Biological Soil Crusts:

Ecology & Management

BIOLOGICAL SOIL CRUSTS:

ECOLOGY AND MANAGEMENT

Technical Reference 1730-2

2001

U.S. Department of the Interior

Bureau of Land Management U.S. Geological Survey
Biological Soil Crusts:

Ecology & Management

This publication was jointly funded by the USDI, BLM, and USGS Forest and

Rangeland Ecosystem Science Center. Though this document was produced through

an interagency effort, the following BLM numbers have been assigned

for tracking and administrative purposes:

Technical Reference 1730-2

BLM/ID/ST-01/001+1730
Biological Soil Crusts:

Ecology & Management

Biological Soil Crusts:

Ecology and Management

Authors:

Jayne Belnap Julie Hilty Kaltenecker

USDI Geological Survey Boise State University/USDI Bureau of Land

Forest and Rangeland Ecosystem Science Center Management

Moab, Utah Idaho State Office, BLM

Boise, Idaho

Roger Rosentreter John Williams

USDI Bureau of Land Management USDA Agricultural Research Service

Idaho State Office Columbia Plateau Conservation Research Center

Boise, Idaho Pendleton, Oregon

Steve Leonard David Eldridge

USDI Bureau of Land Management Department of Land and Water Conservation

National Riparian Service Team New South Wales

Prineville, Oregon Australia

Illustrated by Meggan Laxalt

Edited by Pam Peterson

Produced By

United States Department of the Interior

Bureau of Land Management

Printed Materials Distribution Center

 BC-650-B

P.O. Box 25047

Denver, Colorado 80225-0047

Technical Reference 1730-2

2001
Biological Soil Crusts:

Ecology & Management

ACKNOWLEDGMENTS

We gratefully acknowledge the following individuals for their contributions to the publication of

this text:

Ann DeBolt, USDI Bureau of Land Management

Joe Duft, retired USDI Bureau of Land Management

Russell Krapf, USDI Bureau of Land Management

Otto Lange, Botanical Institute, University of Wuerzburg, Wuerzburg, Germany

Penny Luehring, USDA Forest Service

Cal McCluskey, USDI Bureau of Land Management

Bruce McCune, Oregon State University

Mike Pellant, USDI Bureau of Land Management

Sue Phillips, USDI Geological Survey

Terry Rich, USDI Bureau of Land Management

Melinda Ritacco, USDI Bureau of Land Management

Shaney Rockefeller, USDI Bureau of Land Management

Larry St. Clair, Brigham Young University

Michelle Schmid, USDI Geological Survey

Marcia Wicklow-Howard, Boise State University

Bill Ypsilantis, USDI Bureau of Land Management
Biological Soil Crusts:

Ecology & Management

TABLE OF CONTENTS

1. Introduction to Biological Soil Crusts ................................................................ 1

1.1 Biological Soil Crust Components ............................................................... 3

1.2 Microstructure ............................................................................................. 5

1.3 Morphological Groups ................................................................................. 6

1.4 Differentiating Types of Biological Soil Crusts in the Field........................... 6

1.5 What Biological Soil Crusts Are Not: Physical Soil Crusts ........................... 7

2. Distribution and Factors Influencing Species Composition ............................. 11

2.1 Distribution ............................................................................................... 11

2.2 Factors Influencing Distribution ................................................................ 14

2.2.1 Elevation ........................................................................................ 14

2.2.2 Soils and Topography ..................................................................... 17

2.2.3 Disturbance ................................................................................... 19

2.2.4 Timing of Precipitation .................................................................. 21

2.2.5 Vascular Plant Community Structure ............................................. 22

2.2.6 Ecological Gradients ...................................................................... 22

2.2.7 Microhabitats................................................................................. 23

2.3 Unique Crustal Communities in North America ....................................... 23

2.3.1 Gypsum ......................................................................................... 23

2.3.2 Glades (Lithic and Shallow Soil Sites) ............................................ 24

2.3.3 Thermal Springs ............................................................................ 24

2.3.4 Playas ............................................................................................. 25

2.3.5 Alpine Sod ..................................................................................... 25

2.3.6 Arctic Tundra ................................................................................. 25

2.4 Example: Biological Soil Crusts in Sagebrush Communities....................... 26

3. Ecological Roles ................................................................................................ 29

3.1 Species Composition and Biomass ............................................................. 29

3.2 Carbon Fixation ......................................................................................... 29

3.3 Nitrogen Fixation ...................................................................................... 31

3.4 Albedo ...................................................................................................... 32

3.5 Effects on Vascular Plants ........................................................................... 33

3.5.1 Seed Germination .......................................................................... 33

3.5.2 Plant Establishment and Cover ...................................................... 33

3.5.3 Nutrient Levels in Vascular Plants .................................................. 34

3.6 Water Relations.......................................................................................... 35

3.7 Soil Stabilization ........................................................................................ 40

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Biological Soil Crusts:

Ecology & Management

NOTES 4. Response to Natural Impacts and Human Actions ........................................... 41

4.1 Evolutionary History of Natural Impacts ................................................... 41

4.1.1 Grazing Characteristics .................................................................. 41

4.1.2 Fire ............................................................................................... 42

4.2 Disturbance Effects .................................................................................... 44

4.2.1 Disturbance Severity, Size, Frequency, and Timing ......................... 44

4.2.2 Disturbance Effects on Species Composition ................................. 45

4.2.2.1 Air Pollution ................................................................... 45

4.2.2.2 Oil Spills, Insecticides, and Herbicides ............................ 47

4.2.2.3 Annual Plant Invasion..................................................... 47

4.2.2.4 Fire ................................................................................. 47

4.2.2.5 Mechanical Disturbance.................................................. 49

4.2.2.6 Burial .............................................................................. 50

4.2.3 Disturbance Effects on Nutrient Inputs and Retention .................. 50

4.2.3.1 Carbon Fixation .............................................................. 50

4.2.3.2 Nitrogen Inputs .............................................................. 52

4.2.4 Disturbance and Vascular Plants .................................................... 54

4.2.5 Disturbance and Surface Albedo .................................................... 54

4.2.6 Disturbance and Soil Hydrology .................................................... 55

4.2.7 Disturbance Relative to Wind and Water Erosion .......................... 55

4.2.7.1 Water Erosion ................................................................. 55

4.2.7.2 Wind Erosion ................................................................. 56

4.3 Factors Influencing Natural Recovery Rates ............................................... 56

4.3.1 Sequence of Species Appearance ..................................................... 58

4.3.2 Soil Texture .................................................................................... 58

4.3.3 Climate Regimes ............................................................................ 61

4.3.4 Disturbance Type, Frequency, Severity, and Size ............................. 61

4.3.5 Condition of Adjoining Substrate .................................................. 62

4.3.6 Vascular Plant Community Structure ............................................. 62

4.3.7 Nitrogen and Carbon Fixation ....................................................... 63

4.3.8 Surface Albedo ............................................................................... 63

5. Management Techniques to Maintain or Improve Existing Biological Soil Crusts ........ 65

5.1 Fire ............................................................................................................ 65

5.1.1 Prescribed Fire ............................................................................... 65

5.1.2 Post-fire Management .................................................................... 66

5.2 Livestock Grazing ...................................................................................... 67

5.3 Recreational Use Management ................................................................... 69

6. Monitoring Biological Soil Crusts .................................................................... 71

6.1 Monitoring Methods ................................................................................. 71

6.1.1 Sampling Design and Procedures ................................................... 73

6.1.2 Using Morphological Groups for Monitoring ................................ 73

6.1.3 Quadrat Methods .......................................................................... 74

6.1.4 Line-intercept Methods.................................................................. 75

ii
Biological Soil Crusts:

Ecology & Management

6.1.5 Line-point Intercept Methods ........................................................ 81 NOTES

6.1.6 The Issues of Scale ......................................................................... 81

6.1.7 Voucher Specimens ........................................................................ 81

6.1.8 Statistical Considerations ............................................................... 82

6.2 Other Monitoring Methods ....................................................................... 83

6.3 Impacts of Monitoring ............................................................................... 83

7. Internet Resources for Biological Soil Crusts.................................................... 85

Glossary ....................................................................................................................... 87

References .................................................................................................................... 89

List of Figures

Figure 1.1 Biological soil crusts of the Colorado Plateau ................................... 1

Figure 1.2 Microcoleus vaginatus, one of the predominant cyanobacteria

comprising biological soil crusts ....................................................... 4

Figure 1.3 Physical soil crusts.......................................................................... 10

Figure 2.1 Arid and semi-arid ecoregions of western North America ............... 12

Figure 2.2 Biological soil crusts of the Great Basin and Sonoran deserts .......... 13

Figure 2.3 Biological soil crust forms based on temperature characteristics of

the environment ............................................................................ 15

Figure 2.4 General ecological relationships for biological soil crusts ................ 18

Figure 2.5 Soil stability relative to texture and moisture status ........................ 20

Figure 2.6 Gelatinous and other nitrogen-fixing lichens by relative soil

temperature and calcareous influence ............................................. 23

Figure 3.1 Disturbance plots on the Colorado Plateau showing the difference

 in soil surface color between undisturbed (dark) and disturbed

(light) plots .................................................................................... 32

Figure 3.2 Sandberg bluegrass (Poa secunda) growing in well-developed

biological soil crusts in the northern Great Basin ........................... 33

Figure 3.3 Soil-water relationships with biological soil crusts .......................... 36

Figure 3.4 Biological soil crust and soil characteristics that influence

infiltration ..................................................................................... 39

Figure 3.5 The sheath of Microcoleus vaginatus contains sticky polysaccharides

that entrap soil particles and bind them together............................ 40

Figure 4.1 Comparison of community structure in ecosystems that evolved

 with (midwestern prairie) and without (desert grassland) large

 herds of grazing mammals ............................................................. 41

Figure 4.2 Natural and altered fire cycles in sagebrush communities of the

Great Basin .................................................................................... 43

Figure 4.3 Successional sequence for biological soil crusts ............................... 46

Figure 4.4 Changes in the vascular plant community structure due to

cheatgrass invasion ......................................................................... 48

iii
Biological Soil Crusts:

Ecology & Management

NOTES

Figure 4.5 Susceptibility of biological crusts to mechanical disturbance based

 on dominant morphological group ............................................... 49

Figure 4.6 Resistance of sandy loam soils to wind erosion following disturbance to a well-developed biological soil crust in four time classes ....... 57

Figure 4.7 Resistance of soil surfaces to wind erosion ...................................... 57

Figure 4.8 Generalized recovery sequence for biological soil crusts .................. 59

Figure 4.9 Biological soil crust vulnerability and recoverability based on site

stability, effective precipitation, and disturbance regime ................. 60

Figure 5.1 Soil surface disturbance associated with post-fire revegetation

projects .......................................................................................... 66

Figure 5.2 Biological crust development in a successful seeding in the

northern Great Basin and lack of development in an exotic annual

grass community ............................................................................ 67

Figure 5.3 Native grazers in the western U.S., such as pronghorn antelope

and mule deer, tend to use higher, more productive sites during

summer and move to lower elevation sites for winter forage ........... 68

Figure 5.4 Vehicular disturbance to biological soil crusts................................. 69

Figure 6.1 Quadrat frame used for point-cover estimation .............................. 75

Figure 6.2 Data form for point-intercept using a quadrat method and example

of data collection............................................................................ 76

Figure 6.3 Use of line-intercept for measuring biological crust cover along with

other community attributes (vascular plant cover, litter, bare

mineral soil, rock) .......................................................................... 78

Figure 6.4 Line-intercept data form and example of data collection ................ 79

List of Tables

Table 1.1 Morphological groups for biological soil crust components with

examples of common taxa or groups ................................................ 8

Table 2.1 Soil crust lichens endemic to North America with both wide and

limited distributions....................................................................... 14

Table 2.2 Soil, climatic, and vegetative characteristics for low and mid

elevations in temperate North American arid and semi-arid

ecoregions ...................................................................................... 16

Table 2.3 Dominant biological soil crust components and forms in North

American arid and semi-arid ecoregions ......................................... 17

Table 2.4 Soil crust lichens that are calcium carbonate indicators .................. 21

Table 2.5 Relative cover of biological soil crusts in sagebrush (Artemisia)

vegetation types ............................................................................. 27

Table 3.1 Common nitrogen-fixing lichens and cyanobacteria in cool and

hot deserts of the western U.S. ....................................................... 30

Table 4.1 Percent frequency of biological soil crust organisms on loamy and

sandy soils inside and outside the exclosure at Koonamore

Vegetation Reserve, South Australia, in May 1972 ......................... 51

iv
Biological Soil Crusts:

Ecology & Management

NOTES

Table 4.2 Reduction of nitrogenase activity for new and older disturbances

on silty soils of the Dugway Proving Ground, Utah ....................... 52

Table 4.3 Characteristics of early colonizing lichens....................................... 61

Table 6.1 Attributes of biological crust monitoring strategies......................... 72

Table 6.2 Classification of results from the slake test ..................................... 84

v
Biological Soil Crusts:

Ecology & Management

NOTES

one

Figure 1.1 Biological soil crusts of the Colorado Plateau

(right). The roughened surface is formed by a matrix of

cyanobacterial filaments that stabilizes the sandy soil. This

stabilized surface provides microsites for a diverse lichen

flora (left) in the interspaces between vascular plants that

would otherwise be bare of autotrophic life.

CHAPTER 1

INTRODUCTION TO BIOLOGICAL SOIL CRUSTS

In arid and semi-arid lands throughout the world, vegetation

cover is often sparse or absent. Nevertheless, in open spaces between

the higher plants, the soil surface is generally not bare of autotrophic

life, but covered by a community of highly specialized organisms

(Fig. 1.1). These communities are referred to as biological soil crusts,

or cryptogamic, cryptobiotic, microbiotic, or microphytic soil crusts

(Harper and Marble 1988; West 1990). Biological soil crusts are a

complex mosaic of cyanobacteria, green algae, lichens, mosses,

microfungi, and other bacteria. Cyanobacterial and microfungal

filaments weave through the top few millimeters of soil, gluing loose

particles together and forming a matrix that stabilizes and protects

soil surfaces from erosive forces (Cameron 1966; Friedmann and

Galun 1974; Friedmann and Ocampo-Paus 1976; Belnap and

Gardner 1993). These crusts occur in all hot, cool, and cold arid and

semi-arid regions. They may constitute up to 70% of the living cover

in some plant communities (Belnap 1994). However, biological soil

crusts have only recently been recognized as having a major influence

on terrestrial ecosystems.

Globally, this consortium of soil biota has many similarities in

function, structure, and composition, in spite of their unconnected
Biological Soil Crusts:

Ecology & Management

NOTES and seemingly dissimilar environments. Crusts are found in an

astonishing variety of habitats throughout the world:

in desert and semi-desert plant communities, ranging from shrub and

succulent deserts to open woodlands; in steppe formations in both

the northern and southern hemispheres; in the gaps between

evergreen shrubs and in forests in the Mediterranean-type climate;

and on open ground or between alpine or tundra vegetation. On a

small scale, biological soil crust communities are found in open types

of vegetation in temperate climatic regions; for example, they are

frequently present in (and often restricted to) areas of a few square

meters in xerothermic local steppe formations in central Europe and

in the pine barrens of the eastern United States.

In rangelands, biological soil crusts can be viewed from

functional, structural, and compositional perspectives. They function as

living mulch by retaining soil moisture and discouraging annual weed

growth. They reduce wind and water erosion, fix atmospheric nitrogen,

and contribute to soil organic matter (Eldridge and Greene 1994).

Structurally, biological crusts are a rough, uneven carpet or skin of low

stature (1 to10 cm in height). Below ground, lichen and moss rhizines,

fungal hyphae, and cyanobacterial filaments form a matrix that binds soil

particles together (Belnap 1995). Horizontally, soil crusts occupy the

nutrient-poor zones between vegetation clumps in many types of aridland vegetation. Compositionally, biological soil crusts are diverse. In

many arid and semi-arid communities there are often many more species

associated with the biological soil crust at a given site than there are

vascular plants (Rosentreter 1986; Ponzetti et al. 1998).

Rangeland managers in North America have historically used

key indicator plants for determining the ecological trend and health

of vegetation (USDA 1937; Stoddart et al. 1943). Biological soil

crusts can also be used as indicators of ecological health. In addition,

they act as indicators of abiotic factors, such as the presence of

calcareous soils. Crustal organisms read environmental factors

differently from and on separate time scales than do vascular plants

(McCune and Antos 1982). Most crustal organisms are biologically

active during the cool seasons when the soil surface is moist

(Rosentreter 1986). In contrast, vascular plants are active in spring

and summer when air temperatures are above freezing.

Unlike vascular plants, crustal organisms, particularly lichens,

are not greatly influenced by short-term climatic conditions. This

makes them ideal indicators of long-term environmental factors.

Therefore, each community component can provide information that

may complement, explain, or indicate something about a site’s

characteristics and disturbance history for rangeland management and

evaluation. Just as plants increase or decrease with livestock grazing,

many biological soil crust components are good indicators of physical

two
Biological Soil Crusts:

Ecology & Management

disturbance, such as by livestock, human foot traffic, or motorized NOTES

vehicles (Belnap 1995).

Land managers have been slow to use biological soil crusts in

rangeland evaluations. Descriptions of vegetation or “habitat types”

used by public land management agencies sometimes include

biological soil crusts (e.g., Daubenmire 1970; Hironaka et al. 1983)

but rarely present them as a dominant factor even when they are

prevalent. This is partly because of perceived difficulties with

identification (see Chapter 6). The problem of identification is

exacerbated by the small size of these organisms, which often lack

reproductive structures due to the harsh environments where they

grow. However, identification problems can be substantially reduced

by grouping organisms by function or general morphological

characteristics (see sections 1.3 and 6.1.2).

1.1 Biological Soil Crust Components

Biological soil crusts are usually composed of multiple,

unrelated organisms that occur together on the soil surface. The

various types of organisms that comprise the crust share some

interesting physiological traits. They are all capable of drying out and

temporarily suspending respiration without negative effects, unlike

vascular plants that either die or must regrow new tissue. These types

of organisms are referred to as “poikilohydric.” Most of them

equilibrate their water content with the atmospheric humidity or soil

surface moisture content. Poikilohydric organisms generally become

photosynthetically active very quickly, producing carbohydrates or

sugars minutes after wetting. However, most species still require high

levels of hydration for optimal physiological functioning. The

moisture content threshold for activity is species specific and helps

determine the distribution of the various taxa that make up the

biological crust. Many of these organisms perform under a variety of

light intensities and prefer to dry out rapidly. Because they lack a

waxy epidermis, crustal organisms also tend to leak nutrients into the

surrounding soil upon wetting and drying.

Biological soil crusts have both macro- and microscopic

components. Components that comprise these crusts are common in

desert soils throughout the world. They will be discussed throughout

this document in the context of their roles when they combine with

other organisms as part of a biological soil crust. The term “total soil

crust” will be used to refer to the combination of organisms. When

visible cover (i.e., moss and/or lichen cover) alone is being discussed

(thus excluding cyanobacteria), this will be specifically stated.

Bacteria are a diverse group of primitive, single-celled

organisms. Bacteria can be either autotrophic (i.e., they synthesize

three
Biological Soil Crusts:

Ecology & Management

NOTES carbon compounds from inorganic sources) or heterotrophic

(i.e., they utilize carbon-containing substrates, such as organic matter

in soil, for food). Some bacteria contribute to soil fertility by fixing

nitrogen. Others are important in decomposition.

Microfungi occur either as free-living organisms or in

mycorrhizal associations with plant roots. Free-living microfungi

function as decomposers. They also contribute substantially to the

living biomass in soils. Fungal filaments (hyphae) bind soil particles

together, increasing soil water-holding capacity.

Cyanobacteria (“blue-green algae”) are primitive filamentous or

single-celled bacteria that can photosynthesize and, under anaerobic

conditions, fix atmospheric nitrogen into a form that is available to

higher plants (NH

4+

). Cyanobacteria can be heterocystic (i.e., they

have special cells where nitrogen fixation takes place), or non-heterocystic (i.e., they lack these specialized cells). One of the most

common cyanobacteria worldwide is the non-hetero-cystic,

filamentous species Microcoleus vaginatus. Microcoleus can be seen

with a 10x hand lens on the edge of a broken clump of soil. Under

higher magnification, Microcoleus occurs as a cluster of filaments

surrounded by a gelatinous sheath (Fig. 1.2). Single-celled

cyanobacteria appear as small, blackish cells mixed with surface soil.

When soils contain high amounts of cyanobacteria, they often have a

slightly to highly roughened surface due to the organisms’ binding of

soil particles, coupled with processes like frost-heaving and erosion.

For nitrogen fixation to occur in non-heterocystic cyanobacteria, the

organisms need to be in an anaerobic environment, created by

layering of cyanobacterial filaments just beneath the soil surface.

Green algae are light green to black, single-celled,

photosynthetic organisms. Algae that occur on or just below the soil

surface dry out in a vegetative condition and become physiologically

functional when moistened. They do not rely on resting spores to

regrow after dry periods, as do aquatic algae that

inhabit ephemeral ponds or lakes. Therefore, they

are well adapted to living and reproducing in dry

desert environments. Algae are difficult to observe

without a microscope (100x to 400x), but sometimes

give the soil surface a green tint. Their growth period

is often linked to cool, moist weather, and they may

be difficult to detect when dry.

four

Figure 1.2 Microcoleus vaginatus, one of the predominant cyanobacteria comprising biological soil crusts.

Microcoleus exists as a cluster of filaments, surrounded by a

gelatinous sheath (seen here with soil particles attached).

The living filaments can migrate through the soil, leaving

abandoned sheath material and a stabilized soil matrix

behind. (2000x magnification)
Biological Soil Crusts:

Ecology & Management

Bryophytes are tiny non-vascular plants. This group includes NOTES

both mosses and liverworts. Having greenish leaves when moist,

mosses are generally easy to identify. They reproduce by spore

capsules that rise above the leaves. The spore capsules greatly simplify

the identification of genera and species. However, in arid

environments, mosses often lack reproductive structures and will

reproduce asexually by simple or specialized fragmentation.

Therefore, arid-land mosses are often difficult to identify in the field.

Liverworts come in two general forms: thalloid or leafy. The thalloid

form has a greenish-black thallus or flat, narrow ribbon of

dichotomously branching material. Thalloid liverworts are adnate to

the soil surface, and some can fold in half, almost disappearing from

view (they look like thin black lines on the soil surface when dry).

Leafy liverworts are rare in arid environments. They look like mosses

but are smaller, and when dry the leaves are very black . Both types of

liverworts reproduce by spores and by specialized asexual structures

called “gemmae.” The spores are often produced in a structure within

the upper surface of the liverwort and look like a black or smutty spot

on a green background. The asexual gemmae are variable in size (1 to

3 mm), but are usually green, circular structures that easily break off

the parent plant when mature.

Lichens are fungi that capture and cultivate algae or

cyanobacteria, resulting in a new morphological entity. Lichens are

typically used in ecology courses to illustrate the concept of

symbiosis, as the algae or cyanobacteria provide the fungus with

energy in the form of carbohydrates produced by photosynthesis,

while the fungus provides protection from desiccation. Lichens come

in a wide variety of shapes, sizes, and colors. They generally have an

outer fungal layer, which, if cut in cross section, reveals a layer of algal

cells. Lichens can cover the soil surface like a layer of skin or they can

be three dimensional and leaf-like in appearance. Lichens occur in a

variety of colors, including green, red, brown, white, and black.

Sexual reproduction is limited to the fungal partner. Reproductive

structures are generally round, dark-colored sessile disks on the lichen

surface. Lichens can also reproduce asexually (as the combined

organism) and have several types of specialized fragmentation

structures to facilitate this type of dispersal. Lichens that reproduce

asexually can colonize disturbed sites much more rapidly than those

species that lack this ability.

1.2 Microstructure

The dominant components of biological soil crusts are photosynthetic and therefore require sunlight. When soils are dry, the bulk

of the cyanobacterial biomass is 0.2 to 0.5 mm below the soil surface,

five
Biological Soil Crusts:

Ecology & Management

NOTES where sufficient light for net carbon gain is available but UV exposure

is reduced. Microcoleus vaginatus, which lacks UV-screening pigments,

migrates to the surface for short time periods when soils are moistened

and returns to subsurface zones as they dry (Fig. 1.2). Populations of

Scytonema and Nostoc, cyanobacteria containing UV-screening pigments,

are more commonly found directly on the soil surface.

1.3 Morphological Groups

Morphological groups consist of organisms that are similar

in shape and general appearance. The crust morphology largely

determines its ecological function relative to water infiltration,

erosion, water retention, and resistance and resiliency to disturbance

(Eldridge and Rosentreter 1999). Morphological groups also convey

an image of a particular organism. Table 1.1 outlines the major

morphological groups for biological crusts. Ecological function and

management implications relative to morphological groups will be

discussed in detail in later chapters.

1.4 Differentiating Types of Biological Soil Crusts in the Field

Biological soil crusts are usually composed of various

organisms and morphological groups, unless the crust is in an earlysuccessional stage. However, one or two morphological groups will

normally dominate the crust. The following are examples of

morphological groupings:

• Cyanobacterial crusts are dark colored or black. When moist,

the organisms may be visible as black filaments on and near

the soil surface.

• Green algal crusts are not always visible, but might appear as a

green cast on the soil surface when it is moist.

• Moss crusts are easily observed as a furry carpet with patches of

green, gold, brown, and/or black.

• Liverworts are difficult to detect and usually occur in a mosaic

with other dominant organisms. Close examination with a

hand lens will reveal tiny black ribbons that become more

obvious with moistening.

• Lichen crusts can be identified by their diversity of shapes and

colors. The lichen morphological groups form an anatomical

gradient from a low, simple morphology to taller and threedimensional growth forms.

• Crustose lichens are flat and fused to the substrate.

• Gelatinous lichens are usually black and may appear flat or

three-dimensional. They become jelly-like in texture when

six
Biological Soil Crusts:

Ecology & Management

NOTES

seven

moistened and will swell to several times their size when wet.

This is an important group to identify from a functional

perspective because they have cyanobacteria as their

phytobiont and therefore fix atmospheric nitrogen.

• Squamulose lichens occur as small individual flakes or

scales that often grow in colonies or clusters.

• Foliose lichens are leaf-like and loosely appressed to the

substrate. In dry habitats, foliose lichens inhabit the relatively

moist microsites under plant canopies and on north aspects.

• Fruticose lichens are three dimensional and are often

upright, branched, or thread-like.

1.5 What Biological Soil Crusts are Not: Physical Soil Crusts

Non-biotic soil surface crusts are a major structural feature in

many arid regions. Their properties and manner of formation have

been studied for many years, primarily because of their detrimental

effects on agricultural crops. These crusts are transient soil-surface

layers (ranging in thickness from less than 1 mm to a few cm) that are

structurally different from the material immediately beneath them.

Physical crusts reduce water infiltration and can prevent the

emergence of vascular plant seedlings (Fig. 1.3).

The most important process in the formation of non-biotic

crusts is generally raindrop impact, which breaks up soil aggregates on

unprotected surfaces. Smaller particles wash into spaces between

larger particles, clogging soil pores and reducing infiltration rates by

as much as 90%. This can occur within the first few minutes of a

rainstorm. As drying takes place, surface tension pulls soil

components together, forming a dense, strong layer. Thackett and

Pearson (1965) showed that physical crusts formed under simulated

rainfall had a dense surface layer 1 to 3 mm thick, coated with a thin

layer of well-oriented clay. The crust was underlain by a more porous

structure, and the water permeability of the underlying material was

about five times that of the surface 0 to 5 mm. Rain-formed crusts are

thicker when the raindrops are larger because these larger drops have

more energy and “blast” deeper holes, destroying the original

structure to a greater depth. In general, rain-formed crusts are less

than 5 mm thick. This layer is often harder than the rest of the soil

because compounds such as salts, lime, and silica are deposited at the

surface as water evaporates. Because large pores are absent, the crust

usually has low saturated hydraulic conductivity and limits

infiltration. This increases water runoff and soil erosion.

Soil aggregate structure is also destroyed by machinery or the

hooves of grazing animals. Trampling moist soils destroys existing soil
Biological Soil Crusts:

Ecology & Management

NOTES

eight

Table 1.1 Morphological groups for biological soil crust components with examples of common taxa or groups.

Morphological Group Description and Representative Taxa

cyanobacteria colonies are black to blue-green

and visible primarily when moist

(Microcoleus vaginatus)

algae primarily occur as single-celled

organisms; difficult to detect

(coccoids)

BRYOPHYTES:

short moss mosses <10mm in height (Bryum

spp.,Ceratodon purpureus)

 tall moss mosses >10mm in height

(Tortula ruralis)

 liverwort flat, narrow ribbon or green-black

dichotomously branching material

on the soil surface (Riccia)
Biological Soil Crusts:

Ecology & Management

NOTES

nine

Table 1.1 (continued)

Morphological Group Description and Representative Taxa

LICHENS:

 crustose lichen crust-like growth tightly attached to

the substrate (Lecanora muralis)

 gelatinous lichen blackish, jelly-like when moistened

(Collema coccophorum)

 squamulose lichen discrete flakes that are round or

ear-shaped, convex or concave, and

often have lobed margins

(Psora decipiens)

 

foliose lichen “leafy,” tending to be flattened with

definite upper and lower surfaces

(Peltigera occidentalis)

fruticose lichen three-dimensional, ropey or

branching, without definite upper

and lower surfaces (Aspicilia hispida)
Biological Soil Crusts:

Ecology & Management

NOTES

ten

Figure 1.3 Physical soil crusts. Vascular plant establishment is limited to

cracks between polygons on physically crusted areas due to an impenetrable

surface layer.

aggregates by compacting them into a comparatively impermeable

surface layer. These compacted surfaces have reduced infiltration rates

and increased surface runoff. In this sense, they function

hydrologically in a manner similar to raindrop-induced crusts.

Physical crusts may form on soil of almost any texture except

coarse sandy soils containing very low silt and clay (Lemos and Lutz

1957). Soils especially susceptible to crusting are those with low

organic matter and high silt, sodium, or calcium carbonate content.

These characteristics are all related to soils with low structural and

aggregate stability. Organic matter increases aggregate stability

(through gluing of particles and moderation of forces that reduce

aggregate stability), and places where plant residues are incorporated

generally become planes of weakness, decreasing crust strength. Silts

form strong bonds on drying, thus forming a hard crust. Silts also

have low swelling and shrinking properties, so that the crust does not

crack or disintegrate by itself.

Intensive grazing is often used to break up physical crusts.

However, this result is short-lived, as the soil surface is resealed after

the first minutes of an intense rainstorm. To effectively address a

physical soil crusting problem, livestock grazing systems must

promote greater soil aggregate stability (Thurow 1991). Therefore,

management systems that promote soil surface protection (through

plant and biological soil crust cover) and increase soil organic matter

are the only lasting solution to physical soil crust reduction

(Blackburn 1983).
Biological Soil Crusts:

Ecology & Management

NOTES

eleven

CHAPTER 2

DISTRIBUTION AND FACTORS INFLUENCING SPECIES

COMPOSITION

2.1 Distribution

Biological soil crusts in North America are diverse and are

most evident in arid and semi-arid ecoregions (Fig. 2.1, 2.2). They are

also found on shallow lithic sites and in alpine habitats throughout

the continent and in many early-successional vegetation types in

moister ecoregions. Some crustal organisms are good indicators of a

specific vegetation type or ecoregion. Others, including Microcoleus,

Nostoc, Collema, Psora decipiens, Cladonia spp., and Bryum spp. are

common to many different geographic, climatic, and vegetation

types. For instance, the same soil lichens (Collema, Placidium, Psora)

dominate crusts of both the Sonoran and Great Basin deserts, as well

as areas of South Africa and Australia, although vascular plant species

and climate vary greatly between these regions. While most algae,

lichens, and bryophytes are cosmopolitan, a few are endemic and may

be common on a local or regional level (Table 2.1). For example,

several of the squamulose lichens, such as Placidium squamulosum,

Psora decipiens, Psora cerebriformis, and Psora tuckermanii, have very

broad geographic ranges. Others, including Catepyrenium congestum

and Psora montana, are endemic to North America or have narrow

geographic ranges within the continent. Squamulose lichens have

more endemic species worldwide than any other lichen

morphological group.

Similarities in species composition may occur due to similar

environmental conditions during the active growth period for crustal

organisms. For example, alpine biological crusts actively grow in

summer under moisture and temperature conditions that are similar

to winter environmental conditions in the Sonoran Desert. Most

crust growth occurs during wet, cool periods. In most North

American deserts, this is generally late fall to early spring.

The appearance of biological crusts is variable. Physical

structure of biological crusts is very similar in the hot deserts of the

world, such as the Atacama, Sonoran, Chihuahuan, and Australian

deserts, while very different from the physical structure in cool and

cold deserts, such as the Colorado Plateau, Great Basin, and the

Arctic (Fig. 2.3). Pinnacled crusts are found in areas where crusts are

dominated by cyanobacteria, freezing temperatures are common, and

vascular plant roots are lacking. Winter freezing heaves the soil

upward, forming pinnacle-type microtopography. Subsequent
Biological Soil Crusts:

Ecology & Management

twelve

Figure 2.1 Arid and semi-arid ecoregions of western

North America. Areas shaded in black are cool deserts,

while grey-shaded areas are hot deserts.
Biological Soil Crusts:

Ecology & Management

thirteen

Figure 2.2 Biological soil crusts of the Great Basin

(top) and Sonoran (bottom) deserts. Compare the

overall appearance of these crusts with those of the

Colorado Plateau (Fig. 1.1).
Biological Soil Crusts:

Ecology & Management

NOTES Table 2.1 Soil crust lichens endemic to North America with both wide and limited distributions.

Note that there are very few endemic species.

Species Distribution Reference

Aspicilia californica California Chaparral Rosentreter 1998

Aspicilia filiformis western North America Rosentreter 1998

Aspicilia reptans North America Looman 1964

Catapyrenium congestum Great Basin Bruess and McCune 1994

Psora montana western North America Timdal 1986

Psora tuckermanii Great Basin Timdal 1986

Texosporium sancti-jacobi western North America McCune & Rosentreter 1992

fourteen

NOTES differential growth and erosion results in highly dissected pinnacles to

10 cm high (e.g., Colorado Plateau). Where freezing is common but

soils are held in place with a heavy cover of lichens, mosses, and

vascular plant roots, soil surfaces have a rolling rather than pinnacled

topography (e.g., northern Great Basin). Where freezing is rare, crusts

are flat when only cyanobacteria are present or rugose where lichens

and/or mosses occur (e.g., the Sonoran Desert; Fig. 2.2, 2.3). In nonfreezing regions, a layer of pebbles often covers cyanobacterial

portions of the crust.

On the broad scale of western North America there are several

different vegetation zones or ecoregions that contain biological soil

crusts as major components (Fig. 2.1, Table 2.2, 2.3). The ecoregions

used here are similar to those used by Bailey (1998). The most

prevalent arid soils in North America are silt-loams and are more

susceptible to physical disturbance than are clay-loams common to

many other arid regions in the world, such as the red clay-loams of

Australia.

2.2 Factors Influencing Distribution

2.2.1 Elevation

Total crust cover is inversely related to vascular plant cover,

as less plant cover results in more surface available for colonization

and growth of crustal organisms (Fig. 2.4-A). Thus, when all crust

types are combined (cyanobacterial, moss, lichen), cover is greatest at

lower elevation inland sites (less than 1,000 m) compared to midelevation sites (1,000 to 2,500 m; Hansen et al. 1999; Fig. 2.4-B).

However, relative lichen and moss cover increases with elevation and

effective precipitation until vascular plant cover precludes their

growth (Fig. 2.4-C). Crust organisms have reduced water and nutrient
Biological Soil Crusts:

Ecology & Management

NOTES

fifteen

Figure 2.3 Biological soil crust forms based on temperature characteristics of the environment.

HOT DESERTS

flat

rugose

COOL DESERTS

rolling

pinnacled
Biological Soil Crusts:

Ecology & Management

NOTES

sixteen

Table 2.2 Soil, climatic, and vegetative characteristics for low and mid elevations in temperate North American arid and semi-arid ecoregions. Adapted from Bailey (1995).

Ecoregion Dominant Soil Average Annual Moisture Mean Annual Predominant Vegetation

(Bailey’s Ecoregion Soil Order Moisture/ Precipitation Season/Form Temperature Evolutionary Type

Province) Temperature (mm) Range (°C) Disturbance

COOL DESERTS

Columbia Basin mollisol xeric/mesic 230-635 winter/rain 4-14 drought perennial

(temperate semidesert) spring/rain grassland

Great Basin aridisol aridic/mesic to cryic 180-380 winter/snow 4-13 drought sagebrush steppe

(temperate semidesert spring/rain

and desert)

Colorado Plateau alfisol xeric/thermic 205-510 winter/snow 4-13 drought shrubland

(temperate semidesert spring/rain woodland

and desert) summer/rain

Great Plains mollisol ustic/mesic 485-735 winter/snow 8-15 fire, grazing prairie

(temperate steppe and spring/rain

dry steppe) summer/rain

HOT DESERTS

Mojave Desert aridisol aridic/hyperthermic 100-150 winter/rain 13-29 drought shrubland

(tropical/subtropical desert

on sand)

Chihuahuan Desert aridisol aridic/thermic 205-325 summer/rain 10-18 drought shrubland

(tropical/subtropical

semidesert and desert on sand)

Sonoran Desert aridisol aridic/thermic 75-255 summer/rain 10-24 drought mixed thorn scrub

(tropical/subtropical desert fall/rain

on sand)

COASTAL CHAPARRAL

California Chaparral mollisol xeric/thermic 255-635 winter/rain 16-18 fire chaparral

(Mediterranean dry steppe)
Biological Soil Crusts:

Ecology & Management

NOTES

seventeen

Table 2.3 Dominant biological soil crust components and forms in North American arid and semi-arid ecoregions.

Ecoregion Dominant Biological Crust Components Crust Morphology

Columbia Basin tall mosses, green algae rolling

Great Basin moss, lichen rolling

Colorado Plateau non-heterocystic cyanobacteria (Microcoleus),

nitrogen-fixing lichens (Collema) pinnacled

Great Plains vagrant and foliose lichens flat to rugose

or rolling

Mojave Desert non-heterocystic cyanobacteria (Microcoleus),

nitrogen-fixing lichens (Collema), squamulose

lichens, short mosses flat to pinnacled

Chihuahuan Desert heterocystic cyanobacteria (Nostoc, Schizothrix),

short moss flat

Sonoran Desert heterocystic cyanobacteria (Nostoc, Schizothrix),

gelatinous (nitroen-fixing) lichens (e.g., Collema),

squamulose lichens, short mosses flat

California Chaparral heterocystic cyanobacteria (Nostoc, Schizothrix),

lichens, liverworts flat

needs compared to vascular plants and can withstand the harsh growing NOTES

conditions found in plant interspaces (Anderson et al. 1982a).

The positive relationship between biological crust cover and

available soil surfaces has been amply demonstrated (Rogers 1972;

Harper and Marble 1988; West 1990; Eldridge 1993b; Johansen

1993). As harsh environmental conditions limit vascular plant cover,

greater cover of crusts in lower elevation sites probably occurs in spite

of, not because of, these conditions.

2.2.2 Soils and Topography

Stable or embedded rocks near or at the soil surface can

increase the percent crust cover by perching water and armoring the

surface from physical disturbances (Fig. 2.4-D). Shallow soils often

support a wide variety of cyanobacteria, lichens, and mosses, regardless of

soil texture (Fig. 2.4-E).

Soil texture heavily influences the species composition of

biological crust communities. The more stable, fine-textured soils

(such as gypsum and silty loams) support greater cover and more

varied populations of cyanobacteria, lichens, and mosses (Fig. 2.4-F)
Biological Soil Crusts:

Ecology & Management

NOTES

eighteen Figure 2.4 General ecological relationships for biological soil crusts. High Low

Low High

TOTAL CRUST COVER

VASCULAR PLANT COVER

A B

High Low

Low High

TOTAL CRUST COVER

ELEVATION

C

High Low

Low High

TOTAL MOSS/LICHEN COVER

ELEVATION

E

High Low

Shallow Deep

TOTAL CRUST COVER

SOIL DEPTH

F

High Low

Fine Coarse

TOTAL CRUST COVER

SOIL TEXTURE

D

High Low

Low High

TOTAL CRUST COVER

PERCENT ROCK COVER

Stable Rocks

Unstable Rocks Loose Rocks

Embedded
Biological Soil Crusts:

Ecology & Management

than less stable, coarse-textured soils (Kleiner and Harper 1977b; NOTES

Hansen et al. 1999). Coarse-textured soils may have only large

filamentous cyanobacteria that are highly mobile (such as

Microcoleus). However, once coarse-textured soils are sufficiently

stabilized by larger cyanobacteria, other crustal organisms can then

colonize, including smaller green algal and cyanobacterial taxa (such

as Scytonema and Nostoc). Regardless of soil type, the first lichen to

colonize is generally Collema, followed by Placidium (and Caloplaca in

the northern Great Basin). In more unstable soils, lichens and mosses

may be found only under vascular plants, where some protection

from sediment burial is provided, or on north slopes, where greater

moisture availability favors growth. Soil surface stability is influenced

by texture (percent of sand, clay, silt), depth, and moisture content

(wet, moist, dry). Sand and silt are more susceptible to surface

disturbance when dry, while clay is highly stable (Fig. 2.5).

In later successional stages on stable surfaces, common lichen

species include Fulgensia bracteata, F. desertorum, Squamarina

lentigera, Diploschistes muscorum, and Psora spp. Common moss

genera include Tortula, Bryum, and Grimmia. As soil stability

increases, rich communities of cyanobacteria, lichens, and mosses

become more widespread, covering all surfaces not occupied by

vascular plants or rock.

Soil chemistry can also influence crust cover and composition.

Calcareous and gypsiferous soils generally support high coverage of

species-rich crust with some taxa being excellent indicators of soil

chemistry (Table 2.4). Often physical crusts form stable surfaces that

perch soil moisture, and given long periods without physical

disturbance, will support both biological soil crusts and lichens or

mosses normally found on rocks. Other abiotic factors that influence

relative cover of microbiotic crusts are slope and aspect. Crustal

organisms are only active when moist, and most active when warm.

Therefore, north and east slopes generally favor crustal development

in lower elevation desert regions. Slope angle does not generally affect

crust cover or species richness, except where the slope or soils are

unstable (Rosentreter 1986; Kaltenecker et al. 1997; Belnap and

Bowker, unpublished data).

2.2.3 Disturbance

Intensity and type of soil surface disturbance, along with time

since disturbance, influence the composition of biological crusts (see

Chapter 4). The presence, absence, and abundance of early- or latesuccessional taxa can provide information regarding a site’s disturbance

history. This information, combined with data on vascular plant

community composition, can assist the land manager in understanding a

site’s history, potential productivity, and ecological integrity.

nineteen
Biological Soil Crusts:

Ecology & Management

NOTES

twenty

Figure 2.5 Soil stability relative to texture

and moisture status. Arrows indicate

increasing soil stability. No arrow indicates

similar stability.

Soil Texture

Sand Clay Silt

High High High

Medium-High Low Medium

Medium Low Medium

Soil Moisture Content

Low High Medium-Low

Dry Moist Wet Frozen

Greater % Silt

Greater % Sand

Greater % Clay

WET

Sand Silt

Clay

DRY

Sand Silt

Clay

MOIST

Sand Silt

Clay

Sand Silt

Clay
Biological Soil Crusts:

Ecology & Management

NOTES

twenty-one

Table 2.4 Soil crust lichens that are calcium carbonate indicators. Adapted from McCune and Rosentreter 1995.

Low calcium carbonate High calcium carbonate

Acarospora schleicheri Aspicilia fruticulosa

Arthonia glebosa Aspicilia hispida

Aspicilia reptans Buellia elegans

Aspicilia filiforma Caloplaca tominii

Cladonia borealis Collema tenax

Diploschistes muscorum Fulgensia spp.

Leptochidium albociliatum Heppia lutosa

Megaspora verrucosa Phaeorrhiza nimbosa

Ochrolechia upsaliensis Psora cerebriformis

Placynthiella spp. Psora decipiens

Psora nipponica Psora tuckermanii

Xanthoparmelia wyomingica Toninia sedifolia

Intense disturbance results in bare soil. Severely, newly,

or frequently disturbed soils are generally dominated by large filamentous cyanobacteria (Anderson and Rushforth 1976; Johansen

et al. 1981, 1984; Johansen and Rushforth 1985; Harper and Marble

1988). When disturbance is less severe, less frequent, or some time

has elapsed since the disturbance, crusts are generally in some midsuccessional state, with some lichens and mosses present. Most of

these species reproduce asexually, a life-history strategy that increases

the probability of establishment (Rosentreter 1994). If disturbance

continues, crusts will stay in early-successional stages (i.e.,

cyanobacteria only).

2.2.4 Timing of Precipitation

Dominance of biological crusts is highly influenced by

seasonal precipitation patterns. Ecoregions that receive summer

monsoons (e.g., the Sonoran Desert) tend to have a greater diversity

of heterocystic cyanobacteria (such as Lyngbya, Calothrix, Schizothrix,

and Nostoc) and lower lichen abundance. Lichens in these areas

generally include the gelatinous genus Collema and squamulose

genera Placidium and Peltula (Table 2.3). Large thalloid liverworts are

more common in warm deserts than in the cool deserts of North

America. In cool deserts, crusts are dominated by non-heterocystic

cyanobacteria (such as Microcoleus vaginatus) and a diverse lichen

flora, including Acarospora schleicheri, Aspicilia spp., Candelariella

NOTES
Biological Soil Crusts:

Ecology & Management

NOTES spp., Collema spp., Diploschistes muscorum, Endocarpon pusillum,

Placidium spp., and Psora spp. These lichens physiologically prefer

cool-season moisture and are adapted to lower light intensities

common during winter months. The Columbia Basin, which receives

more moisture and has more winter rain than the Great Basin, has a

greater abundance of mosses than lichens (Ponzetti et al. 1998).

Biological soil crusts in regions influenced by fog, such as

portions of the California Chaparral, support fruticose lichens that

intercept moisture from the air (Dendrographa and Schizopelte). Biological

crusts in many regions are best developed in interspaces between shrubs.

In contrast, fog deserts show the best crust development under shrubs,

due to the moisture intercepted by plant structure or by rock surfaces.

2.2.5 Vascular Plant Community Structure

The vertical and horizontal vascular plant structure of many arid

and semi-arid vegetation communities optimizes growth of biological soil

crusts. In cooler regions, greater structural diversity of vascular vegetation

generally results in greater compositional diversity of biological crusts.

Vascular plants create windbreaks and shade, influencing how much

moisture and light reach the soil surface. They also trap leaf litter, keeping

the interspaces free of substantial or persistent litter cover (Rosentreter

and McCune 1992).

Invasive exotic plants generally decrease the structural diversity of

native vascular plant communities by creating monocultures of densely

spaced plants and by homogenizing litter distribution. They also lead to

decreased biological crust cover and species richness in most ecosystems

(Rosentreter 1994; Kaltenecker 1997). In addition, if moisture is retained

under a litter layer for long periods while temperatures are warm, lichens

can become parasitized by ubiquitous molds (Rosentreter 1984).

2.2.6 Ecological Gradients

Some lichens form natural replacement series along the same

ecological gradients that influence vascular plants, although some lichen

taxa are not good indicators of site conditions due to broad ecological

amplitudes. For example, gelatinous lichens are most common in aridic

calcareous sites and mesic non-calcareous sites (Fig. 2.6). Sites with frigid

soil temperature regimes (mean annual temperature less than 8oC) lack

significant cover of gelatinous lichens. The genus Leptogium might be

present; however, Peltigera and Massalongia tend to be more common.

Some species display a shift in substrate preference in different ecoregions.

For example, Leptochidium albociliatum occurs on mosses in the Great

Basin, while in the Columbia Basin it is more common and occurs on

bare mineral soil (Rosentreter 1986; Ponzetti et al. 1998).

twenty-two
Biological Soil Crusts:

Ecology & Management

2.2.7 Microhabitats NOTES

Biological soil crusts can also create their own microstructure,

which influences species distribution. Mosses create microsites that

are very low in nutrients, low in calcium carbonate, and that retain

moisture longer than bare mineral soil. This creates favorable sites for

lichens that may benefit from the improved moisture regime and do

not require many soil nutrients. Common examples include

Leptogium lichenoides, Massalongia carnosa, and Peltigera spp. that

grow only on mosses and are found in many sagebrush (Artemisia)

steppe habitats.

2.3 Unique Crustal Communities in North America

Special geologic features or soils create conditions that

promote growth of biological soil crusts. Some of these sites support

crusts containing uncommon associations or rare species. These

unique crustal communities are not common across the landscape

but are at times locally abundant. Some unique crustal communities

are discussed below.

2.3.1 Gypsum

Gypsiferous outcrops are comprised of soils that are fine

textured and have high concentrations of sulfate and calcium.

Vascular plant cover at these sites is generally sparse with limited

species diversity. Gypsiferous soils can support well-developed

twenty-three

▲

▲

▲

▲

▲

▲

▲

▲

Figure 2.6 Gelatinous and other nitrogen-fixing lichens by relative soil temperature and calcareous influence.

ARIDIC MESIC FRIGID

warm season grasses cool season grasses cool season grasses

mixed salt desert shrub bluebunch wheatgrass Idaho fescue

calcareous non-calcareous non-calcareous

Heppia lutosa

Peltula richardsonii

Collema coccophorum

Collema tenax

Leptochidium albociliatum

Leptogium lichenoides

Massalongia carnosa

Peltigera spp.
Biological Soil Crusts:

Ecology & Management

NOTES biological crusts with a high lichen diversity (Anderson and

Rushforth 1976; St. Clair et al. 1993), although sites in the hot

deserts of the southwestern U.S. lack the species diversity of the cool

deserts to the north. Common species include Diploschistes diacapsis,

Psora decipiens, Collema tenax, Placidium squamulosum, Buellia

elegans, and Squamarina lentigera. Several lichens are restricted to

gypsiferous soils, including Acarospora nodulosa var. nodulosa (St. Clair

and Warrick 1987), Gypsoplaca macrophylla (Timdal 1990), and

Lecanora gypsicola (Rajvanshi et al. 1998). Gypsiferous sites are

worthy of protection because of their high potential for cover and

biological crust diversity. These sites are often threatened by mining

activity due to the commercial value of gypsum.

2.3.2 Glades (Lithic and Shallow Soil Sites)

Glades are widespread across North America but are limited

both locally and in overall area. Lithic and shallow soil sites are often

colonized by biological crusts because these sites are extremely

droughty and vascular plant growth is limited. In eastern forests,

shallow disturbed or compacted soil (e.g., along roadsides) may be

dominated by biological soil crusts, with Baeomyces rufus or B.

fungoides often forming extensive crustal colonies. Pine barrens

generally support extensive biological crusts. In western and more

northern boreal forest openings, Cladonia spp., Peltigera spp., and

occasionally Multiclavula spp. are dominant. These genera are also

common on disturbed soil along roadways and cutbanks throughout

the non-arid portions of the continent. Some lithic sites in wet

climates are leached of soil nutrients and are often dominated by

gelatinous lichens such as Leptogium or nitrogen-fixing cyanobacteria

such as Nostoc.

2.3.3 Thermal Springs

Some parts of western North America contain extensive active

or dormant thermal (hot) springs. Conditions in these areas are harsh,

limiting vascular plant growth. Soil temperatures are often extremely

high in summer and warm in winter. Soils containing thermal springs

are often infertile, as warm water leaches many minerals, including

calcium, magnesium, nitrogen, and phosphate. Toxic heavy metals

can be present in the water and thus get deposited on the soil surface.

These areas are covered with specialized thermic cyanobacteria when

moist. When dry, they support either non-specialized biological crusts

or crusts characteristic of gypsum.

twenty-four
Biological Soil Crusts:

Ecology & Management

NOTES

twenty-five

2.3.4 Cold Desert Playas

Small ephemeral pools form in parts of the Pacific Northwest

where summers are hot and dry and winters are cool and moist

(Rosentreter and McCune 1992). These sites are located on poorly

drained basalt or other rock types, often barren of vascular vegetation

and usually surrounded by some species of sagebrush (Artemisia).

Drainage may be impaired by rock type or frozen soils, causing water

to pond seasonally in flat, exposed areas. In late winter and early

spring these sites experience cool, yet above freezing, temperatures for

portions of the day. These conditions allow for slow evaporation of

standing water. Such sites are sometimes referred to as playettes, as

they are similar to but smaller than large seasonal lakes called playas.

Playas support either attached or vagrant forms of Dermatocarpon

miniatum and Nostoc. Nostoc commune found in these sites includes

the common filamentous form, the colonial-eggs form, and

occasionally the long, hair-like unattached form (referred to as var.

flagellaris). Small playettes covered with fine silts can be colonized by

Nostoc and by crustose lichens more typically occurring on rocks, such

as Lecanora muralis (Rosentreter 1986).

2.3.5 Alpine Sod

Biological soil crusts are either dominant or common

community components in many alpine and subalpine sites in North

America. Alpine and subalpine sites contain some species common to

the continent’s arid regions. In contrast, a few lichens are

characteristic of alpine crusts alone and include Lecidoma demissum,

Solorina spp., Lepraria neglecta, Ochrolechia frigida, and Thamnolia

spp. (DeBolt and McCune 1993). A genus that occurs on soil or rock

in both alpine and arctic sites is Stereocaulon, which fixes nitrogen and

is common worldwide in montane and alpine habitats (Lamb and

Ward 1974). In the subalpine zone, biological crusts are often

associated with areas where snowdrifts occur. Lichens characteristic of

snow-patch sites are Solorina crocea and Lepraria neglecta (McCune

1998). Cyanobacterial crusts are also common in these areas and are

generally dominated by Nostoc species (Reisiegl 1964).

2.3.6 Arctic Tundra

Alpine and tundra soils are often dominated by fruticose

lichens (Cladina and Cetraria) and graminoids. Some areas are

dominated by fruticose lichens that start out as crust communities in

the early stages of succession. Tundra areas in North America have

poor drainage due to frozen ground and are similar to other portions

of the northern hemisphere, including Greenland. In some arctic

regions there are also large areas of dry steppe-like vegetation, open
Biological Soil Crusts:

Ecology & Management

NOTES Dryas heaths, and fell-fields with extensive coverage of biological crusts.

Cyanobacterial crusts are commonly extensive and often dominated by

Nostoc species (Hansen 1997).

2.4 Example: Biological Soil Crusts in Sagebrush Communities

Sagebrush species are common in western North America,

particularly in the Great Basin and Columbia Plateau vegetation zones.

Some of the same characteristics that influence sagebrush taxa

distribution also influence biological crust development. Relative cover of

biological crusts in various sagebrush vegetation types is presented in

Table 2.5. Biological soil crusts tend to be lacking in sagebrush types that

occur on seasonally flooded soils (silver, alkali, and fuzzy sagebrush;

Winward 1980), as flooded soils create anaerobic conditions that are not

well tolerated by lichens. Heavily saline soils also lack lichen cover,

although moss is sometimes present if the salt concentration is not too

great. Mountain, subalpine, and xeric big sagebrush types often lack

significant biological crust cover due to dense vascular vegetation and

accumulating plant litter. Other sagebrush types support higher biological

crust cover unless soil surfaces are greatly disturbed or the current

vegetation is in an early-successional stage.

Some lichens are good indicators of late-successional stages in

sagebrush communities. The dual gradient theory proposed by McCune

(1993) for lichen species succession in forested habitats applies well to

arid and semi-arid regions in that species respond to time (age) and

moisture in similar successional trajectories. Therefore, the length of time

since the last major site disturbance or an increase in effective soil

moisture will both provide suitable ecological conditions to support

specific lichen species. This is why biological soil crust communities in

different ecoregions recover at different rates (Table 2.2, Chapter 4).

Late-successional indicator species in sagebrush-steppe include

Acarospora schleicheri, Massalongia carnosa, Pannaria cyanolepra (type b),

Trapeliopsis wallrothii, Trapeliopsis sp. nov. (McCune, unpublished), and

Texosporium sancti-jacobi. Some lichens are only present in latesuccessional communities because they grow upon other lichens or

mosses. For example, Acarospora schleicheri grows upon Diploshistes

muscorum, which in turn parasitizes the lichen genus Cladonia. Therefore,

Cladonia can be considered a keystone organism influencing a site’s

diversity. Massalongia carnosa primarily grows on mosses and is not

present until mosses become well established within a site. Texosporium

sancti-jacobi is restricted to old-growth sagebrush communities and

occurs only on decaying organic matter (McCune and Rosentreter 1992).

Other lichens that commonly occur on decayed organic matter but are

not restricted to old-growth sites include Buellia papillata, B. punctata,

Caloplaca spp., Lecanora spp., Megaspora verrucosa, Ochrolechia upsaliensis,

Placynthiella spp., and Phaeophyscia decolor.

twenty-six
Biological Soil Crusts:

Ecology & Management

Table 2.5 Relative cover of biological soil crusts in sagebrush (Artemisia) vegetation types. NOTES

HIGH biological crust cover LOW biological crust cover

Tall Sagebrush

Wyoming big sagebrush subalpine big sagebrush

(A. tridentata ssp. wyomingensis) (A. tridentata ssp. spiciformis)

basin big sagebrush xeric big sagebrush

(A. tridentata ssp. tridentata) (A. tridentata ssp. xericensis)

mountain big sagebrush** mountain big sagebrush**

(A. tridentata ssp. vaseyana) (A. tridentata ssp. vaseyana)

silver sagebrush

(A. cana)

three-tip sagebrush

(A. tripartita)

Short Sagebrush

low sagebrush alkali sagebrush

(A. arbuscula) (A. longilobia)

black sagebrush fuzzy sagebrush

(A. nova) (A. papposa)

stiff sagebrush

(A. rigida)

Bigelow sagebrush

(A. bigelowii)

fringed sage

(A. frigida)

**Biological crust cover high or low depending on site characteristics.

twenty-seven
Biological Soil Crusts:

Ecology & Management

NOTES

twenty-nine

CHAPTER 3

ECOLOGICAL ROLES

In a given ecoregion, ecological roles of biological soil crusts

can vary widely in their importance and will depend on crust

composition and biomass, as well as characteristics of the specific

ecosystem being considered.

3.1 Species Composition and Biomass

The relative importance of biological crusts and their different

ecological roles within an ecosystem is highly dependent on relative

cover of various crustal components. For example, carbon inputs are

higher when mosses and lichens are present than when the crust is

dominated by cyanobacteria. Nitrogen inputs are higher with greater

cyanobacterial biomass and greater cyanolichen cover. Water

infiltration and soil surface stability are related to cyanobacterial

biomass as well as moss and lichen cover.

3.2 Carbon Fixation

Biological soil crusts can be an important source of fixed

carbon on sparsely vegetated areas common throughout western

North America (Beymer and Klopatek 1991). While vascular plants

contribute organic matter to soils directly beneath them, large

interspaces between plants receive little plant material input. Where

biological soil crusts are present, carbon contributed by these

organisms helps keep plant interspaces fertile, providing energy

sources for soil microbial populations.

Crustal organisms are physiologically active only when wet,

and metabolic functions begin almost immediately. Respiration

begins less than 3 minutes after wetting, while photosynthesis reaches

full activity after 30 minutes or more. Soil oxygen concentrations, a

by-product of photosynthesis, reach steady states within 1 to 2 hours

of wetting (Garcia-Pichel and Belnap 1996). Rain during hot months

can place lichens in a state of carbon deficiency, as the lichens can dry

out before photosynthesis has begun. Jeffries et al. (1993a, b) showed

cyanobacterial crusts from the Colorado Plateau had positive carbon

balances only during prolonged wet periods, such as winter and

spring, and that short-term wet-dry cycles, which often occur in lowelevation inland sites during summer, resulted in net carbon loss for

both Microcoleus- and Scytonema-dominated crusts (this would also be

expected for Collema and other soil lichens). Consequently, timing
Biological Soil Crusts:

Ecology & Management

NOTES and duration of precipitation may heavily influence carbon budgets

in crustal organisms and thus influence the distribution of individual

species.

Winter-rain deserts (e.g., Mojave and Great Basin) have a rich

soil lichen flora, with the cyanobacterial flora heavily dominated by

Microcoleus. In contrast, summer-rain deserts (e.g., Sonoran) have

only a small subset of the lichens found in winter-rain deserts, while

the cyanobacterial flora is a complex community co-dominated by up

to 10 species. Different responses to timing and amount of

precipitation and temperature may mean species composition will

shift in response to predicted climate changes. Since many areas that

are currently winter-rain dominated are predicted to receive increased

summer rain, effects of climate change may be reflected in different

soil crust composition than that currently found in the summer-rain

versus the winter-rain deserts.

Soil lichens have several different strategies to optimize carbon

gain (Lange et al. 1992, 1994, 1997). While photosynthesis is

activated at low-water content for all species, each varies in response

to higher water content. Cyanolichens, such as Collema tenax, showed

dramatic drops in carbon fixation when water content exceeded 60 to

80%. Conversely, green algae generally showed little, if any,

depression at high-water content. Water-holding capacity varies

between lichens and will also influence time of carbon fixation for a

given species.

Table 3.1 Common nitrogen-fixing lichens and cyanobacteria in cool and hot deserts of the western U.S.

Cool Deserts Hot Deserts

Lichens

Collema coccophorum Collema coccophorum

Collema tenax Collema tenax

Heppia Heppia

Leptochidium albocilitatum Peltula

Leptogium

Massalongia carnosa

Pannaria

Peltigera

Polychidium

Cyanobacteria

Microcoleus vaginatus (non-heterocystic) Schizothrix (heterocystic)

Nostoc (heterocystic) Nostoc (heterocystic)

Scytonema (heterocystic) Scytonema (heterocystic)

thirty
Biological Soil Crusts:

Ecology & Management

Photosynthetic rates are also dependent on temperature NOTES

(Rychert et al. 1978; Lange 1980; Nash et al. 1982; Nash and Moser

1982; Lange et al. 1997). Photosynthetic rates increase with

increasing temperatures to about 28oC, above which rates decline

dramatically. Collema tenax is an exception, with no such decline seen

up to 36oC.

3.3 Nitrogen Fixation

Nitrogen concentrations are known to be low in desert soils

compared to other ecosystems. Total atmospheric input over the

past 10,000 years has been conservatively estimated at about 3 kg

nitrogen/m2, with 77% lost through wind erosion, ammonia

volatilization, nitrification, and denitrification (Peterjohn and

Schlesinger 1990). Extensive surveys in cool deserts have revealed

only a few nitrogen-fixing plants (Farnsworth et al. 1976; Wullstein

1989). Since nitrogen can limit net primary productivity in many

desert ecosystems (Ettershank et al. 1978; James and Jurinak 1978;

Romney et al. 1978; Nobel et al. 1988), maintaining normal nitrogen

cycles is critical to soil fertility and prevention of desertification

(Dregne 1983).

Cyanobacteria and cyanolichens can be an important source

of fixed nitrogen for plants and soils in desert ecosystems (Evans and

Ehleringer 1993; Belnap 1995). Many biological soil crusts in western

North America are dominated by nitrogen-fixing cyanobacteria and

lichens (Table 3.1). These organisms are capable of both light and

dark nitrogen fixation (Rychert and Skujins 1974; Pearson et al.

1981; Paerl 1990; Belnap 1994). Nitrogen fixation is highly

dependent on past and present water and light regimes, as well as

species composition (Rychert et al. 1978; Belnap 1994). Fixation

rates are highest after photosynthesis has replenished lichen carbon

stores. For most lichen species, nitrogen-fixation rates increase with

temperature to 25o

C, given sufficient moisture. Since nitrogenfixation rates depend on the cover of specific crust species, timing,

extent, and type of past disturbance are also critical factors (Belnap

1995, 1996). Nitrogen-fixation rates in biological soil crusts have

been estimated at 2 to 365 kg/ha annually (Mayland et al. 1966;

MacGregor and Johnson 1971; Rychert and Skujins 1974; Eskew and

Ting 1978; Jeffries et al. 1992), with recent studies indicating that

higher rates are probably more accurate (Belnap, unpublished data).

Still, rates are expected to vary greatly, depending on the species

present and environmental conditions.

Five to 88% of nitrogen fixed by Nostoc has been shown to

leak into the surrounding substrate (Magee and Burris 1954; Silvester

et al. 1996; Belnap et al. 1997). Nitrogen released from crustal

thirty-one
Biological Soil Crusts:

Ecology & Management

NOTES

organisms is readily taken up by surrounding vascular plants, fungi,

and bacteria (Mayland and MacIntosh 1966; Mayland et al. 1966;

Stewart 1967; Jones and Stewart 1969). Vascular plants growing in

biologically crusted soils show higher tissue concentrations of

nitrogen than plants grown in uncrusted soils (Harper and Pendleton

1993; Belnap 1994, 1995; Belnap and Harper 1995). As with carbon,

crusts contribute nitrogen to soils both under plants and in plant

interspaces, thereby counteracting the tendency of these nutrients to

concentrate around perennial plants.

3.4 Albedo

Soil albedo is a measure of the energy reflected off the soil

surface. Dark surfaces absorb more energy, are warmer than light

surfaces, and thus have lower albedo. Because soils with welldeveloped biological crusts are dark, they can be up to 23o

F warmer

than adjacent crusted surfaces (Fig. 3.1; Belnap 1995; Belnap,

unpublished data). Disturbance of biological crusts increases albedo

and therefore decreases soil temperatures. This affects many

ecosystem processes. Metabolic processes, for example, including

photosynthesis and nitrogen fixation, decrease at lower temperatures

(see Sections 3.2 and 3.3; Nash 1996). Lower soil temperatures

decrease microbial activity, plant nutrient uptake rates, and soil water

evaporation rates; delay seed germination time; and decrease seedling

growth rates. Timing of these events is often critical in deserts, and

modifications can reduce species fitness and seedling establishment,

which in turn may eventually affect plant community structure (Bush

and Van Auken 1991). Changes in albedo can also affect animal

populations. For instance, foraging times are often partitioned among

ants, arthropods, and small mammals on the basis of soil surface

temperature (Doyen and Tschinkel 1974; Wallwork 1982; Crawford thirty-two

Figure 3.1 Disturbance plots on the

Colorado Plateau showing the

difference in soil surface color between

undisturbed (dark) and disturbed

(light) plots. Note that the undisturbed

plots have greater surface roughness than

the disturbed plots.
Biological Soil Crusts:

Ecology & Management

1991). In addition, many small desert animals are weak burrowers, NOTES

and soil surface microclimates are of great importance to their

survival (Larmuth 1978). Consequently, altering surface temperatures

can affect desert ecosystems at numerous levels.

3.5 Effects on Vascular Plants

3.5.1 Seed Germination

Biological crusts can affect vascular plant germination. While

small cracks and crannies on the soil surface are generally sufficient for

small-seeded plants to lodge and germinate, most large-seeded plants

need soil or plant litter cover to germinate. In deserts where plant litter is

often limited in interspaces, large native seeds generally have self-burial

mechanisms (such as hygroscopic awns) or are cached by rodents. Many

exotic species, however, lack self-burial mechanisms. Because biological

crusts stabilize soils, germination of such seeds can be inhibited in sites

with well-developed crusts and low plant litter, as was recently

demonstrated for the annual exotic grass, cheatgrass (Bromus tectorum), in

both the field and laboratory (Larsen 1995; Kaltenecker et al. 1999a;

Belnap, unpublished data). Though crusts can reduce germination of

some seeds, seedling germination per se has not been shown to limit

species density in desert plant communities. Rather, studies worldwide show

vascular plant cover and structure are most often controlled by water and/or

nutrient availability rather than other site factors (Mabbutt and Fanning 1987;

Tongway and Ludwig 1990; Dunkerley and Brown 1995).

3.5.2 Plant Establishment and Cover

Biological soil crusts have not been shown to present a barrier

to seedling root penetration once seeds germinate (Belnap, unpublished data; Pendleton and Meyer, unpublished data). This is in

contrast to physical crusts, which often inhibit root penetration. Areas

thirty-three

Figure 3.2 Sandberg bluegrass

(Poa secunda) growing in welldeveloped biological soil crusts in

the northern Great Basin.
Biological Soil Crusts:

Ecology & Management

NOTES

thirty-four

that have been lightly trampled (to keep crusts from developing) over

a 5-year period that included both wet and dry years show no

additional plant establishment (Belnap, unpublished data; Fig. 3.1).

Many authors have shown that seedling establishment of both forbs

and grasses is either not affected or is increased by the presence of

biological soil crusts, on both coarse- and fine-textured soils (Fig. 3.2;

St. Clair et al. 1984; Harper and St. Clair 1985; Eckert et al. 1986;

Harper and Marble 1988; Lesica and Shelley 1992; Belnap 1993).

Numerous authors have reported that biological crusts either

do not compete with vascular plants (Kleiner and Harper 1972,

1977a, b; Anderson et al. 1982b; Jeffries and Klopatek 1987; Beymer

and Klopatek 1992) or that vascular plant cover is enhanced by the

presence of a biological crust (Dadlich et al. 1969; Graetz and

Tongway 1986; Rosentreter 1986; Mucher et al. 1988; Carleton

1990; Ladyman and Muldavin 1994; Ladyman et al. 1994). This

suggests that the presence of plants can aid the survival of crustal

components, perhaps because of microclimate conditions associated

with clumped perennial vegetation (such as decreased soil surface

temperatures, increased surface moisture due to shading, and

decreased wind speed on the soil surface). A few authors have

speculated that there is a negative relationship between cover of

biological crusts and vascular plants (Savory 1988 reviewed in West

1990). However, this view is not supported by the numerous studies

that have directly addressed this issue. Rather, it appears more likely

that biological crusts simply cover soils unoccupied by vascular plants.

3.5.3 Nutrient Levels in Vascular Plants

Plants growing in biologically-crusted soil often show higher

concentrations and/or greater total accumulation of various essential

nutrients than do plants growing in adjacent, uncrusted soils (Belnap

and Harper 1995; Harper and Belnap in press). In one study, leaf

tissue nitrogen was 9% higher in the perennial shrub Coleogyne, 31%

higher in the perennial forb Streptanthella, and 13% higher in the

annual grass Festuca (Vulpia), for plants growing in biologicallycrusted soil compared to plants in adjacent uncrusted soil. Dry

weight of Festuca plants in the crusted soil was twice that of plants in

the uncrusted soil (Belnap 1995; Belnap and Harper 1995). Nitrogen

concentrations in leaf tissue of the biennial Mentzelia multiflora was

higher in plants growing in a biologically-crusted area relative to

plants from an adjacent sand dune that lacked a crust. In greenhouse

experiments, nitrogen levels in Sorghum halepense were higher when

the plant was in pots with cyanobacteria than when in pots without

cyanobacteria. Dry weight of plants in pots with cyanobacteria were

up to four times greater than in pots without cyanobacteria (Harper
Biological Soil Crusts:

Ecology & Management

and Pendleton 1993). Other authors have obtained similar results NOTES

with other species (Shields and Durrell 1964; Brotherson and

Rushforth 1983; Pendleton and Warren 1995).

Several mechanisms may explain this enhanced effect on

vascular plant growth and nutrient status. Material blowing across

biological soil crusts can become trapped, either accumulating within

low pockets in the microtopography or sticking by exudate to

cyanobacterial sheaths. These sheaths are phototropic, and if not

buried too deeply, will push through loose soil and organic matter,

further trapping or entangling soil in the process (Fryberger et al.

1988; Campbell et al. 1989; Gillette and Dobrowolski 1993). Danin

et al. (1989) proposed a positive feedback loop in which crusts

trapping fine particles create nutrient-rich microsites, increasing

germination and growth of vascular plants, which in turn reduces

erosion potential and increases accumulation of washed or windblown soil.

In addition, cyanobacterial sheath material is often coated

with negatively charged clay particles. These clay particles are more

nutrient rich than sand (Black 1968), as they bind positively charged

macronutrients and prevent them from leaching through the soil

profile (Belnap and Gardner 1993). Lange (1974) demonstrated that

compounds in the gelatinous sheath material of several cyanobacteria

taxa were able to chelate iron, copper, molybdenum, zinc, cobalt, and

manganese. Four of the five genera shown to possess this ability

(Anabaena, Anacystis, Lyngbya, and Nostoc) are commonly represented

in biological crusts of western North American deserts (Shields and

Durrell 1964). It is also possible that the nutrient differences result

from thermal effects, as crusted soils are darker and warmer than

uncrusted soils; nutrient uptake by vascular plants would occur at a

higher rate.

Herbivores and other consumers may benefit directly from

the enhanced nutrient status of plants grown in healthy, biologicallycrusted soils (Belnap and Harper 1995). Indirect effects include

positive correlations between soil mycorrhizae and microarthropod

populations with the presence of well-developed biological soil crusts

(Harper and Pendleton 1993; Belnap, unpublished data).

3.6 Water Relations

The effect of biological soil crusts on soil-water relations is

highly site dependent. Level of crustal development (e.g., cover and

relative composition of cyanobacteria, lichen, moss), climatic regimes,

the amount of surface roughness, time since destructive disturbance,

soil texture, and soil structure can all heavily influence hydrologic

cycles at a given site. Soil texture is especially important and can

thirty-five
Biological Soil Crusts:

Ecology & Management

NOTES override any effect of biological soil crusts. For instance, soils high in

shrink-swell clays have relatively low infiltration rates, regardless of

biological crust presence or absence.

Research conducted under a variety of soil and climate

conditions around the world shows variable and interactive effects of

biological crusts and soil properties. Biological crusts influence soil

hydrology because they alter soil surfaces. Aggregate stability, soil

structure, organic matter, and rough microtopography are all

attributes associated with superior hydrologic properties, and

biological soil crusts enhance these attributes in soil (Booth 1941;

Fletcher and Martin 1948; Shields and Durrell 1964) on both microand macro-scales (Belnap and Gardner 1993; Belnap 1994).

Infiltration rates are controlled by the interaction of waterresidence time on a soil surface and the permeability of that surface.

thirty-six

Figure 3.3 Soil-water relationships with

biological soil crusts. In the cool deserts

(Colorado Plateau, Great Basin),

roughened soil surfaces created by

biological crusts act to impede overland

water flow, resulting in increased

infiltration. When biological soil crusts

are removed, or with flat crusts in the hot

deserts (Sonoran, Mojave, Chihuahuan),

water runs over the soil surface unimpeded and infiltration is reduced.
Biological Soil Crusts:

Ecology & Management

Biological crusts can produce a rough surface microtopography (less NOTES

than 100 mm vertically) that acts as detention structures for water

(Danin and Barbour 1982). This is especially pronounced in cool

deserts (e.g., the Colorado Plateau), where frost-heaving is common

and crusts are highly pedicelled. In these regions, biological crusts

generally increase water infiltration (Fig. 3.3; Loope and Gifford

1972). In warm deserts, where frost-heaving is not present and

biological soil crusts are flat, infiltration rates depend more on soil

type and climatic regimes.

While greater surface roughness can increase water pooling

and residence time, the presence of mucilaginous cyanobacteria in soil

crusts can also decrease soil permeability. These cyanobacterial

components of biological soil crusts rapidly swell up to 13 times their

dry volume (Shields and Durrell 1964; Campbell 1977), potentially

closing flow pathways through the soil surface. Scanning electron

microscopy shows sufficient openings in sandy loam soils for water

flow (Belnap and Gardner 1993), but concentration of silt and clay

particles in the crust, in combination with swelling, may restrict

infiltration even in sandy soils when they are wet (Verrecchia et al.

1995). Some authors speculate that the hydrophobic nature of some

crustal components contributes to soil surface sealing (Bond 1964;

Dulieu et al. 1977; Walker 1979; Rutin 1983; Jungerius and van de

Meulen 1988); however, others have shown crustal organisms at some

sites are highly hydrophilic (Kidron et al. 1999).

Overall, infiltration rates appear to be increased in regions

where crusts are pinnacled and decreased in regions where crusts are

flat. This has been reported for sandy soils in Arizona (Brotherson

and Rushforth 1983), Australia (Bond 1964; Rogers 1977; Stanley

1983; Graetz and Tongway 1986; Eldridge 1993a, b), and Israel (Yair

1990). In New South Wales, Australia, infiltration rates increased on

a transect through a sandy-loam soil as grazing decreased and

biological soil crust cover increased. However, compared with

adjacent ungrazed soils with a continuous cover of lichens and

mosses, the lightly grazed areas showed greater infiltration (Eldridge

1993b). Infiltration studies conducted on sandy soils in Oklahoma

and Kansas (Booth 1941) and silt-loams and clayey silt loams in

Arizona (Faust 1970, 1971) failed to find any influence of biological

soil crusts on infiltration rates.

The influence of biological soil crusts on effective hydraulic

conductivity (the rate at which water moves through the soil profile

under field conditions) is also variable. Lichen-dominated crusts in

Utah showed decreased hydraulic conductivity (Loope and Gifford

1972). Lichen and algal crusts in Australia were associated with high

conductivity (Greene et al. 1990). Three-year-old algal crusts in Utah

and non-disturbed crusts in Nevada showed no discernible influence

on conductivity (Williams 1993; Dobrowolski 1994).

thirty-seven
Biological Soil Crusts:

Ecology & Management

NOTES Biological soil crusts have varied effects on soil moisture at

any given time. However, research results have been variable and

again appear to depend on soil type and crust composition. The

crust’s ability to seal the soil surface and reduce evaporation because

of its associated high clay and silt concentrations has been repeatedly

proposed (Danin 1978; Brotherson and Rushforth 1983; Williams et

al. 1995a). While some studies show soil moisture is greater under

well-developed crusts (Harper and Marble 1988; Meyer and GarciaMoya 1989; Verrecchia et al. 1995; Belnap et al. 1996), other studies

have found decreased soil moisture under crusted surfaces (Harper

and Marble 1988).

Though overall infiltration of precipitation is critical for plant

growth, the location of water entering the soil can also be critical in

maintaining plant community structure. Recent work has shown that

the pattern of water infiltration and runoff is important in

maintaining vegetative community structure, especially in hyper-arid

zones where rainfall is too low to support homogenous distribution of

vegetation. Instead of being distributed across the landscape, plants

are concentrated in bands that occur perpendicular to the water flow.

These bands or vegetation clumps capture water, nutrients, and fine

soil particles. The bigger the bands, the more effective they are in

resource capture; however, their overall size is limited by rainfall and

nutrients. Biological soil crusts often cover interband soils. When

these interband areas are disrupted, water infiltration can increase

between vegetated areas; thus, less water is available for the

vegetation, often deep-rooted, woody perennials (Eldridge et al.

2000). This decrease in water availability may lead to changes in plant

community structure by enabling establishment of shallow-rooted

species in the interband areas or by die-off of the perennial shrubs.

Over time, changes in hydrology and plant community structure

result in modification of site potential and ecological function. A

similar situation applies to exotic plant invasions and their effects on

site function. In the Great Basin, invasive exotic annual grasses

modify the native community’s patchy structure to a more

homogenous structure (Kaltenecker 1997). The resultant increases in

plant and litter density in interspaces previously occupied by

biological crusts affect moisture infiltration, which may further

facilitate changes in community structure because less moisture

becomes available for deeply-rooted perennial plants.

As can be seen above, the impact of biological soil crusts on

hydrologic cycles can be highly variable and can result from a

combination of site, soil, and crust factors (Fig. 3.4). However, lack of

standardized data collection methods and descriptions of soil,

biological crust, and climatic characteristics at study sites makes

comparison of research results difficult. These shortcomings have

thirty-eight
Biological Soil Crusts:

Ecology & Management

NOTES

thirty-nine

Figure 3.4 Biological soil crust and soil characteristics that influence infiltration.

Hot Desert (flat, rugose) Cool Desert (pinnacled, rolling)

Low High

INFILTRATION

COVER

cyanobacterial

crust

moss/lichen 

crust

Low High

INFILTRATION

COVER

cyanobacterial

crust

moss/lichen 

crust

flat rough

INFILTRATION

SURFACE ROUGHNESS

fine course

INFILTRATION

SOIL PARTICLE SIZE

All Deserts
Biological Soil Crusts:

Ecology & Management

NOTES undoubtedly contributed to much of the seemingly contradictory

published information.

3.7 Soil Stabilization

Biological soil crusts are unambiguously effective in reducing

wind and water erosion of soil surfaces (Belnap and Gillette 1997,

1998; McKenna-Neumann et al.1996). Wind can be a major erosive

force in deserts, as sparse vegetation leaves large patches of soil

unprotected by plant litter or vegetative cover (Goudie 1978).

Interspace soils are most often stabilized by rocks or biological soil

crusts. Biological crusts play an important role both in preventing soil

loss and facilitating soil accretion. The degree to which different types

of crusts protect the soil surface from both wind and water erosion is

hierarchical in the following manner: bare soil < cyanobacterial/algal

crust < lichen crust < moss crust (Tchoupopnou 1989; Kinnell et al.

1990; Eldridge and Greene 1994; Belnap and Gillette 1998).

Polysaccharides exuded by cyanobacteria and green algae, in

combination with lichen and moss rhizines, entrap and bind soil

particles together, increasing the size of soil aggregates (Fig. 3.5). As

soil aggregates enlarge, they become heavier, have a greater surface

area, and are more difficult for wind or water to move. Biological soil

crusts keep otherwise loose sandy soils in place on steep slopes; they

also stabilize pockets of very shallow soil (Bond and Harris 1964;

Marathe 1972; Gayel and Shtina 1974; Danin and Yaalon 1980;

Schulten 1985; Graetz and Tongway 1986; Campbell et al. 1989;

Danin et al. 1989; Belnap and Gardner 1993). Rough

microtopography creates a still-air boundary layer that protects the

soil surface from wind erosion. Detention dams pool and slow

overland water flow, giving sediment time to settle out, reducing

sediment loss (Fig. 3.3; Brotherson and Rushforth 1983; Alexander

and Calvo 1990).

forty

Fig. 3.5 The sheath of

Microcoleus vaginatus contains

sticky polysaccharides that entrap

soil particles and bind them

together.
Biological Soil Crusts:

Ecology & Management

NOTES CHAPTER 4

RESPONSE TO NATURAL IMPACTS AND HUMAN ACTIONS

4.1 Evolutionary History of Natural Impacts

Euro-American settlement of western North America in the

19th and 20th centuries changed the type, timing, and intensity of

natural disturbance. The following is a discussion of the nature of two

processes—grazing and fire—in presettlement ecosystems.

4.1.1 Grazing Characteristics

Soil and plant characteristics of low- and mid-elevation arid

and semi-arid ecosystems in North America west of the Rocky

Mountains indicate that these ecosytems evolved with low levels of

soil surface disturbance. This is suggested by these ecosystems’

dependence on nitrogen provided by the biological soil crusts (Evans

and Ehleringer 1993; Evans and Belnap 1999) and by dominant

bunchgrasses’ lack of adaptation (such as tillering, secondary

compounds, or high tissue silica content) to grazing (Mack and

Thompson 1982; Martin 1975; Stebbins 1981). Large mammal

numbers would have been low due to limited surface water and sparse

vegetation. Limited water availability would have restricted use of

lower elevations to winter seasons, as is seen today (West 1988;

Parmenter and Van Devender 1995). Dung beetles, present globally

in other systems with large mammal herds, are also lacking in western

North America (Fig. 4.1; Mack and Thompson 1982). Shallow soils

can also limit distributions of burrowing vertebrates and invertebrates, further reducing soil surface disturbance. Current natural

Figure 4.1 Comparison of community structure in ecosystems that evolved with (midwestern prairie) and without

(desert grassland) large herds of grazing mammals. forty-one
Biological Soil Crusts:

Ecology & Management

NOTES disturbance by large mammals and burrowing fauna in western North

American deserts is greatest in the Sonoran and Chihuahuan Deserts,

moderate in the Mojave Desert and Great Basin, and low in the

Colorado Plateau. Because of their evolutionary history, these regions

appear to be more negatively affected by soil surface disturbances

than regions like the Great Plains that evolved with higher levels of

surface disturbance (see section 4.2).

4.1.2 Fire

In arid and semi-arid native communities, plants have patchy

distributions that result in discontinuous fuel and mosaics of various

fire intensities (Whisenant 1990). Biological soil crusts provide little

fuel to carry a fire through interspaces, thereby acting as “refugia” to

slow the spread of fire and decrease its intensity (Rosentreter 1986).

Unburned islands of vascular vegetation and biological soil crust

provide propagules for reestablishment in burned areas (Fig. 4.2).

Johansen et al. (1993) observed that the crust’s structural matrix was

left intact following low-intensity fire, indicating that a lightly burned

crust still functions to maintain stability against erosive forces for

both vascular plants and biological soil crusts during the recovery

period.

Fire is a natural determinant of the sagebrush-steppe potential

vegetation types. Historical fire patterns helped create mosaics of

successional stages in both vascular plant and biological soil crust

communities. More productive sites generally have fire-return

intervals of less than 30 years (Burkhardt and Tisdale 1976; Arno and

Gruell 1983; Fisher et al. 1987). Although this time period might

constrain biological soil crust recovery, higher precipitation enhances

regrowth (see section 4.3.3; McCune 1993). Fifty to 100 years has

often been cited as the average return interval in shrub-steppe regions

(Wright et al. 1979; Peters and Bunting 1994) and is adequate to

restore biological soil crust components. However, there is no direct

evidence for fire being a cyclic disturbance in lower elevation shrubsteppe. Here, shrub die-off from insect, disease, and winter kill is

common and is probably responsible for most shrub turnover

(McArthur et al. 1990). Historic fires were probably small, low

intensity, patchy, and uncommon because of sparse and discontinous

vegetation. Fire was not historically a common disturbance in most

salt-desert shrub types (West 1994), except for the more productive

types supporting Great Basin wildrye (Elymus cinereus). Likewise,

sparsity of vegetation in the southwestern deserts resulted in long firereturn intervals and low-intensity fires (Allen 1998).

forty-two
Biological Soil Crusts:

Ecology & Management

NOTES

forty-three

Figure 4.2 Natural and altered fire cycles in sagebrush communities of the Great Basin. Fire at natural frequencies

(50-100 year intervals or longer) occurred in a mosaic of intensities over the landscape due to discontinuity of fuel.

This allowed natural regeneration of both the vascular plant and biological soil crust communities. Invasion of exotic

species such as cheatgrass into these communities resulted in continuous fine fuels and larger, more intensive fires.

Cheatgrass will often dominate the community following fire. Revegetation with perennial species can help reestablish a more natural community structure.

or
Biological Soil Crusts:

Ecology & Management

NOTES 4.2 Disturbance Effects

Disturbance can directly and indirectly affect many aspects of the

structure and function of biological crust communities, including cover,

species composition, and carbon and nitrogen fixation. The impact of a

given disturbance depends on its severity, frequency, timing, and type, as

well as the climatic conditions during and after it.

Methods of assessing impacts of, and recovery from, disturbance have been highly variable. Generally, measurements have been

limited to visual estimates of crust cover. However, Belnap (1993)

showed visual assessment can only accurately assess moss and lichen

cover, and cannot be used to measure the degree of recovery of

cyanobacterial biomass, soil stability, and/or physiological functioning

of crustal organisms. In addition, some studies have only considered

total crust cover but have not delimited the relative cover of

cyanobacteria, mosses, and lichens. The relationship between total

crust cover and disturbance impacts can be weak, as cyanobacterial

cover generally increases, while moss and lichen cover decreases, after

disturbance. This often makes total crust cover a poor measure of the

dynamics of soil crust recovery. Differentiating between crustal

components is also important because alteration of species

composition can heavily influence the crust’s ecological functioning

(Eldridge 1998). Comparing recovery rates from different studies can

be problematic, as factors known to control recovery rates (such as

site stability and precipitation following disturbance) are often not

reported. More importantly, disturbance severity is seldom quantified.

Studies generally report disturbance levels as “light,” “moderate,” or

“heavy” without any definition of these categories; thus, what is

“moderate” in one study may be considered “heavy” in another. As

studies cover a large range of climatic zones, soil types, and levels of

disturbance, and as there has been no standard for measuring crust

recovery, it is not surprising that in the literature recovery rates have

ranged widely (2 to more than 3,800 years), and either appear to

show no pattern or often appear contradictory (Anderson et al.1982a;

Callison et al. 1985; Jeffries and Klopatek 1987; Cole 1990; Belnap

1995, 1996; Belnap and Warren 1998).

4.2.1 Disturbance Severity, Size, Frequency, and Timing

The impact of a disturbance is affected by its severity, size,

frequency, and timing. While most compressional disturbances (such

as from vehicles and trampling by people or animals) result in similar

types of impacts, severity can vary widely depending on disturbance

source. For instance, vehicles and trampling exert compressional and

shear forces; however, these forces are much greater for vehicles than

trampling. In addition, vehicles often turn soils over and bury crustal

forty-four
Biological Soil Crusts:

Ecology & Management

NOTES

forty-five

organisms, while trampling tends to only compress the surface.

Vehicle tracks often channel water off-site and thus slow or prevent

recovery (Webb and Wilshire 1983). Intensifying physical impacts

(such as high-intensity, short-duration grazing) is deleterious to

biological soil crust cover and its species richness (Johansen 1993).

Disturbance that removes or kills crustal organisms results in greater

impact and slower recovery than disturbance that leaves crushed crust

material in place. As lichens and mosses are less tolerant of

disturbance than cyanobacteria, frequent disturbance can maintain

the biological soil crust at a low-successional stage (e.g., dominated by

cyanobacteria; Fig. 4.3, Harper and Marble 1988).

Disturbance timing can affect the degree to which the cover

and species richness of a biological crust is reduced. Soils have

different intrinsic soil strengths that vary with moisture content (see

Fig. 2.5). Soils with little tendency to form aggregates, such as sands,

are more susceptible to compressional stresses when dry. Fine-textured

soils or those with inorganic crusts are more vulnerable to

compressional disturbance when wet (Webb and Wilshire 1983).

Crust components are brittle when dry, and the connections they

make between soil particles are easily crushed. Thus, compressional

disturbances can severely affect the crust’s ability to stabilize soils,

especially in dry sandy and silty soils. On silty soils of the Great

Basin, early wet season (winter) use by livestock has been shown to

have less impact on crust cover and species composition than late

winter or spring use. As crustal species are only metabolically active

when wet and are brittle when dry, disturbance in dry seasons is

generally more destructive, and organisms are less able to recover,

than when disturbed in wet seasons (Harper and Marble 1988;

Marble and Harper 1989). Crusts on clay soils can be an exception,

as they are often more vulnerable when wet (Fig. 2.5).

4.2.2 Disturbance Effects on Species Composition

Disturbance generally results in loss of species diversity,

biomass, and surface cover of biological crust components. The more

severe the disturbance, the greater the loss. Thus, after severe disturbance, the resulting crust community is generally greatly simplified

from multiple species of cyanobacteria, lichens, and mosses to a

community often dominated by one or a few species of cyanobacteria.

4.2.2.1 Air Pollution: A few studies have addressed the

impact of air pollutants on soil lichens in desert environments. No

differences have been found in species composition near pollution

sources when compared to control sites. It is reasoned that lichens

with thalli closely appressed to the soil surface, a condition common
Biological Soil Crusts:

Ecology & Management

NOTES Figure 4.3 Successional sequence for biological soil crusts. This example is for Microcoleus vaginatus-dominated

crusts on the Colorado Plateau. Sequences in other ecoregions are similar but may involve different taxa.

(Illustration by Gloria Brown)

forty-six
Biological Soil Crusts:

Ecology & Management

NOTES

forty-seven

to most desert soil crust lichens, are less susceptible to damage by air

pollutants than lichens whose tissues are more exposed to air. In

addition, most desert soils are very alkaline, and thus thought to

buffer acidity from pollutants (Sheridan 1979; Nash and Sommerfeld

1981).

4.2.2.2 Oil Spills, Insecticides, and Herbicides:

No known studies have directly addressed the effects of oil, oil

dispersants, or insecticides on species composition of intact soil

crusts. However, there has been a great deal of work on individual

cyanobacteria, green algae, and mosses isolated from soil crusts. These

experiments have shown that crustal species are differentially affected,

depending on the compound and the species tested. Thus, exposure

to these agents could potentially alter species composition of crusts

(Metting 1981).

One study addressed herbicide effects on intact biological soil

crusts. Direct application of two glyphosate herbicides (Roundup®

and Accord®) on moss-dominated biological soil crusts had no shortterm negative impact on bryophyte cover (Youtie et al. 1999). In fact,

bryophyte cover decreased significantly in control plots due to litter

buildup from exotic annual grasses that had invaded the site (see

4.2.2.3 below), while cover stayed the same or increased slightly in

treated plots. However, repeated treatments are often required to

effectively control weedy species. There is little information on the

effects of repeated application or long-term effects of glyphosate and

other herbicides. Therefore, caution should be used when applying

these chemicals to remnant native areas supporting biological soil

crusts (Youtie et al. 1989).

4.2.2.3 Annual Plant Invasion: Invasion of exotic

annual plants into perennial plant communities can pose a long-term

threat to biological soil crusts, as the crust-dominated interspace

between perennial plants is often heavily invaded. Surveys in invaded

communities show rich perennial moss/lichen communities are

quickly replaced with only a few species of annual mosses and

cyanobacteria (Kaltenecker 1997; Belnap and Phillips in press). The

mechanism by which this shift occurs is not known, but probably

results from a decrease in available soil surfaces (via increased cover of

live plants and litter; Fig. 4.2, 4.4), higher cover of plant material

shading the soil surface, and/or increased fire frequency (Kaltenecker

1997; Kaltenecker et al. 1999a; Youtie et al. 1999).

4.2.2.4 Fire: Biological crusts are generally killed by

hot ground fires, resulting in loss of biomass and visible cover

(Johansen et al. 1993). Frequent fires prevent recovery of lichens and
Biological Soil Crusts:

Ecology & Management

NOTES

forty-eight

Figure 4.4 Changes in the vascular plant community structure due to cheatgrass invasion.
Biological Soil Crusts:

Ecology & Management

NOTES

forty-nine

mosses, leaving only a few species of cyanobacteria (Whisenant 1990;

Eldridge and Bradstock 1994). Damage to, and recovery of,

biological crusts depend on the pre-fire composition and structure of

the vascular plant community and on fuel distribution, fire intensity,

and fire frequency. Historic fire regimes in semi-arid and arid

landscapes generally left small-scale patches of unburned areas

between perennial plants, and/or larger-scale patches of unburned

shrubs across the landscape (Fig. 4.2). This resulted in a mosaic of

successional stages of plants and biological crusts, with propagules

readily available to replenish burned sites. Historic fires were also

relatively infrequent, leaving time for later-successional crustal

organisms to recolonize (Whisenant 1990; Peters and Bunting 1994).

Many semi-arid areas are now invaded by annual weeds, and

unnaturally frequent, large fires that preclude crustal species’

recolonization or succession are common.

4.2.2.5 Mechanical Disturbance: Mechanical

disturbance results from activities such as vehicle traffic (military and

recreational), trampling by livestock and people, and land-clearing

(such as mining). Such uses are increasing exponentially in arid and

semi-arid areas of the world. Effects of mechanical disturbance are

especially noticeable at sites with highly erodible soils and large

topographic relief (Harper and Marble 1988).

Figure 4.5 Susceptibility of biological crusts to mechanical disturbance based on

dominant morphological group. High Low

Low High

Increasing Morphological Complexity

Susceptibility to Mechanical Disturbance

cyanobacteria

crustose lichens

squamulose lichens

gelatinous lichens

short mosses

foliose lichens

liverworts

tall mosses

fruticose lichens

short mosses

foliose lichens

liverworts

tall mosses

crustose lichens

squamulose lichens

gelatinous lichens

cyanobacteria

fruticose lichens
Biological Soil Crusts:

Ecology & Management

NOTES

fifty

Over 30 studies on four continents document that livestock

grazing, vehicle use (both recreational and military), and human

trampling dramatically reduce lichen/moss cover and species richness

of crusts. Resistance to disturbance generally decreases as the

organisms become more morphologically complex (Fig. 4.5; Harper

and Marble 1988; West 1990; Johansen 1993; Eldridge and Greene

1994; Ladyman and Muldavin 1996). Cyanobacteria, the most

resistant to disturbance, are highly mobile and can recolonize

disturbed surfaces rapidly. Lichens use a combination of adaptive

thallus structures and pigments (Blum 1973; Galun et al.1982), water

storage capacities, tolerance of frequent and/or prolonged inundation,

and/or an ability to fix atmospheric nitrogen to increase resistance to

disturbance. For example, Rogers and Lange (1971) showed that

lichens Collema coccophorum and Heppia lutosa (H. despreauxii) were

the least affected by sheep trampling around a watering point

compared to other taxa. These lichens are able to fix atmospheric

nitrogen and to store greater amounts of water than stratified green

lichens (Galun et al. 1982). In a comparison of species inside and

outside a grazing exclosure, vagrant foliose lichens Chondropsis

semiviridis and Xanthoparmelia convoluta, crustose lichens

Diploschistes scruposus and Caloplaca spp., and the squamulose lichens

Peltula imbricata were associated with ungrazed sites (Table 4.1; Crisp

1975; Eldridge and Koen 1998). The gelatinous lichen Collema

coccophorum was the most tolerant of livestock trampling (Rogers and

Lange 1971; Eldridge 1996).

Most soil surface disturbance compacts soils. Compaction

influences soil water and nutrient-holding capacity, which can lead to

changes in soil crust community species composition. These subtle

compositional changes often occur before cover changes are apparent

(Eldridge 1996).

4.2.2.6 Burial: Crust disruption often destabilizes

underlying soils, leaving adjacent crusts vulnerable to burial by windand water-moved sediments. When soils are moist, the large

filamentous cyanobacteria can respond to burial by moving up to 5

mm every 24 hours. When dry, these organisms are not able to move.

Burial kills non-mobile photosynthetic components of the crust,

including mosses, lichens, green algae, and smaller cyano-bacteria

(Campbell 1979). Therefore, burial generally results in a greatly

simplified crustal community.

4.2.3 Disturbance Effects on Nutrient Inputs and Retention

4.2.3.1 Carbon Fixation: Cyanobacterial crusts near coalfired power plants have greater chlorophyll a per unit soil surface area
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-one

Table 4.1 Percent frequency of biological soil crust organisms on loamy and sandy soils inside and outside the exclosure at Koonamore Vegetation Reserve, South

Australia, in May 1972. Adapted from Crisp (1975).

LOAMY SOIL SANDY SOIL

Species Morphological Group Exclosed Grazed Exclosed Grazed p

Lichens

Acarospora smaragdula crustose 2.4 0.5 7.6 2.2 n.s.

Aspicilia calcarea crustose 39.4 20.6 26 26.2 *

Aspicilia calcarea fruticose 13.6 1.1 18.2 2.4 *

Chondropsis semiviridis foliose 0.08 0 0.1 0 n.s.

Collema coccophorum gelatinous 81.4 63.6 21 0 *

Diploschistes scruposus crustose 0.6 0 2.4 35.6 n.s.

Fulgensia subbracteata crustose 20 5 25.2 14.2 *

Psora decipiens squamulose 38.7 15.1 33.8 24.6 *

Psora crystallifera squamulose 11.4 0.7 13.8 2 *

Toninia sedifolia squamulose 13.1 3.8 13.8 7.6 *

Xanthoparmelia convulata foliose 0.04 0 0.1 0 n.s.

Liverworts

Riccia lamellosa liverwort 4.2 0.4 8.2 1.2 *

* Indicates significant effect of exclosure at p<0.10; n.s. = effect of excosure was not significant.
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-two

than crusts away from the plant, implying higher levels of carbon

fixation. This is partially a result of fertilization by nitrogen and sulfur

compounds from effluents (Sheridan 1979; Belnap 1991).

Photosynthesis-inhibiting herbicides show significant impact on Nostoc

growth and nitrogen fixation (Gadkari 1988). In general, herbicides

inhibit growth and reproduction in culture (Metting 1981). However,

effects appear to be more pronounced in the laboratory than the field,

and may be transitory (Prasad et al. 1984).

Alteration in crust species composition will affect total carbon

fixation, as lichens and mosses fix more carbon per unit soil surface

area than cyanobacteria (Phillips and Belnap 1998). Because much of

the carbon fixed by crustal organisms is released into the surrounding

soils (Lewin 1956), crust cover reduction is expected to reduce soil

carbon available for microbial populations that are often carbon

limited. This, in turn, may affect decomposition rates of plant litter,

and thus, levels of nutrients available to vascular plants (Paul and

Clark 1996).

4.2.3.2 Nitrogen Inputs: Power plant effluents have

been shown to decrease nitrogen fixation in Collema and Microcoleus/

Nostoc/Scytonema-dominated crusts (Sheridan 1979; Belnap 1991).

Atmospheric nitrogen deposition may offset reduced nitrogen inputs

from crusts; alternatively, anthropogenic nitrogen deposition may

aggravate nitrogen loss through increased ammonia volatilization and

denitrification. In addition, biological soil crusts release ammonia in

the soils, while anthropogenic nitrogen deposition contains large

amounts of nitrate (Garcia-Pichel, unpublished data). Microbial and

vascular plant species differentially use ammonia and nitrate; thus,

deposition of additional nitrates may alter the dynamics of both soil

and plant communities (Binkley et al. 1997).

Table 4.2 Reduction of nitrogenase activity for new and older disturbances on silty soils of the Dugway Proving Ground,

Utah. All disturbance types resulted in material left on site except “Scalp,” which removed the top 1 cm of crust.

Age of Disturbance Type of Disturbance Percent Reduction in Nitrogenase Activity

New Vehicle 68

New Bike 79

New Foot 62

6 months Vehicle 100

9 months Tank 83

9 months Scalp 95

9 months Rake 81
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-three

Free-living or lichenized Nostoc show stimulation of nitrogen

fixation at low concentrations of, or short exposure to, arsenic, nickel,

lead, palladium, and zinc. However, longer-term exposure to cadmium,

lead, and zinc inhibits fixation (Henriksson and DaSilva 1978). Exposure

to crude oil and oil dispersants decreases nitrogen fixation in Nostoc

(Marowitch et al. 1988). Nitrogen fixation is significantly inhibited in

Nostoc by many insecticides, herbicides, and phenolic compounds tested

(Bhunia et al. 1991; Megharaj et al. 1988).

Mechanical disturbance can result in large decreases in soil

nitrogen through a combination of reduced input and elevated losses

(Peterjohn and Schlesinger 1990; Evans and Belnap 1999). In all soils

tested, disturbance by vehicles, human foot traffic, mountain bikes,

and raking immediately reduces nitrogen input from crusts (25 to

40% on silty soils; 76 to 89% on sandy soils). Over time, nitrogenase

activity can drop by 80 to 100% relative to controls, due to

subsequent death of buried material (Table 4.2; Belnap et al. 1994;

Belnap 1995,1996).

Species composition changes also affect nitrogen inputs, as

cyanolichens (such as Collema) fix an order of magnitude more

nitrogen than the equivalent soil surface area of cyanobacteria. Thus,

the shift from a lichen crust to a cyanobacterial crust can result in less

nitrogen entering the ecosystem, as has been shown in multiple

studies. Jeffries et al. (1992) showed that heavy grazing reduced

nitrogen fixation in sandy soils by 95%. In silty loam soil, Terry and

Burns (1987) showed a 64% reduction of nitrogen fixation in burned

areas, 85 to 94% reduction in grazed areas, and 99% reduction in a

tilled area. Collema cover was reduced 50 to 80% in grazed areas

relative to adjacent ungrazed areas (Brotherson et al. 1983; Johansen

and St. Clair 1986). Expected nitrogen inputs would be reduced

accordingly. Evans and Belnap (1999) showed nitrogen fixation in an

area released from grazing 25 years was still 2.5 times less than an

adjacent, never-grazed area due to reduction in Collema cover.

Decreased nitrogen inputs from crusts can have long-term

impacts on soil nitrogen levels. Jeffries (1989) found 50% less

nitrogen in grazed soils compared to adjacent ungrazed soils. Evans

and Belnap (1999) found a 42% decrease in soil nitrogen and 34%

decrease in plant tissue nitrogen when comparing a previously grazed

(released 30 years previous to the study) site to an adjacent ungrazed

area. In the same area, stable nitrogen isotopes showed that both soil

and plants in the grazed area contained less newly fixed nitrogen than

the ungrazed area, and nitrogen mineralization potential decreased

almost 80% (Rimer and Evans 1997). This has large implications for

ecosystems that are dependent on biological crusts for nitrogen

inputs, such as those on the Colorado Plateau (Evans and Ehleringer

1993).
Biological Soil Crusts:

Ecology & Management

NOTES 4.2.4 Disturbance and Vascular Plants

Crusts can affect vascular plant germination, survival, and

nutrition. Lichen-moss crusts in areas with frost heaving (as in cool

deserts) increase perennial vascular plant seed entrapment,

germination, establishment, survival, biomass, and nutritional status

(Belnap and Harper 1995). In these deserts, disturbance that flattens

frost-heaved surfaces and/or eliminates mosses and lichens can have a

negative effect on some or all of these characteristics. However,

germination and survival of some exotic annual grasses can be

enhanced with biological crust disturbance.

In hot deserts, seed entrapment, biomass, and fecundity of

annual plants is generally increased when smooth cyanobacterial

crusts are disturbed (Prasse 1999), although there are some exceptions

(Gutterman 1994). Disturbance of smooth cyanobacterial crusts

enhances germination of some annual species, while decreasing

germination of others (Zaady et al. 1997). Rugose crusts in hot

deserts have received little study, and no work has addressed the

effects of disturbance on seed entrapment. Effects on germination in

rugose crusts appear to be species dependent. In contrast to smooth

crusts, disturbance to hot desert rugose crusts lessens survival and

biomass of perennial plants (McIlvanie 1942; Crisp 1975).

Biological soil crusts have been shown to influence availability

of many plant-essential nutrients. Crust loss can result in less plantavailable magnesium, potassium, iron, calcium, phosphorus,

manganese, and sulfur (Harper and Belnap in press). The spatial

distribution of nutrients is also affected by disturbance, as disturbance

will alter the relative distribution of biological crust components (i.e.,

lichens, mosses, cyanobacteria) across the landscape (Klopatek 1992).

4.2.5 Disturbance and Surface Albedo

Trampling of dark-crusted surfaces exposes underlying, lighter

soils, thus increasing albedo (reflectance) by up to 50% at most

wavelengths measured (Fig. 3.1). This increased albedo represents an

energy loss from the soil surface of approximately 40 watts/m2 and

can reduce soil temperatures by up to 14o

C (Belnap 1995). Such

changes in reflectance are especially apparent using satellite imagery,

as can be seen in the Negev Desert, along the U.S.-Mexico border,

and in Australia. Much of this difference is attributed to loss of

biological soil crusts (Graetz and Tongway 1986). Large-scale changes

in surface albedo may contribute to changes in regional climate

patterns (Sagan et al. 1979).

fifty-four
Biological Soil Crusts:

Ecology & Management

4.2.6 Disturbance and Soil Hydrology NOTES

The effect of biological soil crust disturbance on soil

hydrology is very site specific. Water infiltration, runoff, and resultant

soil moisture are influenced by soil surface roughness, soil texture,

microtopography, surface albedo and temperature, vegetative cover

(both vascular and non-vascular), and climatic conditions. All these

factors can be altered when biological crusts are disturbed. In general,

disturbance flattens pinnacled and rolling crusts, thus decreasing

water infiltration and increasing runoff (Fig. 3.3). In hot deserts,

disturbance to smooth and rugose lichen- or cyanobacterial-crusted

surfaces can increase overall infiltration rates (Greene et al. 1990;

Eldridge et al. 2000). However, this increase in infiltration may

negatively affect nearby vascular plants that depend on interspace

runoff for survival (see Section 3.6). Soil structure alteration as a

result of soil compaction can also reduce infiltration rates. Available

soil moisture results from a complex interaction of many soil and

plant characteristics, including infiltration, soil moisture-holding

capacity, albedo, plant root density, etc. Only two studies have

addressed the combined effects of crusts on available soil moisture,

with disparate results. One study found increased soil moisture under

intact crusts, while the other found soil moisture increased under

disturbed crusts (reviewed in Harper and Marble 1988).

4.2.7 Disturbance Relative to Water and Wind Erosion

Compressional disturbance damages soil-stabilizing

cyanobacterial sheath material both at and below the soil surface.

The damage below the surface is irreparable, as living cyanobacteria

are not present to regenerate sheath material. As greater than 75% of

photosynthetic biomass and productivity is from organisms living in

the top 3 mm of soils, disturbance that results in even small soil losses

can dramatically reduce site fertility and further reduce soil surface

stability (Garcia-Pichel and Belnap 1996).

4.2.7.1 Water Erosion: Globally, all studies show crust

cover loss significantly increases water erosion of both coarse- and

fine-textured soils. Continuous tracks have a greater impact than nonconnected disturbances (such as hoof prints) because of increased

water flow volume and velocity along the tracks (Webb and Wilshire

1983). Crushing of cyanobacterial connections between particles

results in smaller, isolated soil aggregates that are then more easily

moved by water. In addition, surfaces flattened by compressional

disturbances are less able to reduce water velocity, thereby allowing

fifty-five
Biological Soil Crusts:

Ecology & Management

NOTES movement of larger soil particles and reducing opportunities for

sediment to settle out.

4.2.7.2 Wind Erosion: All studies of wind erosion

indicate that disturbed soils are more susceptible to wind erosion than

undisturbed soils when dry (Fig. 4.6). When crusts are crushed or

absent, soil particle movement is initiated at lower wind speeds, as

resistance to wind erosion increases with better soil crust

development. Well-developed crusts (with lichens and mosses) on

both silt and sandy soils have 2 to 130 times greater resistance to soil

erosion than less well-developed crusts or bare soil (Fig. 4.7; Williams

et al. 1995b; McKenna-Neuman 1996; Belnap and Gillette 1997,

1998; Leys and Eldridge 1998). Vehicle tires result in greater damage

than hooves on a given soil type (Belnap and Gillette 1998).

Decreased resistance of soils to wind erosion is directly

associated with increased sediment movement. All experimental

disturbances applied to crusts show increased sediment production—

up to 35 times that of adjacent undisturbed crusts (Leys and Eldridge

1998; Williams et al. 1995b). Nearby soil crusts are often buried by

blowing sediment, resulting in death of photosynthetic organisms.

4.3 Factors Influencing Natural Recovery Rates

Recovery rates are dependent on many factors, including

disturbance type, severity, and extent; vascular plant community

structure; adjoining substrate condition; inoculation material

availability; and climate during and after disturbance. Comparing

recovery rates reported in the literature is highly problematic, as

studies range over a wide array of climatic zones, soil types, and levels

of disturbance, and these factors are seldom quantified. Thus, most

studies can only be used to define the general range of recovery rates.

Some studies do report climate, soil, and/or disturbance data such

that comparisons are possible (Anderson et al. 1982a; Jeffries and

Klopatek 1987; Callison et al. 1985; Cole 1990; Belnap 1995, 1996;

Eldridge 1996; Belnap and Warren 1998).

Examination of these studies indicates that general recovery

times can be predicted for soil crusts in different environments. Most

recovery time calculations for crust cover assume a linear recovery

rate. On the Colorado Plateau, scalped plots were reassessed 2 to 5

and 10 to 14 years after disturbance. After 2 to 5 years, cyanobacterial

cover was predicted to recover in 45 to 110 years, whereas it recovered

within 14 to 34 years. Thus, linear assumptions greatly overestimated

recovery time. Lack of moss recovery prevented useful estimates at

two of the three sites at the earlier sampling time. At one site, early

estimates were 400 years to recovery, while later estimates were 42

fifty-six
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-seven

Figure 4.6 Resistance of sandy loam soils to wind erosion following disturbance to a well-developed biological soil

crust in four time classes. Threshold friction velocity is the force required to detach soil particles from the surface.

Time classes indicate the length of time since disturbance to the control. Following determination of threshold

friction velocities for controls, treatments were applied as follows: Foot = one pass wearing lug-soled boots; Tire,

1 Pass = one pass of a four-wheel drive vehicle with knobbed tires; Tire, 2 Pass = two passes of a four-wheel drive

vehicle with knobbed tires. Within each time sequence, controls were significantly more resistant to wind erosion

than treated surfaces. Adapted from Belnap and Gillette (1997).

Figure 4.7 Resistance of

soil surfaces to wind

erosion. Resistance

increases with surface

protection and development of the biological soil

crust. The greatest amount

of resistance is demonstrated by soils with welldeveloped biological crusts

characterized by a diverse

lichen flora. Adapted from

Belnap and Gillette

(1998). Threshold Friction Velocity

Threshold Friction Velocity
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-eight

years. As with cyanobacteria, linear extrapolations overestimated

recovery time. In contrast, Collema recovery after 3 years was

estimated at 85 years; after 14 years, the estimate for full recovery was

50 years, a fairly close match. Collema recovery at a second site was

highly variable: after 2 years, recovery time was estimated at 487

years; after 11 years, estimates were site dependent, ranging from 40

to 766 years. Original estimates were therefore both under and over

estimated, depending on individual site characteristics. Sites with

more shade and less sandy soils were quicker to recover than original

estimates predicted, while more exposed sites with sandier soils were

less able to recover than originally estimated (Belnap, unpublished

data).

4.3.1 Sequence of Species Appearance

Cyanobacteria and green algae are generally the first

photosynthetic species to appear on disturbed soils, probably because

of their mobility across soil surfaces and their ability to colonize from

the air (Schlichting 1969). Large filamentous cyanobacteria such as

Microcoleus generally appear first (Fig. 4.3, 4.8), especially on unstable

sandy soils (Ashley and Rushforth 1984; Belnap 1995). This species is

then followed by smaller cyanobacteria and green algae. Occasionally,

desert soils are slightly acidic, and here green algae can be the first to

appear (Johansen 1993).

Lichens and mosses require stable soil surfaces for growth.

This can be provided by a physical crust or by large, filamentous

cyanobacteria. Once soils are stabilized, gelatinous nitrogen-fixing

lichens (e.g., Collema spp.) are generally the first lichens to appear in

disturbed areas, followed by other early-successional lichens and

mosses (Table 4.3; Johansen et al. 1984; Belnap 1993). Earlycolonizing lichens often disperse phytobiont-containing tissue (e.g.,

isidia, soredia, or algal-covered spores), as opposed to bare spores,

thus precluding the need to find phytobionts on-site.

4.3.2 Soil Texture

Soil texture influences crust recovery rates. Recovery of all

crust components is faster in fine-textured soils than in coarsetextured soils, as fine-textured soils are often stabilized by chemical

and rain crusts and retain soil surface moisture longer (Fig. 4.9; as

reviewed in Harper and Marble 1988; Johansen 1993; Ladyman and

Muldavin 1996). Recovery of the site’s wind resistance is also more

rapid in fine-textured soils, probably due to crust formation after

rainfall. While silty and sandy soils show a similar reduction in wind

resistance to vehicle disturbance (83% and 74%, respectively), silty

soils show a 50% recovery of wind resistance after a single large rain
Biological Soil Crusts:

Ecology & Management

NOTES

fifty-nine

Figure 4.8 Generalized recovery sequence for biological soil crusts. Successional stages are illustrated as steps on the diagram. Disturbance input (indicated by arrows) will

cause the crust to revert to an earlier successional state, with severe disturbance resulting in complete destruction of the crust. Recovery time is dependent on environmental

conditions, including effective precipitation (see Chapter 2). Note that recovery times, for mid- and late- successional species in areas with lower effective precipitation are

unknown, as recovery times are so long no estimates are possible.

Late Colonizers

Mid Successional Species

Early Colonizing Lichens and Mosses

Gelatinous Lichens

Small Cyanobacteria and Green Algae

Large Filamentous Cyanobacteria

Severe Disturbance

Resulting in Bare Soil

Geographic Region

(elevation, average annual precipitation) Estimated Years to Recovery

Lower Mojave (<1500 m, 100 mm)

Gelatinous

Lichens

Early

Colonizers

Mid

Successional

Late

Successional

1,200 3,800

High Mojave (1500 m, 200 mm) 200 800

Colorado Plateau (1500 m, 200 mm) 50 500

Northern Great Basin (1000 m, 350 mm) 20 25

unknown unknown

unknown unknown

60 125

unknown unknown

Mid-successional Species
Biological Soil Crusts:

Ecology & Management

NOTES

sixty

Figure 4.9 Biological soil crust vulnerability and recoverability based on site stability, effective precipitation, and

disturbance regime. Vulnerability decreases and recovery rates increase with increased site stability and effective

precipitation, and infrequent disturbance.
Biological Soil Crusts:

Ecology & Management

NOTES

sixty-one

event, while very sandy soils can take up to 10 years for similar

recovery (Belnap and Gillette 1997; Belnap and Herrick, unpublished

data).

4.3.3 Climate Regimes

Crustal organisms are metabolically active only when wet;

thus, recovery is faster in regions and microsites with greater effective

precipitation (Fig. 4.8, 4.9; Johansen et al. 1993; Harper and Marble

1988). Crusts on north and east slopes, as well as at higher elevations,

will recover more quickly than crusts on south and west slopes and at

lower elevations. However, as discussed in Sections 2.2.1, 2.2.5, and

2.2.6, there are thresholds along ecological gradients where adequate

precipitation results in development of a more diverse and dense

vascular flora in lieu of the biological crust.

4.3.4 Disturbance Type, Frequency, Severity, and Size

No work has addressed recovery rates of lichens or

cyanobacteria from air pollution. In general, crusts are highly

susceptible to hot fires; thus, recovery will depend on the size and

intensity of fires. As noted previously in “Disturbance Effects”

(Section 4.2), most compressional disturbances have similar types of

impacts. However, severity of mechanical disturbance can vary widely

with disturbance type. Thus, on similar soils, vehicle tracks generally

have longer recovery times than disturbances that do not churn the

soil or make continuous tracks (Wilshire 1983; Belnap 1996).

Table 4.3 Characteristics of early-colonizing lichens.

Species Phytobiont Reproductive Units

Caloplaca tominii green algae soredia

Cetraria islandica green algae isidia-like cilia

Cladonia chlorophea group green algae soredia

Collema spp. cyanobacteria isidia

Cornicularia normoerica cyanobacteria finger-like lobes

Endocarpon pusillum cyanobacteria algal-covered spores

Leprocaulon microscopum green algae soredia

Leptogium corniculatum cyanobacteria isidia

Peltigera didactyla cyanobacteria / green algae isidia

Peltigera rufescens cyanobacteria spores, unspecialized

thallus fragmentation

Placidium squamulosum green algae / cyanobacteria algal-covered spores
Biological Soil Crusts:

Ecology & Management

NOTES

sixty-two

 Repeated disturbance will generally keep crusts at an earlysuccessional stage (e.g., cyanobacteria-dominated) by preventing

lichen or moss colonization (Fig. 4.8; Belnap 1996).

Disturbance severity often controls recovery rates. In western

North America, site recovery within a given soil and climate type is faster

where disturbances are not severe enough to remove crustal material than

it is at sites where crusts were removed. When crusts are completely

removed, recovery can be excessively slow, especially in areas with low

effective precipitation and/or sandy soils (Fig. 4.9; Eldridge 1996). In

contrast, crusts crushed in place with vehicles, foot traffic, and horses

recover much faster, especially on fine-textured soils.

Because recolonization of disturbed areas occurs mostly from

adjacent, less-disturbed areas, the size and shape of disturbance can

affect recovery rates. This is especially true for lichens. Desert soil

lichens colonize only infrequently through spores; thus, recovery of

lichen cover is mostly dependent on material washing or blowing in

from adjoining areas (Eldridge and Ferris 1999). The rate of lichen

recovery is much slower in areas with higher internal surface areas

relative to perimeter area (Belnap, unpublished data).

4.3.5 Condition of Adjoining Substrate

In general, crust recovery is slower if soils adjacent to

disturbed areas are destabilized. Detached sediment can both bury

adjacent crusts, killing many crustal organisms, and/or provide

material for sandblasting nearby surfaces, thus increasing wind

erosion (Belnap 1995; McKenna-Neumann et al. 1996; Leys and

Eldridge 1998).

4.3.6 Vascular Plant Community Structure

Crusts recover more quickly under shrub canopies than in

adjacent plant interspaces. This is probably due to greater soil

moisture and fertility under shrub canopies. In the Mojave Desert

lichen recovery (after complete removal 50 years previously) was 36%

under shrub canopies, compared with 4% in the shrub interspaces

(Belnap and Warren 1998). A similar pattern was seen in plots

established on the Colorado Plateau, where lichen recovery was 2 to 3

times faster under shrubs than adjacent interspaces (Belnap,

unpublished data).

The growth of biological soil crusts and vascular plants can be

an interactive process (Danin et al. 1989). Eldridge and Greene

(1994) present a conceptual model in which crust development at

sites is strongly linked to restoration of vital soil processes. As primary

colonizers of recovering sites, biological soil crusts enhance microsite

fertility and provide safe sites for seeds. In turn, established vascular
Biological Soil Crusts:

Ecology & Management

NOTES

sixty-three

plants help stabilize soils, provide shade, and reduce wind speeds at

the soil surface, providing conditions conducive to further

development and diversification of the biological soil crusts (Wood et

al.1982; Danin et al. 1989).

4.3.7 Nitrogen and Carbon Fixation

Overall, nitrogen and carbon fixation rates in biological soil

crusts are dependent on its species composition, biomass, and

physical structure. Thus, recovery is dependent on recovery rates of

specific crustal components. Recovery of nitrogen fixation is

dependent on recovery rates of cyanobacteria and cyanolichens.

Nitrogen fixation in crusts can require anaerobic microzones in the

soils, which may be dependent on buildup of cyanobacterial biomass

(Belnap 1996). Carbon fixation increases when lichens and mosses

colonize (Phillips and Belnap 1998). As recovery is faster with higher

available moisture, both nitrogen and carbon fixation rates are

expected to recover faster in cool deserts than in hot deserts, other

factors being equal.

Lichen cover may not fully recover after disturbance. On the

Colorado Plateau, soils previously occupied by cyanolichens were

often occupied by mosses after release from grazing (Belnap,

unpublished data). As long as cyanolichen recovery does not occur,

reduced nitrogen inputs are expected, and soil and plant nitrogen

levels may remain lower (Evans and Belnap 1999).

4.3.8 Surface Albedo

Restoration of normal surface albedos and temperatures will

depend on the restoration of pre-disturbance cover of cyanobacteria,

lichens, and mosses. While cyanobacteria do form a darkish matrix in

which other components are embedded, mosses and lichens are much

darker and can contribute 40% or more of the cover in an

undisturbed crust (Belnap 1993). Consequently, surface albedo

recovery will be controlled by factors that control lichen and moss

recovery.
Biological Soil Crusts:

Ecology & Management

NOTES

sixty-five

CHAPTER 5

MANAGEMENT TECHNIQUES TO MAINTAIN OR

IMPROVE EXISTING BIOLOGICAL SOIL CRUSTS

While total protection from disturbance is often the easiest

way to maintain or improve biological soil crusts, this is not often

possible or desirable. There are many factors to consider in the

management of soil communities, including disturbance type,

intensity, timing, frequency, duration, or extent. Research is needed

to determine realistic biological soil crust objectives by soil type in

most potential vegetation types, but initial estimates can be

determined by using “healthy” reference areas as described by the

National Research Council (1994). Protection of relic sites as

rangeland reference areas is important, as these sites provide baseline

comparisons for ecological potential and future scientific research.

5.1 Fire

Proactive management is needed to prevent unnaturally large

and/or frequent fires in areas where fuel build-up or annual grass

invasions have occurred. Such management actions may include

altering grazing regimes to prevent annual plant invasions, prescribing

fire to prevent fuel build-up, and/or restricting off-road vehicle use.

5.1.1 Prescribed Fire

Used cautiously, prescribed fire can be a useful tool in some

situations for renewing community vigor. However, the site’s ecology

and evolutionary history need to be understood, as burning can result

in conversion of some sites to exotic vegetation. Thus, even though a

community evolved with fire, fire may no longer be desirable because

of exotic plant invasions. Prescribed fire is not recommended for such

sites unless post-fire restoration treatments are planned. Prescribed

fire may be useful on more productive sites with low potential for

exotic plant invasion to reduce high woody vegetation densities

resulting from overgrazing. Other impacts (such as recreational or

domestic livestock use) should be limited following treatment to

allow full site recovery.
Biological Soil Crusts:

Ecology & Management

NOTES

Figure 5.1 Soil surface

disturbance associated

with post-fire revegetation projects. This site

was drill-seeded with a

rangeland drill.

5.1.2 Post-fire Management

Once a site has burned, evaluation is needed to determine

whether recovery will occur naturally or if revegetation is needed.

Many burned sites, particularly those in the Great Basin and

Intermountain regions, require revegetation to stop exotic plant

invasion, and most techniques require some soil surface disturbance

(Fig. 5.1). This may not appear consistent with recovery of biological

crusts. However, failure to treat sites can result in irreversible

dominance by annual species (such as cheatgrass), which prevents the

return of well-developed biological soil crusts (Fig. 4.2, 4.5, 5.2;

Kaltenecker 1997, Kaltenecker et al. 1999a). Once revegetated,

protection from grazing and recreational use is often necessary for

recovery of the biological soil crust and the vascular plant community.

Recovery in these areas can be further facilitated by use of minimaltill or no-till drills or other seeding methods that minimize soil

surface and compressional impacts. Emphasis should be placed on

restoring the native plant community using local ecotypes, if

available.

In the western U.S., revegetation of salt-desert shrub and

lower-precipitation Wyoming big sagebrush communities presents

some unique problems. West (1994) presents evidence that cheatgrass

will remain and potentially increase in these types, as revegetation is

difficult because of arid conditions and unpredictability of wet years.

He suggests that without livestock consumption of cheatgrass,

susceptibility to fire could increase. However, the effective use of

livestock in long-term control of cheatgrass has not been

demonstrated. In addition, such grazing would occur well into the

growing season and thus to the potential detriment of biological

crusts and native vascular plants.

sixty-six
Biological Soil Crusts:

Ecology & Management

NOTES

5.2 Livestock Grazing

Managing for healthy biological soil crusts requires that

grazing occur when crusts are less vulnerable to shear and

compressional forces. Timing for this is highly dependent on soil type

and climatic regimes (see Chapter 4). Crusts on all soil types are least

vulnerable to disturbance when soils are frozen or snow covered.

Biological crusts on sandy soils are less susceptible to disturbance

when moist or wet; on clay soils, when crusts are dry (Fig. 2.5;

Marble and Harper 1989; Memmott et al. 1998). In general, light to

moderate stocking in early- to mid-wet season is recommended. On

low- to mid-elevation sites, winter use is advantageous to most

vascular plants, including riparian communities, and substantially

reduces supplemental feed costs associated with livestock production.

Winter grazing also most closely replicates the grazing strategy of

native herbivores, who use more productive, higher-elevation sites

during summer and lower-elevation sites in winter (Fig. 5.3; Miller et

al. 1994; Burkhardt 1996). It is important to remove livestock well

sixty-seven

Figure 5.2 Biological crust development in a successful seeding in the

northern Great Basin (above) and lack of development in an exotic annual

grass community (below). Note the difference in plant density and the

accumulation of litter on the soil surface that limits biological crust

development in the exotic community.
Biological Soil Crusts:

Ecology & Management

NOTES

Figure 5.3 Native grazers in the western U.S., such as pronghorn antelope

and muledeer, tend to use higher, more productive sites during summer and

move to lower elevation sites for winter forage. They may also move through

areas, rather than staying in one area. This type of seasonality and intensity

minimizes impacts to biological crusts by concentrating use when crusts are

frozen or moist.

before wet season’s end to allow regrowth of crustal organisms before

extended drought. Use when soil surfaces are very wet and muddy

should also be avoided to prevent burying the crust (Kaltenecker and

Wicklow-Howard 1994; Kaltenecker et al. 1999b). Implementation

of rest-rotation strategies that minimize frequency of surface

disturbance during dry seasons and maximize periods between

disturbances will reduce impacts to biological soil crusts.

Dispersal of livestock throughout useable portions of pastures

should also be emphasized. Disturbance of sensitive sites should be

avoided or minimized when they are a small proportion of the overall

pasture or landscape. Ways to disperse livestock include the following:

• Locating water and salt (or other supplements) on sites with

low potential for biological soil crust development and in

areas that discourage livestock from loitering. In many areas,

sites with high rock cover are good options. Livestock trailing

preferences need to be considered when evaluating locations.

• Using brush barriers or fence segments to divert trailing. Sites

with high potential for biological soil crust development are

often not preferred by livestock for forage; however, these

same sites may be open and easy to walk across. Because of

lack of forage, minimal barriers are usually sufficient to

discourage access.

sixty-eight
Biological Soil Crusts:

Ecology & Management

NOTES

Stocking levels and season of use should be ascertained on an

annual basis, jointly by managers and users, with optimal coverage of

both vascular plants and biological soil crusts as the management goal

(Kaltenecker and Wicklow-Howard 1994; Kaltenecker et al. 1999b).

Optimal coverage should be based on site capability and rangeland

health indicators of site stability and nutrient cycling. Livestock

exclusion from reference areas and sites with highly erodible soils or

low vascular plant cover is appropriate to protect biological crusts and

site stability.

5.3 Recreational Use Management

Many recreational activities have impacts similar to those

of livestock use. Therefore, principles relating to management of

livestock disturbance intensity, timing, frequency, duration, or extent

apply to recreational impacts as well. However, there are also major

differences. People are often harder to control than livestock. People

can carry food and water; thus, access to these essentials does not

limit their activities. People tend to go where they want, even in the

presence of barriers such as fences. People also have a greater affinity

for open vegetation, as it is easier to walk or drive through; however,

these same open sites are generally dependent on biological soil crusts

for stability (Fig. 5.4). Education, legal restriction and/or use

stipulations, and compliance activities may require proportionately

sixty-nine

Fig. 5.4 Vehicular disturbance to biological soil

crusts. Recreational disturbances are concentrated

in open interspaces, where crusts are most

abundant and vulnerable. The tracks in the top

photo are approximately 10 years old.
Biological Soil Crusts:

Ecology & Management

NOTES higher management priorities and time requirements than for the

control of livestock.

Concentration of recreational use is generally desirable.

Designated campsite use reduces the impact of haphazard placement

of sites by individuals. Trails minimize the amount of biological soil

crust that is disrupted by trampling. Education can be used to teach

people how to camp in areas without designated campsites (e.g., on

hardened surfaces, such as rocks, or in areas with minimal crust

potential) and how to travel cross-country in areas that lack trails

(e.g., in washes, on rock, on fallen logs).

Recommended management practices include the following:

• Restrict road locations to less sensitive areas. Road

drainage (culverts, water bars) should be designed so that

erosion or sediment fill of adjacent off-site areas is

minimized.

• Promote extensive, low-density uses, such as hiking and

backpacking, during late fall and winter periods. Restrict

access during dry seasons.

• Permit high-density, high-impact uses, such as Christmas

tree and firewood cutting areas, for short durations during

late fall and winter, preferably when soils are frozen. Areas

should be rotated based on a total allowable disturbance

threshold with long recovery periods (greater than 10

years minimum on moderate- to high-resiliency sites,

such as in sagebrush communities [greater than 230 mm

average annual rainfall] in the northern Great Basin)

before redesignation for use. Exclude low-resiliency sites.

• Provide designated trails, and restrict use to trails in highdensity recreational areas.

• Provide interpretive sites and literature on recognition and

value of protecting biological soil crusts at major access

points in areas of extensive or unique crust formation.

• Require an analysis of impacts to biological soil crusts and

appropriate stipulations on all use applications, such as

rights-of-way, oil and gas and other exploration permits,

permits to drill, etc.

seventy
Biological Soil Crusts:

Ecology & Management

NOTES

seventy-one

CHAPTER 6

MONITORING BIOLOGICAL SOIL CRUSTS

Monitoring is the collection and analysis of repeated

observations so that trends can be assessed. Typically, monitoring

is used to evaluate landscape condition changes over time, often in

relation to defined management goals. A monitoring program’s

objectives will determine the position in the landscape at which

measurements will be made and the period over which data will

be collected and assessed. Monitoring is often designed so that

measurements can be made by more than one observer, and the

level of acceptable change is usually determined before monitoring

commences.

Monitoring studies often differ from traditional research

studies in that they are not designed to infer the cause of any

observed changes. Monitoring studies usually lack replication and

often have no controls. Plot sizes and measurement protocols may be

similar in both monitoring and research studies, but the management

objectives are often widely different. The following is a discussion of

field-based monitoring strategies and procedures for biological crusts.

Key attributes for each strategy are summarized in Table 6.1.

6.1 Monitoring Methods

During the past century, rangeland managers have grappled

with methods to assess the health and trend of landscapes (Tueller

1988). Techniques have concentrated primarily on vascular plant

attributes, such as cover, frequency, presence/absence, abundance, and

biomass of various species, particularly perennial plants (Stoddart et

al. 1975; Friedel and Bastin 1988; Friedel et al. 1988; Holechek et al.

1989; Milton et al. 1998). Recently, there has been more emphasis on

soil surfaces, and monitoring techniques to assess soil surface

condition have been developed (Tongway 1994; Pellant 1996;

Whisenant and Tongway 1996; de Soyza et. al. 1997).

While many scientists acknowledge the close links between

biological soil crusts and rangeland condition (Klopatek 1992),

crusts and their component organisms have rarely been recorded

during field-based assessment (West 1990). Early efforts to classify

soil surfaces and include biological soil crusts were developed in the

semi-arid woodlands of eastern Australia (Tongway and Smith 1989).

This assessment showed differences in the stability, erosion, and

nutrient status of soil surfaces, and provided a reliable estimate of

potential productivity of the surface independent of vascular
Biological Soil Crusts:

Ecology & Management

NOTES

vegetation. The method used four classes for soil surfaces: Class 1 =

stable, productive surfaces with high biological crust cover and species

diversity; Class 2 = slightly unstable, moderately productive surfaces

with broken biological crust cover and moderate species diversity;

Class 3 = moderately unstable and unproductive surfaces with low

biological crust cover and species diversity; and Class 4 = very

unstable and unproductive surfaces where biological crusts are absent

(Mucher et al. 1988).

This system was later refined and extended to other landscape

types (Tongway and Hindley 1995) and other soil characteristics,

such as the degree of cracking, surface coherence, microtopography,

and biological crust cover. U.S. workers (Pellant 1996; USDI 1997)

have also included biological crust cover as a component of federal

land monitoring programs. In eastern Australia, the Department of

Land and Water Conservation has been collecting data (including

biological crust cover) on the condition and trend of rangelands since

the mid-1980s (Green 1992). Monitoring staff in Canyonlands

seventy-two

Table 6.1 Attributes of biological crust monitoring strategies. See the text for detailed discussion of each method.

Recording Method: Individual Taxa vs. Morphological Groups

Individual taxa Allows documentation of biodiversity.

More sensitive to change over time.

Morphological groups Quicker, easier identification.

Good evaluation of ecological function.

Less variability.

Less sampling time and intensity to get reliable estimates.

Sampling Methods

Quadrats (using point or cover Useful if clumps are <2 cm diameter or groups/taxa are

highly interspersed.

Use if information on spatial relationships is not desired.

Provides cover estimate.

Line-point Intercept Useful if clumps are <2 cm diameter or groups/taxa are

highly interspersed.

Provides information on spatial relationships.

Provides cover estimate.

Line Intercept Best if clumps are >2 cm diameter and groups/taxa are

not highly interspersed.

Provides information on spatial relationships.

Provides actual measure of cover.

class estimates)

•

•

•

•

•

•

•

•

•

•

•

•

•

•

•
Biological Soil Crusts:

Ecology & Management

NOTES

seventy-three

National Park in southeastern Utah have been collecting cover

estimates for biological crusts since 1987 (Belnap, unpublished data).

6.1.1 Sampling Design and Procedures

Biological crusts are typically measured using standard or

slightly modified rangeland assessment techniques. These are

generally based on measurement of plant and soil attributes within

permanent sampling units (quadrats), stratified within relevant

vegetation or soil zones (Ludwig and Tongway 1992). Stratification

may be necessary where the landscape is composed of a number of

homogeneous land units. On degraded sites, nearby reference areas

should be measured in order to determine the site’s potential for

biological crust development. Biological crusts are likely to be most

pronounced in areas protected from trampling, such as under shrubs

or adjacent to obstacles such as fallen trees and rocks. Shallow, rocky

soils often provide “refugia” for crustal organisms that have been

destroyed by livestock on more productive soils. However, reference

areas should be as identical as possible to their paired disturbed sites

with regards to soil characteristics (texture, chemistry, depth),

placement on the landscape, and vegetation production potential.

6.1.2 Using Morphological Groups for Monitoring

Biological crust organisms are rarely recorded during routine

rangeland monitoring despite increased acceptance over the past

decade of their importance in ecosystem processes (Harper and

Marble 1988; West 1990; Eldridge and Greene 1994; Ladyman and

Muldavin 1996). West (1990) contends that this is due to difficulties

in identifying the organisms in situ, lack of a standardized sampling

procedure with often undue emphasis on laboratory culturing, lack of

conspicuousness of some organisms (particularly when the soils are

dry), and their patchiness in time and space. As identification is

problematic, monitoring can be a daunting task to all but the most

experienced (Eldridge and Rosentreter 1999).

However, strong relationships exist between form

(morphology) of biological crust organisms and their ecological

functioning in relation to landscape processes and disturbance

(Eldridge and Rosentreter 1999). Morphology determines how crust

organisms function ecologically (relative to erosion and water

retention) and their tolerance to, and recovery from, physical

disturbance. Morphological groups have been proposed as surrogates

for species in biological crust monitoring (see Table 1.1; Kaltenecker

1997; Ponzetti et al. 1998; Eldridge and Rosentreter 1999).

Morphological groups are biologically and ecologically

efficient and convey to non-specialists a better image of the
Biological Soil Crusts:

Ecology & Management

NOTES organismal form and its likely impact on soils and landscapes. These

relationships hold true in different regions and continents and

eliminate the need for complex, often confusing changes in

nomenclature. Other advantages of morphological groups for

rangeland monitoring include ease of identification, which is

independent of sexual reproductive structures, and the ability to

monitor sites more quickly with less specialized staff (Eldridge and

Rosentreter 1999). Cover and abundance measures of morphological

groups can be obtained more rapidly than measuring each individual

species. This allows use of the same size plot for varied sites and

assessment of more sites per unit time.

Although morphological groups are appropriate for broadscale or regional monitoring, their use may not allow the detection of

individual species, particularly rare or uncommon taxa. Specific goals

of individual studies (i.e., determination of total diversity vs.

functional diversity vs. site stability) need to be determined prior to

initiation of monitoring. This will determine whether morphological

groups can be used or whether monitoring needs to occur at the

generic/species level. In all cases, crustal organisms are best detected

or identified when moist. When sampling during dry weather, the soil

surface to be measured can be moistened with water mist prior to

reading to make the organisms more visible.

6.1.3 Quadrat Methods

In communities where biological crusts are patchy, lichen

and moss cover is sparse, and/or large areas are being assessed, crust

monitoring is often done using quadrats. These can be located either

randomly or along a line transect. Quadrats can be used to estimate

both biological crust and vascular plant cover (Anderson et al.1982a,

b; Brotherson et al. 1983), using either point-intercept or cover

classes (Fig. 6.1, 6.2). While cover of the various strata within the

community (i.e., biological crusts, herbaceous plants, and shrubs) can

be determined, horizontal spatial relationships between groups of

organisms cannot be defined using quadrat methods. Where

biological crust cover is relatively dense or where personnel turnover

is high, microplot sampling effectively estimates cover (McCune and

Lesica 1992). Reduced quadrat size is also useful if a goal of the study

is to observe greater detail in biological crust composition in relation

to environmental variation within the site. For example, Rogers and

Lange (1971) used 15 x 20-cm quadrats to examine changes in crust

floristics related to stock watering points in semi-arid and subtropical

Australia. Other Australian studies have employed 0.5 m2

 subplots

within larger plots to assess cover and frequency of crustal organisms

(Eldridge and Bradstock 1994; Eldridge 1996; Eldridge and Tozer

seventy-four
Biological Soil Crusts:

Ecology & Management

1996; Eldridge 1999). U.S. workers have adapted a 20 x 50-cm NOTES

quadrat and cover classes traditionally used for vascular vegetation

monitoring (see Daubenmire 1959) for estimating biological crust

cover (Rosentreter 1986; Ponzetti et al. 1998).

Destructive sampling might be necessary if detailed

documentation of biological crust species composition is desired.

Eldridge and Semple (unpublished data) removed small cores (4 cm2

)

from an area of high biological crust cover and diversity and used a

dissecting microscope to record cover and abundance of crustal

organisms. Use of a short-focus telescope is a similar, non-destructive

method (Pickard and Seppelt 1984), but is less accurate.

Within a quadrat, cover and frequency are relatively quick to

assess, can be recorded by taxa or morphological groups, and are often

good indicators of the ecological and hydrological status of the

landscape (Eldridge and Koen 1998). Frequency is determined as the

number of times that an organism or group is recorded within a given

number of sampling units (expressed as a percentage). This measure

describes the abundance and distribution of species and is useful to

detect changes in community composition over time. It is one of the

easiest and fastest methods available for monitoring changes in

vegetation (USDI 1996). Frequency methods are used extensively by

federal land management agencies for monitoring changes in

landscape condition or trend in relation to vascular plants, and can

easily incorporate biological soil crusts.

6.1.4 Line-intercept Methods

Line-intercept has been used in the northern Great Basin in

studies that characterized both the crust and vascular plant

seventy-five

Figure 6.1 Quadrat frame used for point-cover estimation.

This frame is 25 x 25 cm in size. Hits are determined by dropping a

pin vertically from each intercept within the frame.
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Figure 6.2 Data form for point-intercept using a quadrat method (left) and example of data collection (right).
Biological Soil Crusts:

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Biological Soil Crusts:

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communities (Kaltenecker 1997; Kaltenecker et al. 1999a, b;

Kaltenecker, unpublished data). Using line-intercept (see Canfield

1944 for a description of the original methodology developed for

vascular plant communities), taxa or morphological groups (Eldridge

and Rosentreter 1999) are recorded every centimeter along a number

of 10-m or 20-m line transects (Fig. 6.3, 6.4), with cover and

floristics of the biological crust and vascular plant communities

calculated as a percentage of the total line transect length (Rogers

1994). Each line forms a single, independent sampling unit.

Line-intercept is a rapid and efficient method of sampling

when the vegetation community is strongly patterned and is therefore

useful in many arid and semi-arid ecosystems. Line-intercept is easiest

Figure 6.3 Use of line-intercept for

measuring biological crust cover along

with other community attributes

(vascular plant cover, litter, bare mineral

soil, rock). Note the placement of the

transect line close to the soil surface.

Cover of each entity is measured along

the top edge of the tape. A misting of

water makes crustal organisms more

visible and therefore easier to measure.

when organisms have a definitive boundary, and can be difficult to

apply in sites with dense or intermingling vegetation. An advantage of

line-intercept over quadrat methods is the ability to derive linear

spatial relationships between organisms from the data.

Lines should be long enough to include most of the variability

in both the vascular vegetation and biological crust. Optimum line

length should be determined from pilot sampling. It is imperative

that the line is placed in exactly the same location each time to

prevent sampling error associated with repeated measure. Permanent

markers, such as steel stakes or nails, may be located along the line to

ensure correct placement. If vascular vegetation is measured at the

same time, a longer transect might be required with intensive

sampling of the biological crust along a shorter portion of the line

(USDI 1997).
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seventy-nine

Figure 6.4 Line intercept data form (this page) and example of data collection (next page).
Biological Soil Crusts:

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Biological Soil Crusts:

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6.1.5 Line-point Intercept Methods NOTES

The line-point intercept method is similar to line-intercept,

but uses random or predefined points along a line rather than

intercepts, and records only the entity or entities (if recording

over- and understory) associated precisely with that point. Cover

and floristics of the biological crust and vascular plant communities

are then calculated as a percentage of the total number of points.

Line-point intercept is useful for estimating cover in communities

where the crust and vascular vegetation are not strongly patterned.

As with line-intercept, linear spatial relationships can be assessed

using this method.

6.1.6 The Issues of Scale

Size and shape of the sampling unit depends upon the

vegetation type and spatial distribution. Ultimately, the size and

number of quadrats will depend on a compromise between statistical

considerations (see Section 6.1.8) and what is logistically possible.

Generally, in sparsely vegetated landscapes, larger linear quadrats are

useful in decreasing variability because of cover heterogeneity.

Circular quadrats are often preferred to square because they reduce

edge effects. The nature of the impact being investigated will also

influence the monitoring method(s) used. For example, studies

focusing on species composition will require microplot sampling,

while compressional impacts (off-road vehicles, grazing) on biological

soil crusts may be best examined by using repeated photo-points,

remote sensing, or aerial photography interpretation on large plots of

up to several hectares in area.

6.1.7 Voucher Specimens

Representative specimens of each crustal organism should be

collected as vouchers for each site. Voucher specimens should include

all the variation within a morphological group. A site’s biodiversity

can later be assessed by identification of voucher specimens by an

expert and application of diversity indices.

Voucher specimens require specialized preparation to preserve

them for long-term storage (Rosentreter et al. 1988). This often

includes removal of excess soil, stabilization of the underlying

substrate, and fixation of the specimen onto a stiff card to protect it

from breakage. A collection of local or regionally occurring crustal

organisms stored in a herbarium is useful for future identification of

taxa and for describing a study area’s floristics. This enables other

researchers and land managers to evaluate differences in biological

crust composition between regions.

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6.1.8 Statistical Considerations

Statistical considerations are important when designing any

sampling procedure. It is important that the size and number of

sampling units, their placement in relation to landscape

heterogeneity, and their replication (if significance testing is required)

do not compromise any future data analyses. The most efficient

design is usually the one that yields the highest statistical precision

(smallest standard error and narrowest confidence interval around the

mean) for a given amount of effort or money. It is therefore crucial

that a biometrician or a statistician with an ecological background be

consulted prior to establishment of a monitoring program.

While the value of statistical analysis regarding changes in

biological soil crust cover or floristics cannot be undervalued, rigorous

statistical examination might not always be necessary or even

desirable for a monitoring program (Eldridge and Rosentreter 1999).

The danger, however, is that subsequent personnel may wish to look

for statistically significant results using a monitoring protocol that

was not established to allow rigorous examination. Monitoring sites

are frequently unreplicated or pseudo-replicated, and analyses may

therefore result in erroneous conclusions (Hurlbert 1984).

A pilot study may be necessary to determine optimal quadrat

size, the shape or number to be used, or the transect length that will

enable the operator to reduce Type I error (erroneously rejecting a

true null hypothesis). For any site-by-time combination, a speciesarea curve (McCune 1992) can be calculated to give the optimal

sampling intensity needed to estimate parameters for a given

proportion of the population. For example, in semi-arid eastern

Australia, Eldridge and Tozer (1995) found that between six and eight

0.5-m2

 quadrats were needed to adequately describe the biological

crust in a woodland community, while only three quadrats were

required to describe the crust in a grassland.

Species-area curves are likely to vary in relation to landscape

heterogeneity, and the observers must ensure that all site

measurements are made from a homogeneous sample. For example,

in the patterned semi-arid woodlands in eastern Australia, where

groves of trees located on the contour are separated by treeless intergroves, the biological crust cover varies markedly between the two

zones (Eldridge 1999; Tozer and Eldridge, unpublished data).

Monitoring should occur within, rather than across, geomorphic

zones; otherwise, species-area curves are meaningless and will fail to

reveal the optimal number of units necessary to detect differences.

Despite the importance of statistical considerations, the

sampling intensity may depend entirely on sampling efficiency.

This is particularly important in studies where cores or samples are
Biological Soil Crusts:

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NOTES

eighty-three

collected for laboratory identification and analysis. As a broad

generality, one day of collecting and recording crustal organisms in

the field can generate up to 10 days of processing and identification

in the laboratory. Sampling intensity, therefore, is often a compromise

between what is statistically necessary and what is logistically feasible.

6.2 Other Monitoring Methods

As discussed in Chapter 4, changes in biological crust biomass

and function often cannot be assessed by visible cover change (Belnap

1993). Obviously, monitoring of cover changes is the easiest and most

practical method. However, if resources are available, additional

methods should be used to more accurately track biological crust

responses to disturbance or management change. The methods

described below are examples of monitoring that provides

information about biological crusts from a functional perspective.

Determination of cholorophyll a using spectrophotometric or

high pressure liquid chromatography (HPLC) analysis is an efficient

and reliable measure for monitoring changes in photosynthetic

biomass, even when visible cover is difficult to detect (Belnap 1993).

In areas dominated by cyanobacteria and cyanolichens, monitoring

nitrogenase activity is an effective way to determine functional

changes in the biological crust (Belnap et al. 1994; Belnap 1996).

Both methods require destructive sampling, but can provide valuable

information regarding stages in biological crust development.

The slake test is a simple method of determining soil surface

stability under wet conditions. Small soil surface fragments (6 to 8

mm diameter) are immersed in rain-quality water, observed over a

period of several minutes, and rated according to cohesion of

fragments after soaking (Table 6.2). Biological crusts will maintain

cohesion when soaked, while physical or chemical crusts will tend to

slake or disperse (see Tongway and Hindley 1995 for detailed

methods regarding this test). More complicated methods of

determining stability include the use of wind tunnels and

simulated rainfall.

6.3 Impacts of Monitoring

Care should be taken to establish rigorous protocols when

sampling within quadrats and along lines. Because crustal organisms

are susceptible to damage by humans, inadvertent trampling within

the sampling units may create biased results. Studies on biological soil

crusts require that voucher specimens be collected to provide a
Biological Soil Crusts:

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NOTES

eighty-four

Table 6.2 Classification of results from the slake test. Adapted from Tongway (1995).

Class Stability Rating Observed Behavior of Soil Fragment

1 Very unstable Fragment collapses completely in <5 seconds into a shapeless

mass. A myriad of air bubbles is often present.

2 Unstable Fragment substantially collapses over about 5-10 seconds with

only a thin surface crust remaining; >50% of the sub-crust

material slumps to an amorphous mass.

3 Moderately stable Surface crust remains intact; slumping of sub-crust

material is <50%.

4 Stable Entire fragment remains intact after 5 minutes. This level of

stability can remain for many hours.

permanent record of the species found in an area. Removal of voucher

specimens or destructive sampling modifies the community and

should occur away from permanent plots.
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CHAPTER 7

INTERNET RESOURCES FOR BIOLOGICAL SOIL CRUSTS

The Internet has a number of valuable resources for information on biological soil crusts or

the organisms that comprise crusts (lichens, algae, etc.). Below we have listed several of the most

useful web pages currently available.

Web Address Author or Organization Description

www.soilcrust.org

U.S. Geological Survey Biological soil crust information,

(Jayne Belnap) based in Canyonlands National

Bureau of Land Management Park. This web page contains a

National Park Service near exhaustive bibliography for all

things crust-like and a recent

version of this reference.

www.id.blm.gov/iso/soils/index.html

Bureau of Land Management, This site contains an introduction

Idaho State Office to soil biological communities,

including soil crusts and

subsurface micro-and

macro-organisms.

www.ucmp.berkeley.edu/fungi/lichens/lichens.html

University of California, Berkeley, Introduction to lichens.

Museum of Paleontology

www.unomaha.edu/~abls/resources.html

American Bryological and Checklists and floras; links to

Lichenological Society (ABLS) institutions and individuals that

are resources for information on

nonvascular plants and lichens.

mgd.nasce.org/hyperSQL/lichenland

Department of Botany and Plant One of the best introductions to

Pathology, Oregon State Univ. lichens, complete with great

color photos.

ucs.orst.edu/~mccuneb/

Bruce McCune Bruce McCune is a leading

Oregon State University ecologist in North America and

specializes in lichen and moss

ecology and taxonomy.
Biological Soil Crusts:

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NOTES

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Web Address Author or Organization Description

www.ndsu.nodak.edu/instruct/esslinge/chcklst/chcklst7.htm

Theadore Esslinger Cumulative Checklist for the

Department of Botany Lichen-forming, Lichenicolous

North Dakota State University and Allied Fungi of the Continental United States and Canada.

www.fs.fed.us/land/ecosysmgmt/ecoreg1_home.html

USDA Forest Service Descriptions of Bailey’s

Robert G. Bailey ecoregions of the U.S.,

including maps.

www.accessexcellence.org/BF/bf05/johansen/web.html

California Academy of Sciences Text of a talk presented to high

Jeffrey Johansen school teachers for the California

Academy of Sciences. A layman’s

introduction to biological crust,

focusing on algal and

cyanobacterial components and

research on inoculation to

speed recovery following

disturbance.

www.biblio.bio.purdue.edu/www-cyanosite

Department of Biology, Web server for cyanobacterial

Purdue University; Foundation research; includes a link to CyBib,

for Microbiology a bibliographic database containing references on cyanobacteria.
Biological Soil Crusts:

Ecology & Management

GLOSSARY

adnate: Pressed closely against a surface.

aggregate stability: The degree to which a group of soil particles cohere so as to behave

mechanically as one unit.

albedo: A measure of the energy reflected off a surface. Dark surfaces have a lower albedo

than light surfaces; therefore, light surfaces absorb less energy than dark surfaces.

ammonia volatilization: The vaporization of ammonia into the atmosphere.

anaerobic: The absence of oxygen. Also refers to organisms able to live or grow in the

absence of free oxygen.

appressed: Pressed closely against a surface.

autotroph: An organism whose growth and reproduction are independent of external sources of

organic compounds. Carbon compounds are created by autotrophic organisms via the

reduction of carbon dioxide (CO2), with light energy driving the process.

bacteria: Members of a group of diverse

e

and ubiquitous prokaryotic, single-celled organisms.

bryophytes: Tiny plants lacking vascular tissues. This group includes mosses and liverworts.

calcareous: Substrates rich in calcium carbonate, such as limestone or dolomite. Also refers

to soils derived from or containing these substances.

cosmopolitan: Found worldwide.

crustose lichens: Lichens that form a crust-like growth form that is closely applied to the

substrate.

cyanobacteria: “Blue-green” algae; prokaryotic, photosynthetic organisms that generally have

a blue-green tint and lack chloroplasts.

cyanolichen: Lichen that contains a cyanobacterium as its phytobiont.

denitrification: The formation of gaseous nitrogen or gaseous nitrogen oxides from nitrate

or nitrite by microorganisms.

desertification: Land degradation in arid, semi-arid, and dry subhumid regions of the world

resulting from climatic and other natural stresses coupled with human activities.

Processes that lead to desertification include soil loss through wind and water erosion;

changes in soil quality due to salinization, waterlogging, and nutrient depletion; loss

of vegetative biomass and cover; and compositional and structural changes in plant

communities due to exotic plant invasion (Mouat et al. 1995).

foliose lichens: Lichens that are leaf-like in growth form, flattened with definite upper and

lower surfaces, and usually loosely attached to the substrate.

fruticose lichens: Lichens with three-dimensional growth forms that are ropey or branching

and do not have definite upper and lower surfaces.

gelatinous lichens: Lichens that are non-stratified (algal and fungal layers are not distinct)

and that have a jelly-like appearance when moistened. All gelatinous lichens have

cyanobacteria as their phytobiont.

gemmae: Structures produced by liverworts that function in vegetative reproduction.

green algae: Photosynthetic unicellular and multicellular organisms that lack true tissue

differentiation.

heterocysts: Specialized cells occurring in some filamentous cyanobacteria that are the sites of

nitrogen fixation.

heterotrophs: Organisms that require organic compounds for growth and reproduction.

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Biological Soil Crusts:

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hydraulic conductivity (Ke): The rate at which water moves through the soil profile under

field conditions.

hyphae: Fungal filaments.

infiltration: The downward entry of water into the soil.

interspace: The spaces in a plant community between shrubs or trees.

lichen: A symbiotic relationship between a fungus and an algae or cyanobacterium.

lithic: Pertaining to rock; rocky.

liverwort: A small, non-vascular plant.

microfungi: Fungi that occur free-living in the soil or in association with roots of vascular

plants (mycorrhizae).

mucilaginous: Having a sheath composed of sticky polysaccharides surrounding

cyanobacterial, algal, or fungal filaments.

mycorrhizae: A stable, symbiotic association between a fungus and the root of a plant.

nitrification: The process in which ammonia is oxidized to nitrite and nitrite to nitrate.

nitrogen fixation: The conversion of elemental nitrogen (N2) to organic combinations or to

forms readily usable in biological processes.

nitrogenase: The enzyme that catalyzes biological nitrogen fixation.

permeability: The ease with which gases, liquids, or plant roots penetrate or pass through

the soil.

photosynthesis: The process in which light energy is absorbed by specialized pigments of a

cell and converted to chemical energy. The ultimate result of photosynthesis is the

fixation of CO2 and the production of carbon compounds (sugars).

phytobiont: The photosynthetic partner in the lichen symbiosis, i.e., the algal or

cyanobacterial component of a lichen.

poikilohydric: Organisms that are capable of becoming physiologically dormant under dry

conditions.

prokaryotic: Cells that lack a nucleus.

propagules: Material that results from either sexual or asexual reproduction by crustal

organisms and disperses into areas where the biological soil crust has been removed

by disturbance. This material “germinates” and grows to establish a new crust.

resilience: The ability to recover following disturbance.

resistance: The ability to withstand disturbance.

respiration: The biological process whereby organisms oxidize carbon compounds to CO2

and water as a source of energy.

rhizines: Root-like structures of lichens and mosses that function to attach the organism to

the substrate.

saline soil: A soil containing sufficient soluble salts to impair its productivity.

sessile: Attached to a surface without a stalk but not imbedded in the surface.

squamulose lichens: Lichens that occur as discrete flakes or scales that are often round or earshaped. Squamulose lichens may be convex or concave and often have lobed margins.

symbiosis: An obligatory, interactive association between two organisms, producing a stable

condition in which they live together in close physical proximity to their mutual

advantage.

thallus: The vegetative body of a lichen or liverwort.

xerothermic: Dry, hot conditions.

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1. TITLE AND SUBTITLE

TR-1730-2 – Biological Soil Crusts: Ecology and Management

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Jayne Belnap, Julie Hilty Kaltenecker, Roger Rosentreter, John Williams, Steve

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13. ABSTRACT (Maximum 200 words)

This technical reference presents a review of current scientific knowledge on the ecology of biological soil crusts, then integrates this

information into a discussion on monitoring and management of arid and semiarid lands in the western United States. The document

begins with a discussion on the composition, structure, and distribution of biological crusts relative to environmental factors such as

climate and soil characteristics. This information is then synthesized into guidelines on development of monitoring strategies as well as

site-specific management prescriptions regarding livestock, recreation, and other multiple-use activities. The material is presented from

a practical and applied management perspective and is summerized in numerous tables and figures.

14. SUBJECT TERMS

Biological Soil Crusts, Soils, Stability, Hydrology, Nutrient Cycling, Disturbance,

Management, Monitoring, Ecology

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