Bryophytes of Nevada On-line
Moss Floristics in the Deserts of North America

Arid climates in western North America are caused chiefly by the presence of mountain ranges that create rain shadows. These significant ranges include the Sierra Nevada, Cascades, and elements of the Rockies (Great Basin and Mojave Deserts), the Peninsular Ranges and Sierra Madre Occidental (Sonoran Desert), and the Sierra Madre Oriental and Sierra Madre Occidental (Chihuahuan Desert). In addition to these western cordilleras, aridity is attributed to the presence of a subtropical high pressure cell, and the prevailing cold oceanic currents along the western coast of the continent (Smith et al. 1997). Although qualifying definitions of deserts vary widely in the literature, the boundary between desert and semidesert can be recognized along the 120-150 mm isohyet of annual precipitation (Shmida 1985). By this definition, only the western edge of North America's deserts conforms to a true desert, namely the Mojave and western Sonoran Deserts. Compared to other deserts of the world, North American deserts are not only small in area, but youthful in age. As judged by paleobotanical evidence using packrat middens, present floristic compositions extend only to circa 8,000 years ago (Betancourt et al. 1990; Spaulding 1991).

As a whole, the North American deserts are located within the Basin and Range Physiographic Province (Fenneman 1931), a region of parallel fault-block mountains separated by drainage basins. In the Great Basin, Mojave, and Chihuahuan Deserts, the majority of these basins drains internally, with playas frequent. The Sonoran Desert is the exception, exhibiting predominantly external drainage, although intermittent flows are still the rule (Smith et al. 1997). The deserts of North America can be classified into three warm deserts (Mojave, Sonoran, and Chihuahuan) and one cold desert steppe (Great Basin). The Great Basin Desert is characterized by mean winter temperatures below freezing, with snowfall constituting a significant proportion of total precipitation. Bordering the extensive Great Basin Desert to the south, is the relatively small Mojave Desert, which is characterized by its primarily winter rainfall regime (weakly bimodal), cool winter temperatures, middle elevations, the presence of a hexaploid race of the creosote bush, Larrea tridentata (DC.) Coville, and the coincident distribution of Joshua tree (Yucca brevifolia Engelm.; MacMahon & Wagner 1985). Moving from west to east across the three warm deserts, a unimodal winter rainfall pattern (western Mojave Desert) gives way to a summer rainfall pattern that is first bimodal (winter and summer rains, Sonoran Desert), and then unimodal with summer rainfall (Chihuahuan Desert). Such variation in precipitation in deserts has major implications for poikilohydric organisms, since it directly influences soil moisture: winter rains moisten the soil for a much longer duration than summer rains. Seed plant floristic composition in these deserts is thought to result primarily from these differences in rainfall patterns (Smith et al. 1997). Defining features of the Sonoran Desert include relatively low elevation, warm winter temperatures, bimodal rainfall regime, the occurrence of a tetraploid race of Larrea tridentata, and the coincident distributions of three tree species, ironwood (Olneya tesota Gray), paloverde (Cercidium floridum Benth.), and little-leaf paloverde (C. microphyllum (Torr.) Rose & Jtn.). Defining features of the Chihuahuan Desert include summer rainfall, higher elevations, the prevalence of perennial grasslands, the occurrence of a diploid race of Larrea tridentata, and the coincident distributions of two perennials, tarbush (Flourensia cernua DC.) and mariola (Parthenium incanum H.B.K. The presence of high mountain ranges in deserts creates ecological elevation gradients over which plant assemblages change drastically over short distances. The lower tree line is encountered as one enters pinyon-juniper woodland, and the "desert" is left behind. Above this point, at least five well defined vegetation zones occur, including a mesic evergreen forest at the highest elevations (Magill 1982). Thus the southwestern deserts contain a variety of nondesert communities.

A salient feature of deserts is that evaporation offsets much of the gains in water input from precipitation (Bailey 1981). Deserts are characterized by sporadic and episodic rainfall, in which a drought of several months can be followed by a high intensity rain event that may represent a third of the annual precipitation. As a result, the coefficient of variation (CV; ratio of standard deviation to mean) of annual precipitation, among the world's biomes, is highest in warm deserts and second highest in cold deserts (Frank and Inouye 1994). That is, variation in rainfall is highest in deserts than any other biome. Similarly, along an elevation gradient in the desert, annual rainfall patterns will become less variable as elevation increases. Much of the heavy rains in deserts that is not lost from evaporation can be lost via runoff, rather than stored in soils. Low relative humidities and high surface temperatures translate into quickly drying surface soils throughout most of the year in warm deserts. Because of this unpredictability in rain events, seed plants of the most arid regions have evolved phenological schedules that "maintain flexible life histories that can opportunistically respond to sporadic inputs of moisture into the system" (Smith et al. 1997).

In general, desert soils are dry for most of the year, are distinctly low in organic content, N, and P, are rich in inorganic ions composing carbonates and gypsum, are high in calcium, and are alkaline. One consequence of low soil moisture contents in combination with high soil alkalinity is a slow decomposition rate that translates into a deficiency in important macro- and micronutrients (Evenari 1985). Low elevation desert soils are further impacted by excessive livestock grazing, which has by many accounts resulted in losses in biodiversity and significant disruptions to ecological processes (Fleischner 1994). Personal observations in the Mojave Desert in California and Nevada indicate that, where livestock grazing is presently occurring or has recently occurred, soil crustal communities are severely damaged or absent entirely, and the bryophytes are restricted to boulder and rock outcrops. A sharp contrast exists between north-facing and south-facing slope aspects: bryophytes may be absent entirely from surfaces along a south-facing aspect that exceeds 20 degrees, whereas rich moss communities can occur on north-facing sloping soils or, more commonly rock outcrops (Nash et al. 1977). Net annual primary productivity of plant communities in the Mojave Desert is among the lowest in the world: 13-57 g m^-2 yr^-1 for creosote-bursage scrub. Water availability is the most important factor limiting primary productivity, with nitrogen second (Smith and Nowak 1990).

The Great Basin Desert is far better known floristically than the warm deserts of North America. The monumental Utah flora of Seville Flowers (1973) includes not only Utah, but elements from surrounding states, making this treatment the only comprehensive description of mosses in the North American arid west. He lists 256 species of mosses, among 77 genera, as occurring in Utah. When the "intermountain west" is defined to include Utah, Nevada, southern Idaho, southwestern Wyoming, southeastern Oregon, northern Arizona, and the "indefinite fringe of western Colorado and adjacent New Mexico", the region includes 342 species from 122 genera and 39 families (Spence 1988). The latter list approximates the natural boundaries of the Great Basin Desert as considered here, but is somewhat more inclusive in its eastern and southern boundaries and includes non-desert regions (Smith et al. 1997). The more northerly regions of the Great Basin Desert (southeastern Washington, eastern Oregon, northeastern California, and northwestern Idaho) can be accessed through Lawton (1965, 1971). Individual state lists for this region can be accessed for Colorado (Weber 1973), Idaho (McCleary and Green 1971), Nevada (Lawton 1958), Oregon (Christy et al. 1983), and Wyoming (Eckel 1996).

The natural boundaries of the northern Mojave Desert include the two southernmost counties in Nevada, the southwesternmost county in Utah, and the northwesternmost county in Arizona. The bryofloristics of this region were recently summarized to include 75 species of mosses (Stark and Whittemore, in press). As with other treatments in arid North America, most of these species are montane and thus not actually inhabitants of the desert. Nevertheless, well over half of these species belong to the more xeric families Pottiaceae, Grimmiaceae, and Orthotrichaceae. The former list is derived from portions of state treatments/checklists (Lawton 1958; Haring 1961; Flowers 1973) and more recent distributional studies in Nevada (Nash et al. 1977; Lavin 1981, 1982). In contrast, the bryoflora of the California Mojave and California Sonoran Deserts is virtually unknown. Drawing primarily from the southern California checklist of Harthill et al. (1979) and historical statewide data (e.g., Koch 1950), 25 species are derived. As presently construed, the California Mojave Desert and California Sonoran Desert add only 8 species to the Mojave Desert as a whole, a number which is clearly too low and points up inadequate exploration.

The Sonoran Desert stretches from southeastern California south throughout most of Baja California, east through most of southern Arizona, and south through most of the state of Sonora, Mexico. This is a region of considerable diversity of mosses, inclusive of perhaps over 200 species (Bartram and Richards 1941; Haring 1961; Johnsen [undated]; McCleary 1954, 1959, 1962). However, the "true desert" regions within this large area are probably low in species diversity, as evidenced by, for example, a survey of Baja California (Bowers et al. 1976), which is estimated to have in the neighborhood of 120 species (Delgadillo 1993). Similarly, the true desert plant communities identified by McCleary (1962) determined that less than six percent of the Arizona species occur in low elevation deserts. Although Johnsen [undated] lists 342 species as occurring in Arizona, the northern half of the state does not lie within the Sonoran Desert, and probably has a considerably higher diversity than the southern portion of the state.

The vast Chihuahuan Desert stretches from west Texas and southern New Mexico, south through portions of Chihuahua, Coahuila, Durango, Zacatecas, Nuevo Leon, and San Luis Potosi. Our knowledge of the Chihuahuan Desert derives from a mixture of statewide compilations (Bartram 1949; Whitehouse and McAllister 1954; Mahler 1978; Delgadillo and Cardenas 1979, 1987; Cardenas and Delgadillo 1984) and addenda (Ireland et al. 1981, 1984) coupled with intensive work in localized parks and ranges (Little 1937; Magill 1976; Stark and Castetter 1982, 1986, 1987. The flora of Big Bend National Park in the eastern Chihuahuan Desert represents the most detailed study in the mosses of this region (Magill 1976). The author lists a total of 105 species in 58 genera, and indicated that the mosses are principally derived from families of South American origin. As noted in Delgadillo (1993), there is marked similarity between the moss floras of Zacatecas, Mexico, to that of west Texas and New Mexico. In what may be typical of warm deserts in North America, elements of the northern Mexican deserts include a large group of mosses that are widely distributed (i.e., deserts and non-deserts; Delgadillo 1993).

Besides the specific checklists noted above, workers in American deserts have been immeasurably assisted by Zander's (1993) treatment of the entire Pottiaceae, the publication of the Mexican bryoflora (Sharp et al. 1994), unpublished keys by Norris for California (see Shevock treatment, this volume), and keys to dominant genera in the region, such as key desert genera Didymodon (Zander 1999) and Crossidium (Delgadillo 1975).

Didymodon nevadensis Zand. was recently described (Zander et al. 1995) from the southern Nevada Mojave, and is restricted to gypsum formations. Although not a localized endemic, aside from three outlying populations in non-desert sites of North America, it has only been reported from large populations north of Lake Mead. Despite a few reports from outside deserts, Crossidium seriatum Crum & Steere remains a globally rare species that is sporadically distributed across the Mojave, Sonoran, and Chihuahuan Deserts. It occurs on gypsum and calcareous soils. Trichostomum sweetii (Bartr.) Stark appears to be a southwestern endemic, known only from seven populations worldwide (Stark 1996). A recent attempt to refind the type collection from which Bartram (1945) described the species, was unsuccessful; this indicates that, at least in the Mojave Desert, populations are not extensive. Syntrichia chisosa (Magill, Delgad. et Stark) Zand. occurs in the Chihuahuan Desert, and, similar to other globally rare bryophytes, it is characterized by few, widely disjunct populations, in this case Mexico, southern Africa, and the Chihuahuan Desert (Magill et al. 1983). However, in Mexico this species may be locally abundant (Mishler 1994; pers. comm., C. Delgadillo).

The genus Entosthodon deserves attention from conservation concerns, for there are apparently at least four species with very limited distributions. Entosthodon planoconvexus (Bartr.) Grout is apparently known from only two populations worldwide, one of which is in the Mojave Desert of southern Utah, and the other in the Sonoran Desert of Arizona (Pima County; Haring 1961; Flowers 1973). Entosthodon sonorae (Card.) Steere is a calcareous southwestern endemic found in the Sonoran Desert of northern Mexico and Arizona, with an outpost in west Texas (Magill 1976). A third species in this genus, E. tucsonii (Bartr.) Grout is apparently known only from Pima Co., Arizona (Haring 1961). Finally, E. wigginsii Steere occurs in southern Utah and Arizona (Flowers 1973). Perhaps notable, the latter species is not listed in a list of Arizona mosses (Johnsen, undated). Grimmia americana Bartr. is known from only two populations worldwide, one in west Texas and the other in Arizona (Crum & Anderson 1981; Crum 1994). The close resemblance among a trio of desert species (G. americana, G. anodon Bruch & Schimp. in B.S.G., and G. plagiopodia Hedw.), the latter two of which are common, no doubt has hindered interpretation of this rare endemic. Grimmia moxleyi Williams in Holz., although rarely reported from the Mojave (Harthill et al. 1979), will probably turn out to be be a common associate of G. orbicularis Bruch ex Wils. The former species is endemic to the southwestern U.S. and adjacent northern Mexico (Greven 1999).

Pseudocrossidium crinitum (Schultz) Zand. is known from only a single locality in southern Nevada (Lawton 1958), and a single locality in southern Utah (Spence 1987). However, in the Chihuahuan Desert of southern New Mexico and west Texas, it can be the dominant moss in the desert lowlands (Magill 1976; Stark and Castetter 1987). This species, and at least four others (Tortula bartramii (Steere in Grout) Zand., Syntrichia chisosa (Magill, Delg. & Stark) Zand., S. pagorum (Milde) Amann, and Didymodon nevadensis), are of interest in that they are not known to reproduce sexually in the desert southwest due to the absence of male plants.

Differences in degree of mesicness across the three warm deserts are indicated by the diversity of leafy hepatics. For example, in the Organ Mountains of the Chihuahuan Desert, four species were recovered (Stark and Castetter 1982). However, not a single leafy hepatic has yet been reported for the Mojave Desert (Stark and Whittemore 2000), despite the existence of high elevation mountain ranges. In this vein, it is not surprising that the Sonoran and Chihuahuan Deserts are more diverse than the drier Mojave.

Excellent reviews exist of the six principal habitat types inhabited by desert mosses (Scott 1982) and aspects of reproduction in desert bryophytes (Longton 1988). In addition, an introduction to the Utah deserts in provided in Flowers (1973). The author readily admits to adequate familiarity with only a single desert, the Mojave, with some experience in the northern Chihuahuan.

The blackish superficial crusty covering often observed atop desert soils, while inconspicuous, has great ecological value to the community. It consists of not only mosses, but elements from as many as four different kingdoms. Lichens with cyanobacterial symbionts fix atmospheric nitrogen. Populations of cyanobacteria may also inhabit the axils of Syntrichia and Grimmia moss leaves (pers. obser.). In addition to providing the primary source of nitrogen for the ecosystem, the desert crust functions in preventing soil erosion, retaining soil water, and enhancing seedling germination (St. Clair et al. 1984; Lesica & Shelly 1992; Eldridge 1993; Evans & Ehleringer 1993; Eldridge & Rosentreter 1999). Once disturbed, desert cryptogamic crusts may take many decades to reestablish (Belnap 1993).

Perhaps the most impressionable features that the desert makes upon a bryologist are the dominance of the Pottiaceae and Grimmiaceae, the absence of hepatics and pleurocarps, and the tremendous changes in moss community structure along an elevation gradient. Lawton (1971) lists 17 species as characteristic of the "arid transition zone" (the northern reach of the Great Basin Desert), of which 6 are in the Grimmiaceae, and 4 in the Pottiaceae. Similarly, Flowers (1973) describes the "typical desert mosses" inhabiting the southern Utah desert (Mojave Desert) as consisting of 28 species. Of these 16 belong to the Pottiaceae, and 3 to the Grimmiaceae. Magill (1976) describes the creosote bush desert habitat in the Chihuahuan as dominated by six genera in the Pottiaceae, with pleurocarpous mosses notably absent. From the entire national park (Big Bend), the above author indicates that the family Pottiaceae comprises the largest family, including over 40% of all species. Stark and Castetter (1982) identify 17 species of mosses as occurring in the lowland deserts aside the Organ Mountains of the Chihuahuan Desert. Of these, 11 are in the Pottiaceae, and 5 in the Grimmiaceae. Harthill et al. (1979) list 25 species as occurring in deserts or desert ranges in the California Mojave and Sonoran Deserts. Of these, 14 are in the Pottiaceae, and 6 in the Grimmiaceae. Finally, McCleary (1962) categorized 31 species in Arizona as desert mosses, with 19 pottiaceous and 5 grimmiaceous species. The Pottiaceae (76 species) and Grimmiaceae (38 species) comprise two of the three largest moss families in the state of Arizona (McCleary 1954). In one of the few reports from a locality that is entirely desert (i.e., no included montane regions), McCleary (1959) discussed 11 species of mosses from Papago Park in the Sonoran Desert, including 6 in the Pottiaceae and 2 in the Grimmiaceae. Similarly, the bryophytes of the Jornada Experimental Range in the northern Chihuahuan Desert consisted of 23 species, of which a majority belonged to these two dominant families (Little 1937). Other deserts of the world exhibit similar patterns (e.g., Downing and Selkirk 1993).

Data from Stark and Castetter (1987) contrast a low elevation site in the Chihuahuan Desert with the high elevation site in the adjacent Organ Mountains. At the low elevation desert site, the authors recovered a mean population density (the number of species occurring in a clump or patch of contiguous soil mosses) of less than 0.01 m^-2 and an absence of pleurocarpous species, whereas at the high elevation site mean population density was 8 m^-2 and > 50% of the species were pleurocarpous. Population structure changed markedly with elevation: species diversity increases, density of populations increases, and the frequency of acrocarpous populations decreased. Notably, the number of species per aggregrate population of plants, while increasing along the elevation gradient, was comparable at the low desert site to the highest elevation site, indicating clustered populations in the desert. Such changes along elevation gradients are reflected in physiological changes in species desiccation tolerance, with low elevation species of Syntrichia exhibiting greater desiccation tolerance than high elevation congeners (Oliver et al. 1993).

Prospects of low diversity in deserts probably attract few bryophyte plant collectors. Indeed, low deserts often exhibit markedly lower numbers of mosses than high elevation areas (e.g., Lawton 1971; Flowers 1973; Magill 1976). However, this can be deceiving. Recently one of Lawton's (1958) collecting sites was revisited in the Mojave Desert, where she had collected and reported fewer than five species, and three times the number of species were recovered. Bryologists accustomed to collecting in mesic sites can easily overlook the presence of blackish soil crusts.

Finally, I wish to address the perception that a significant proportion of desert mosses is ephemeral, annual, or transient species that evade, rather than tolerate, drought. Whereas some flowering desert annuals have accelerated life cycles resulting in seed set in as little as six weeks after germination (Smith and Nowak 1990), unpublished field investigations of the Mojave Desert mosses inhabiting the harshest low elevation habitats (where annual precipitation averages 100 mm yr^-1), indicate a well developed pattern of perennial life history. Species in the genera Pterygoneurum, Crossidium, Tortula, Syntrichia, Grimmia, Weissia, and even Funaria (F. muehlenbergii Turn.) all appear to be perennial, and capable of tolerating long periods of drought. The tremendous physiologic desiccation tolerance capabilities of desert mosses have been extensively studied using Syntrichia ruralis as the model species (e.g., Bewley 1979; Oliver 1991).

Acknowledgments. The author thanks the National Geographic Society (grant number 5429-95) for providing partial travel costs; the National Park Service for funding floristic investigations in Clark County, NV (Libby Powell); the Bureau of Land Management, Las Vegas for granting a collecting permit (Gayle Marrs-Smith); and the University of Nevada for providing a New Investigator Award. In addition, Alan Whittemore and Claudio Delgadillo provided invaluable input to this paper.

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-----. 1999. A new species of Didymodon (Bryopsida) from western North America and a regional key to the taxa. Bryologist 102: 112-115.

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This page is part of the
Bryophytes of Nevada On-line
web site, with content contributed by
Dr. Lloyd Stark
Plant Ecologist, Bryologist and Assistant Professor
University of Nevada at Las Vegas
and
James R. Shevock
University of California, Berkeley, and
California Academy of Sciences, San Francisco
and hosted by the
Nevada Natural Heritage Program
on the
State of Nevada web server