrogen fix,

and decreases in soil stability. The inability to stabilize loose grain soils could lead to accelerated

dust storm activity in the Southwest and go on to cripple the economies affected. Whether such

a severe consequence could result from a single step is unlikely. Unfortunately, increased

exploitation of the arid regions for commercial

and recreational reasons will only bring more

footsteps. Footsteps will give way to vehicle

trails and trails may lead to a landscape easily

destroyed, but not easily recovered.

Recovery

The recovery time for soil crusts vary

with region and what status defines recovery.

Figure 4 shows the factors influencing recovery. In

returning to a productive state, favorable

conditions can lead to redevelopment in 20

years (Belnap 2003). As Figure 5 proves, the

limited moisture in deserts like the Mojave,

make lichen reformation impossible within the

limits of our lifetime.

Figure 4: Depicts the variable affecting recovery of cryptobiotic soils

Figure 5: Shows time estimated for BSCs to recover in dry regions.

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Soil Crust Engineering

Through decades of great research, land managers are becoming increasingly aware of

the time needed for recovery and now seek strategies to accelerate the recovery of soil crusts.

Recent research (Bowker and Belnap, 2005) identified repeated correlations between

micronutrient dense soils and moss and lichen abundance, suggesting micronutrient

supplementation may be an answer. However, researchers stress that more experiments need to

be designed. They hope to find nutrients advantageous to cyanobacteria, to help provide a more

complete soil crust remedy. Additionally, designed experiments need to find any negative effects

supplementation would have on desert ecosystems (Bowker and Belnap, 2005).

Conclusion

While researchers toil with the problem of trying to ensure biological soil crust recovery,

trail steppers similar to me should do our best to try not to disrupt anything further. Soil crusts

have much to offer us in return for just leaving it alone. It is a bargain that should ensure its place

holding down the desert floor. Land management specialists who are responsible for the safety of

the land should consider these studies, and consider the impact they have upon the landscapes of

tomorrow.

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References

Belnap, J. 1991. Sensitivity of desert cryptograms to air pollutants: soil crusts and rock lichens. In: D. Mangis, J. Baron, and K. Stolte. Acid rain and air pollution in desert park areas. Technical Report NPS/NRAQD/NRTR-91/02. Tucson, AZ: National Park Service. 112-119 p

Belnap, J. and Gillette, D. A. 1998. Disturbance of Biological Soil Crusts: Impacts on Potenital Wind Erodibility of Sandy Desert Soils in Southeastern Utah.v. 8. Pg 355-362.

Belnap, J., 2003, The World at Your Feet: Desert Biological Soil Crusts: Frontiers in Ecology and the Environment, v. 1, no. 4, p. 181-189.

Bowker, M. A., Belnap, J., Davidson, D. W., and Phillips, S. L., 2005, Evidence for Micronutrient Limitation of Biological Soil Crusts: Importance to Arid-Lands Restoration: Ecological Applications, v. 15, no. 6, p. 1941-1951.

David, N. C., 1990, Trampling disturbance and recovery of cryptogamic soil crusts in Grand Canyon National Park, 1990.

Eldridge, D., 2000, Ecology and Management of Biological Soil Crusts: Recent Developments and Future Challenges: The Bryologist, v. 103, no. 4, p. 742-747.

Horodyski, RJ and Knauth, LP. 1994. Life on land in the Precambrian. Science263: p. 494-25

Leung, Y.F., and Marion, J. L., 2000, Recreation Impacts and Management in Wilderness: A State-of-Knowledge Review: USDA Forest Service Proceedings, v. 5, p. 23-48.

Rosentreter, R., M. Bowker, and J. Belnap. 2007. A Field Guide to Biological Soil Crusts of Western U.S. Drylands. U. S. Government Printing Office, Denver, Colorado.

USGS Fact Sheet FS-065-01, July, 2001.