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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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