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Microbiotic crusts, also known as cryptogamic crusts, microphytic
crusts, and cryptobiotic crusts (St. Clair & Johansen 1993), cover extensive portions
of the arid and semiarid regions of the world. These crusts consist of water-stable soil
aggregates held together by algae, fungi, lichens, mosses, and rarely, liverworts. Over
the past 25 years, researchers have gathered evidence that these communities are
ecologically important, particularly with reference to retarding soil erosion, increasing
soil fertility, increasing infiltration, and interacting with vascular plant germination
and nutrient uptake (for reviews see Harper and Marble 1988, Johansen 1993, Metting 1991,
West 1990). Although most researchers agree that the microbiotic crusts are ecologically
important, it is becoming increasingly evident that not all crusts are alike in terms of
their physical, chemical, and biotic properties, and that contributions to the stability
and fertility of the ecosytems in which they occur varies greatly from site to site. In
other words, benefits and properties observed in one site can not be attributed to all
sites.
Microbiotic crusts are distributed throughout western North America. They are best
developed on silty soils of elevated electrical conductivity in the semiarid regions of
the Great Basin and Colorado Plateau (Anderson et al. 1982a, 1982b). Soils derived from
the reddish sandstones of the Colorado Plateau commonly have well lichenized, pediceled
crusts in undisturbed areas (Fig. 1, Harper & Marble 1988, Kleiner & Harper 1972,
Johansen 1993), whereas those in the Great Basin support well lichenized polygonal crusts
with a vescicular surface horizon (Fig. 2, Harper & Marble 1988, Johansen 1993,
Johansen & St. Clair 1986, St. Clair et al. 1984). Bryophytes are present in all
semiarid crust sites, but become dominant over lichens in more northern sites, such as the
northern part of the Great Basin and the Lower Columbia Basin (Johansen et al. 1993,
Rosentretter, personal communication). Microbiotic crusts in hot deserts are often less
well developed, being thinner and more fragile than their cool desert counterparts.
Lichens and bryophytes are rarer and compositionally different. Many regions in hot
deserts have coarser soils and desert pavement (a stone-covered surface with very little
Figure 1. Pediceled crust from Navajo Figure 2. Polygonal vescicular crust from
National Monument, AZ, typical of. Dugway Proving Grounds, UT, typical of Colorado Plateau
sites. Great Basin sites.
exposed soil). These regions can have a subsurface algal crust, but lack the surface
topography of the siltier soils. In all regions, crusts are highly susceptible to
disruption by rangefire and the trampling effects of grazing livestock, and are greatly
reduced in
diversity and coverage in areas exposed to such disturbance (Anderson et al. 1982b,
Belnap 1996, Brotherson et al. 1983, Johansen et al. 1984, Klopatek 1993, Marble &
Harper 1989, Rogers and Lange 1971, St. Clair et al. 1986).
Despite their putative ecological importance, our understanding of the species
composition of microbiotic crusts is exceptionally poor. Floristic studies of microbiotic
crusts have been sporadic and nearly always incomplete. No single researcher has the
taxonomic expertise to study all microphytic components of the crust (lichens, bryophytes,
eukaryotic algae, cyanobacteria, fungi). Anderson and Rushforth (1976) examined multiple
phyla, but even their comprehensive effort neglected some groups (fungi and coccoid green
algae). Most studies focus on one of three components: algae, lichens, or mosses.
Cameron was the first researcher to conduct extensive algal floristic surveys of
crusted and uncrusted arid-land soils. He studied the soil algae throughout southern
Arizona (Cameron, 1960, 1961, 1962, 1963, 1964a, 1964b) as well as other North American
hot desert regions (Cameron and Blank, 1966). Although Cameron examined soils from
numerous sites, his work is limited because site-specific lists are not given, so species
richness in any one given soil is impossible to assess. Furthermore, his primary expertise
was in cyanobacteria, and his treatment of eukaryotic algae is very limited in taxonomic
scope. Even given these limitations, his work represents one of the best taxonomic
studies, with over 400 samples being collected to produce a list of 72 algal taxa. Work by
others during this period has similar limitations (Durrell 1959, 1962, Shields &
Drouet 1962, Hunt & Durrell 1966).
Cyanobacteria as observed in rough cultures of moistened soil and diatoms
(Bacillariophyta) were the taxonomic groups studied by Rushforth and his coworkers (Table
1). Without use of unialgal isolations, identification of chlorophyte and xanthophyte
coccoid algae is impossible. Furthermore, although the taxonomy of the chlorophyte
coccoids was under study and revision (Archibald & Bold 1970, Bischoff & Bold
1963, Brown & Bold 1964, Chantanachat & Bold 1962, Deason & Bold 1960, Groover
& Bold 1969), the taxonomy of this group was confusing and beyond the expertise of
this research group. With only moistened soils, they found 5-16 cyanobacterial taxa per
site, while 6-31 diatom taxa per site were found in prepared diatom slides (Table 1).
Total algal taxa ranged from 13 to 48 per site.
Starting with his work in the Lower Columbia Basin, Johansen and colleagues (Johansen
et al., 1993, Flechtner, in press, Flechtner et al., in press) began to work with
chlorophyte and xanthophyte algae in a systematic, thorough fashion. Utilizing plating and
isolation techniques, a total of 14-47 chlorophyte taxa were found per site. Flechtner
isolated cyanobacteria as well, and numbers of taxa recovered in this phylum also
increased, such that total algal species richness in these studies varied from 46-101,
over double that found in previous studies. When identical protocols are used, comparisons
between sites become more meaningful, and it is evident that the high desert sites in Utah
have many more algal taxa than the hot desert sites further south (Table 1). We have found
new species of green algae (Flechtner et al. in review, Flechtner in press) and new genera
and species of cyanobacteria in our recent ecological studies of microbiotic crusts. Every
locality we have studied in detail has yielded taxa new to science (unpublished
observations).
Several lichen floras and checklists for the Intermountain Area have been published
(Egan 1972, Nash & Johnson 1975, Newberry 1991, St. Clair & Newberry 1991,
Schroeder et al. 1975, Shushan & Anderson 1969). However, very few studies have dealt
directly with soil lichens. Anderson and Rushforth (1976) published the first list of
terricolous lichens from Utah. Specimens were collected from 34 sites in three general
areas. Most of the sites (24) were located in the Great Basin, five were in gypsiferous
areas of Washington County, while the balance were located in pristine, open grassy areas
in Canyonlands National Park. They reported a total of 17 lichen species in 11 genera. St.
Clair & Warrick (1987) found a new species record (Acarospora nodulosa) for
North America in gypsiferous soils. Gypsiferous soils have also been the source of new
species (Rajvanshi et al. in press), a new genus (Timdal 1990), and a new family (Timdal
1990) of lichens. Distribution patterns of vagrant species in several lichen genera have
recently been published (Rosentretter 1993, Rosentretter & McCune 1992). Vagrant
lichens often live on soils, and are highly susceptible to disturbance by fire and grazing
livestock (Rosentretter 1997). Three recent monographic works (Thomson 1987, 1989, Timdal
1986, Breuss & McCune 1994) have added valuable taxonomic and ecological information
about two of the more abundant soil genera in western North America (Psora and Catapyrenium).
Finally, St. Clair et al. (1993) published a summary paper describing the lichens of soil
crust communities in the Intermountain Area of the western United States. They cited a
total of 34 species in 17 genera from soil crust communities.
Several moss floras and checklists for selected areas in western arid lands have been
published (Flowers 1973, Harthill et al. 1979, Magill 1982, Nash et al. 1977, Spence 1988,
Weber 1973). Taxonomic treatments of selected groups of mosses (familes, genera) for this
region have also been published (Mishler 1994, Stoneburner 1985, Stoneburner & Wyatt
1985, Zander 1993). Studies on the ecology and phenology of selected species have been
sporadic (Alpert & Oechel 1985, Mishler & Oliver 1991, Stark 1996, 1997, Stark
& Castetter 1987, 1995), and new species descriptions uncommon (Spence 1987, Zander et
al. 1995). However, most of these studies only tangentially take up study of the
bryophytes of microbiotic crusts. To our knowledge, there are no publications that
specifically target microbiotic crust bryophytes from a species compositional standpoint.
Rosentretter and McCune have put together informal and unpublished lists of crust
bryophytes, and we will consult these lists and possibly their personal collections to
complement our own studies.
When all of the studies containing species-specific information about
microbiotic crusts are considered, it is apparent that a few cosmopolitan species
dominate. These species include: Microcoleus vaginatus, Nostoc commune, Schizothrix
calcicola (cyanobacteria), Hantzschia amphioxys, Luticola mutica, Pinnularia
borealis (diatoms), Collema tenax, Fulgensia desertorum, Psora decipiens,
Catapyrenium lachneum (lichens), and Tortula ruralis, Bryum species (mosses). A
second, much larger tier of species are common in some regions, but not all areas.
Finally, about half of the taxa are rare, occuring in one or only a few sites. Studies in
which all major components of the crust (cyanobacteria, eukaryotic algae, lichens, mosses,
fungi) have been examined by taxonomic experts for the respective groups have never been
conducted. Algal studies generally have not looked at the lichens or mosses, and those
that have have been limited to incomplete algal analysis combined with lichen
characterization and limited bryophyte coverage (Johansen & St. Clair 1986, Johansen
et al. 1984). This lack of comprehensive studies has made it difficult to determine if
associations between taxa exist. Connections between species composition and ecological
process have also been impossible to detect, although such connections are suspected,
particularly with regard to soil fertility, soil stabilization, fragility, and soil water
relations (Evans & Johansen 1999).
Most studies of microbiotic crust species composition have been conducted in Utah
(Table 1). Well-developed crusts are present in undisturbed pockets of rangeland all the
way up to British Columbia. Rosentretter and McCune have both studied lichens of some of
these areas (Rosentretter 1993, Rosentretter & McCune 1992), but the northern crusts
have been virtually unstudied with regards to their algal components. Studies on
microbiotic crusts in the Chihuahuan, Sonoran, and Mojave deserts are very limited and
incomplete, and our preliminary work in these regions suggests that many undescribed taxa
and new North American records are likely present in these deserts. Due to the difficulty
of identifying most microbiotic species, microbiotic crust communities of all regions of
the semiarid and arid western United States presently need characterization.
Table 1. Species richness within phyla for site-specific soil studies in North America.
For each study, a citation and state (ST) where the study took place are given. Groups
studied include: Cyanobacteria (CY), Chlorophyta (CH), Xanthophyta (XA), Bacillariophyta
(BA), Eustigmatophyta and Euglenophyta (EU), fungi (FU), lichens (LI), and Bryophyta (B).
When zeroes are recorded, proper protocols for examining that taxonomic group were used,
but no representatives were found.
Citation |
ST |
CY |
CH |
XA |
BA |
EU |
FU |
LI |
B |
S |
Shields and Drouet, 1962 |
NV |
12 |
4 |
|
|
|
|
|
|
16 |
Hunt and Durrell, 1966 |
CA |
17 |
5 |
|
|
|
48 |
|
|
70 |
Anderson and Rushforth, 1976 |
|
|
|
|
|
|
|
|
|
|
Virginia Park, Canyonlands |
UT |
5 |
|
|
8 |
|
|
5 |
1 |
19 |
Gypsiferous soils, near Hurricane |
UT |
8 |
1 |
|
26 |
|
|
13 |
2 |
50 |
Antelope Valley |
UT |
5 |
|
|
16 |
|
|
13 |
4 |
38 |
Johansen et al., 1981 |
AZ |
10 |
|
|
20 |
|
|
|
|
30 |
Johansen et al., 1984 |
UT |
15 |
2 |
|
31 |
|
|
5 |
4 |
57 |
Ashley et al., 1985 |
UT |
16 |
4 |
|
24 |
1 |
|
|
|
45 |
Johansen and Rushforth, 1985 |
UT |
14 |
1 |
|
7 |
|
|
|
|
22 |
St. Clair et al., 1986 |
UT |
9 |
1 |
|
6 |
|
|
1 |
|
17 |
Grondin and Johansen, 1993 |
CO |
17 |
4 |
1 |
5 |
|
|
|
|
27 |
Johansen et al., 1993 |
WA |
13 |
47 |
9 |
8 |
|
|
|
|
77 |
Flechtner, 1999 |
|
|
|
|
|
|
|
|
|
|
Yuma Proving Ground |
AZ |
19 |
23 |
1 |
3* |
0 |
|
1* |
|
47 |
San Nicolas Is. |
CA |
23 |
23 |
1 |
|
1 |
|
|
|
48 |
Fort Bliss |
NM |
10 |
14 |
0 |
|
0 |
|
6* |
|
30 |
Dugway Proving Ground |
UT |
42 |
37 |
8 |
14* |
0 |
|
4* |
|
105 |
Buckhorn Wash, San Rafael Swell |
UT |
49 |
34 |
8 |
5* |
0 |
|
|
|
96 |
Wedge Overlook, San Rafael Swell |
UT |
47 |
14 |
0 |
13* |
0 |
|
8* |
|
82 |
Flechtner et al., 1998 |
MX |
18 |
35 |
2 |
11 |
1 |
|
|
|
67 |
Rajvanshi et al. in press |
UT |
|
|
|
|
|
|
23 |
|
23 |
___________________________________________________________________________________
*Numbers of taxa from unpublished studies.
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