Cryptobiotic Crust in the Sonoran Desert
Updated 3/23/10

These are communities of cyanobacteria, green algae, lichens, mosses, liverworts, and microorganisms that colonize the surface of bare soil.  "Cryptobiotic" means "hidden life."  Crusts often go unnoticed unless they are very extensive or colorful, and some do not even look alive.  But they are vital to the health of soils and ecosystems.  Cryptobiotic crust is best known (and probably most studied) from the protected lands of the national parks of the Colorado Plateau, where it forms dark lumpy patches on the red soil.  But it is equally important to desert, prairie, and tundra ecosystems, and also colonizes bare ground in humid temperate environments.  The most complex and spectacular "old growth" crusts take decades to develop.  They are miniature forests with dozens of species of cyanobacteria, green algae, mosses, and lichens over a dark layer of organic-rich soil.  The damp soil is alive with earthworms, snails, millipedes, insects, and microorganisms, and nourishes grasses, wildflowers, and even trees.
You must look beyond the more obvious landscape features and learn to see in a different way.  Seek subtle differences in color and texture on bare soil between the rock outcrops, among a scatter of pebbles, or under thorny trees and inconspicuous shrubs.  You will discover a tiny, intricate and surprisingly beautiful hidden world.
Crusts hold the soil in place and protect underlying sediments from erosion.  They pioneer soil development on bare inorganic sediments, absorbing water and enriching the surface with nutrients and organic matter.  This creates a favorable environment for seeds to germinate and for insects and other soil organisms to live.  Crusts enable the land to recover more quickly after a fire.  Crust organisms such as lichens and dried mosses are vulnerable to burning and can be killed even in a relatively cool, fast-moving grass fire.  But the cyanobacteria often survive.  Grasses, shrubs, and crust organisms regenerate faster in this living substrate than they would on barren ground.
They are extremely susceptible to destruction by crushing and trampling.  Once damaged, they may take many years to grow back.  Meanwhile, several feet of sediment may be washed or blown away.  Areas that have been stripped of cryptobiotic crusts are vulnerable to erosion, flooding, deflation, dust storms, invasion of exotic weeds that thrive on disturbed soil, and/or chemical impoverishment due to loss of organic material and precipitation of evaporite minerals.  Hikers and horseback riders who venture off established trails can damage crusts.  This is a localized issue that can be reduced through education and regular trail maintenance.  Offroad vehicles venturing off established routes are a more widespread and serious problem.
PUBLIC-LANDS RANCHING IS A MAJOR THREAT to cryptobiotic crusts and nearly all Western U.S. plant communities.  Cattle strip vegetation and leave only the most inaccessible crevices untrampled.  Cryptobiotic crust cover on public land is often measurable only in square inches, usually under shrubs or in cracks between rocks that are too steep for cattle.  Well-developed crust communities flourish only where cattle are excluded.  The ranching culture perpetuates several myths about crusts:  vast areas of desert are thought to be "naturally" devoid of crusts, and crusts may be deliberately destroyed because they "compete" with grass or "prevent" grass from growing.
Specimens illustrated are from public land in southern Arizona:  State, USFS, BLM, and Saguaro National Park.


Dark patches of cyanobacteria on old grazing land:  Sawtooth Mts., AZ.
CYANOBACTERIA ("blue-green algae") are the first stage in the development of the soil itself,
and an essential part of natural revegetation.  Their cells fix nitrogen, enriching clay and sand with the
elements necessary for plant life.  Gelatinous sheaths protect the cells from the dry air and bind the young soil together.
NOTE recent vehicle tracks that have damaged this crust.

Cyanobacteria swell dramatically when wet.
Dry (left) and wet (right).  Both photos are 3 mm.  Sawtooth Mts.

Lichens are the most colorful, conspicuous, and easily identified soil crust organisms.
A lichen is composed of a fungus and a cyanobacteria or green alga living in sybiosis.
The photosythetic alga provides nutrients.  The fungus provides the shape, color, and spores.
The main body of the lichen is called a THALLUS.  APOTHECIA are conspicuous reproductive structures.

Collema   Black Jelly Lichen
Dry (left), wet (center) - both photos 2 mm.  Growing on ground (right), 3 cm.
Cyanobacterium Nostoc is the photobiont for this nitrogen-fixing genus.  Appears early in
crust communities, after cyanobacteria but before other lichens or mosses.

Placidium  Brown Earthscale
Lobed brown scales to 1cm, white underneath, green when wet.  Black sunken perithecia.
The most commonly seen Sonoran Desert crust lichen. 

Psora icterica  Green Earthscale 
Yellowish green even when dry.  Brown or black apothecia are usually present.  On granitic soil.

Psora crenata Pink Earthscale
Conspicuous on limestone soils.  Forms hard mounds or lumps.  Thick, cracked, convoluted scales are powdery white and pinkish orange, with round black apothecia.

Psora decipiens (Blushing Scale) is similar but flatter, darker orange, and the scales have frayed whitish edges.

Psora tuckermanii:  Uncommon on north-facing limestone soil or rock.  Requires more shade than other Psora species. 
Brown to reddish-brown scales have white rims.  Cinnamon-brown apothecia may or may not be present.
They can be difficult to see since they are nearly the same color as the thallus.
Xanthoparmelia chlorochroa:  A wandering groundcover!
This vagrant lichen is abundant on the High Plains of Wyoming,
Nebraska, and Colorado.  It collects in windblown piles among
shrubs and boulders, and provides winter food for antelope.
Large 3-D Anaglyph Photo


Liverwort (Marchantia sp.)   Dry (left) and wet (right).  Both photos 4 cm.

Liverworts are primitive spore-bearing plants related to mosses.  Most species are found in cool wet
environments.  A few have become adapted to life on desert soil in humid, shady rock crevices.
 They can shrivel and go dormant during the dry months, and revive for a few days or weeks after a rain.

CLICK THUMBNAIL to view a 4"x4" patch of dry crust
from the Tucson Mountains.  Photo shows true moss,
Selaginella spikemoss, Placidium and Collema lichens,
cyanobacteria film, and felsic volcanic soil.
Leaf litter is from ocotillo and yellow palo verde.

Webpage, text, and photos by Lorena B. Moore.
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