playas in instances where the water table was high;
zonation of desert bacterial and algal species has been
where the water table was low, the nearest vegetation
documented across moisture and other environmental
to the playa was on alluvial slopes. Lichvar et al. (1999)
gradients in some field studies and in laboratory stud-
ies. Surface films of Microcoleus sp. (a crust-forming
reported a narrow seasonal band of "pseudohalo-
phytes" composed of annual vegetation from Deadman
blue-green alga) have been reported on the mud at Race-
Dry Lake, Twentynine Palms, California.
track Playa, California (references cited in Stone 1956),
The causal factor in the apparent negative associa-
and several related species have been reported on oth-
tion between many species of desert plants and flood-
er playas (Brostoff et al. 1996, Brostoff 1998). Further,
ing is problematic; further, the degree to which desert
the presence of microalgae remnants such as Phacotus in
vegetation provides information helpful in identifying
cores is associated with the presence of standing water
inundated areas is unknown. Dobrowolski et al. (1990)
in the recent geologic history of playas (Enzel et al. 1992).
reviewed plant responses to soil saturation and flood-
Biotic crusts have been used as possible positive
ing in desert systems. They cited studies of plants such
and negative indicators for WoUS (Lichvar and Spre-
as creosote bush, big sagebrush (Artemisia tridentata),
cher 1996) in southern California, and crust types and
and green rabbitbrush (Crysothamnus nauseosus ssp.
presence have been shown to be related to inundation
viridulus), which quickly succumbed to experimental
cycles in Canadian playas (Renaut 1993). Further, for
flooding and to elevation of the water table to a 10-cm
vernal pools in California, Riefner and Pryor (1996) re-
(4-in.) depth, presumably because of anoxia adversely
port concentric distribution of the same crusts as occur
affecting their roots. They noted other species tolerant
in many playas, and speculate that the organisms and
of flooding: Atriplex torreyi and greasewood (Sarco-
crusts they produce would be useful for delineation.
batus vermiculatus). While the latter could not survive
Although biologically induced soil crusting has been
six-month or longer inundation, the former pair could.
documented in the technical literature for nearly 100
The authors of the review expressed surprise at these
years, only in the past few years, with a recognition of
results since greasewood is commonly considered to
the importance of their ecological function, has it been
be a phreatophyte and tolerant of high water tables,
given attention (Johansen 1993). These crusts, which
whereas saltbush and rabbitbrush are thought to have
in very arid areas may replace macrophytes as domi-
more xeric-adapted ancestry.
nants, are referred to as cryptobiotic, cryptogamic, al-
In perhaps the work of most direct application to
gal, microphytic, cyanobacterial, microfloral, or biolog-
playa delineation, Ganskopp (1986) followed population
ical crusts. Cryptobiotic crusts are water-stable soil
responses (big sagebrush, green rabbitbrush, and
aggregates held together by algae, fungi, lichens or
greasewood) to inundation and groundwater level at
mosses, and the substances they produce. As a group
varying elevations above the water level. Leaves of all
they are characterized by generally darker coloration and
three species perished immediately upon inundation. Big
greater adhesion than crusts formed strictly by physi-
sagebrush and green rabbitbrush were intolerant of either
cal and chemical processes. Johansen (1993), in a review
any inundation or water tables within 10 cm (3.93 in.) of
article, mentions three common forms of cryptobiotic
the surface. The former died soon after surface inunda-
crusts: 1) smooth and flat forms dominated by algae, 2)
tion, the latter tolerated surface flooding for up to one
rough, uneven crusts dominated by lichen, and 3) pedi-
week. Greasewood tolerated surface flooding for 40
cled algal and algallichen crusts. The first form is found
days. In a study of vegetation patterns on an Owens
typically in areas of ephemeral ponding and may or may
Lake playa (California), Dahlgren et al. (1997) reported
not develop into the other two forms; the lichen crusts
that the ultimate limitation to plant distribution is shal-
usually occur on silty, often saline soils; the last form is
low anoxic groundwater restricting rooting depth.
common in pinyonjuniper and sagebrush communities.
The vegetation of playas under consideration here
Among the organisms comprising these crusts are blue-
differs markedly from that of other areas; the playa lakes
green algae, green algae, diatoms, euglenas, lichens,
of western Texas have suites of characteristic vegeta-
fungi, mosses, liverworts, and bacteria. They are a bio-
tion, some species of which may not be found elsewhere
logically diverse assemblage showing high spatial and
(e.g., Reed 1930).
temporal variation. Several species of cyanophytes are
common in these crusts, but they are most often domi-
nated by the filamentous blue-green alga Microcoleus
BIOTIC SOIL CRUSTS
vaginatus. Other typical genera include Phormidium,
Plectonema, Schizothrix, Nostoc, Tolypothrix, and Scy-
Much as variations in the species abundance and
tonema. In some soils with a high concentration of gyp-
diversity of macrophytic vegetation may be used as indi-
cators of certain environmental conditions, microbial
sum, diatoms may dominate.
communities can as well. Distinct small-scale surface
Crust types have been broadly classified into two
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