Cryptobiotic Crust in the Sonoran Desert
Updated 6/6/07


CRYPTOBIOTIC CRUST is also called BIOLOGICAL SOIL CRUST.
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.
WHERE CAN I FIND CRUSTS?
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.
WHAT GOOD ARE THEY?
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, worms, 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 substrate than they would on barren ground.
CRYPTOBIOTIC CRUSTS ARE FRAGILE.
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 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 proper trail maintenance.  Offroad vehicles 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 public lands 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, and BLM.

Cyanobacteria

Dark patches of cyanobacteria on old cotton fields & grazing land: Sawtooth Mts., AZ.
CYANOBACTERIA ("blue-green algae") are the pioneers of cryptobiotic crust formation, the first stage in the natural
revegetation of barren ground.  Their gelatinous sheaths absorb water and bind sediments together.
They are among the oldest and most primitive life forms on earth, and were almost certainly the first to colonize the land.
NOTE vehicle tracks that have damaged this crust.

 
Cyanobacteria from the Sawtooth Mts. locality.
Dry (left) and wet (right).  Both photos are 3 mm.

Lichens
Lichens are the most colorful, conspicuous, and easily identified 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 and color,
and grows spores and various other reproductive structures.
See lichen photos from Nags Head Woods & Buxton Woods, North Carolina, on my OCEAN FOREST page.

Collema tenax   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.  A lichen "pioneer" in
crust communities, appearing after cyanobacteria but before other lichens or mosses.

 
Placidium lacinulatum 
Also called Catapyrenium lachneum and Dermatocarpon lachneum:
Lobed brown scales to 1cm, white underneath, green when wet.  Black sunken perithecia.
The most conspicuous Sonoran Desert crust lichen.  With Collema tenax, Peltula spp.,
and Psora crenata, forms a common and distinctive crust community.

 
Psora icterica:  Yellowish-green even when dry,
with white edges and brown or black apothecia.  Photo 3 cm.

 
Psora crenata:  Convoluted pink and white scales with round black apothecia at the edges.
Individual scales are 3-8 mm, entire patch is typically 2-6 cm.
Psora decipiens is similar but is dark orange and the scales have frayed edges..

 
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

Liverworts

Liverwort #1:  Liverworts are fragile creeping plants related to mosses.
In the desert, they are found on soil near damp shady rocks.

 
Liverwort #2  Dry (left) and wet (right).  Both photos 4 cm.

 
Liverwort #2 (similar to first photo, but larger, with spore-bearing structures):  LEFT:  15 cm  RIGHT:  1 cm

 
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.

 
DUNE CRYPTOS - Outer Banks, NC
In photo:  cyanobacteria, green algae, young moss,
Cladonia lichen, and fallen spikelet of Sea Oats
(Uniola paniculata), a characteristic dune grass.
Cryptos anchor the sand, fix nitrogen, and provide
moist seedbeds for salt-tolerant succulents and shrubs.

Cryptobiotic crusts are more noticeable in dry climates
but unique crust communities are also found in humid
environments, especially where soils are dry, cold, or
poor in nutrients.

Site designed and maintained by Lorena B. Moore.   About Me
E-mail if you have comments or if you can identify the liverwort species in the photos.
Permission is granted for noncommercial use of photos with credit and a website link.
Artwork is copyright © 2002 by Lorena Babcock Moore and may NOT be used without permission.
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