J. Hattori Bot. Lab. No. 82: 157- 169(July1997)

THE BIOGEOGRAPHY OF THE BRYOPHYTES OF THE SEMI-ARID STEPPE OF SOUTH-CENTRAL BRITISH COLUMBIA, CANADA

TERRY T. MclNTOSH1

ABSTRACT. The biogeography ofbryophytes of the semi-arid steppe of south-central British Colum­ bia Province, Canada, was discussed under three major elements: the widespread elements, endemic elements and disjunctive elements. Possible causal explanations and illustrative bryophytes are pro­ vided for each of the three elements to elucidate the complex issues ofbiogeography involved.

INTRODUCTION This paper presents a biogeographical discussion of the bryophytes collected by Mcin­ tosh (1986) in his survey of the semi-arid steppe and associated open Pinus ponderosa Dougl. forests of south-central British Columbia. The area that was investigated included the dry portions of the Fraser, Thompson, and Nicola Rivers to the north and west of the study area, and the Okanagan and Similkameen Rivers in the south and east. Seventy two species of mosses and five species of liverworts were collected during the survey. Species are grouped and discussed here within three broad biogeographical cate­ gories: the Widespread Element, the Endemic Element, and the Disjunctive Element. These elements have been derived from a number of sources, including Godfrey (1977), Noble (1982), Schofield (1969, 1980, 1988a, 1988b, 1992), Schofield and Crum (1972), and Schuster (1984). The classification of arid land elements developed by Frey and Kurschner (1988) was not used for the following reasons. Firstly, since each tax.on has a unique evolutionary and distributional history, I prefer to keep any characterization of elements at as general a level as possible; afterall, elements are somewhat artificial groupings. As with terms such as bio­ mes or communities, elements serve only as general guides towards the understanding of regional evolutionary history. By employing the "Circum-Tethyan" and "Xerothermic-Pan­ gaean" elements of Frey and Kurschner (1988), too much of a causal restriction is placed on the biogeographical understanding of the ranges of taxa within the elements. Although I concur with Frey and Kurschner (1988) that the disjunctions in these elements may be ex­ tremely ancient, one cannot exclude more recent arid land expansions or the occurrence of long distance dispersal events. The biogeographic study of bryophytes is encumbered with a number of limitations, including paucity of collections, incomplete taxonomy, possible misidentification of mater­ ial, an inadequate fossil record, and site disturbance (Schofield 1992). All of these factors must be considered when discussing bryophyte distribution patterns. It is because of these limitations that many of the ideas in the following discussions are speculative, but poten­ tially valuable as long as the limitations are fully understood (Noble 1982).

1 Herbarium, University of British Columbia, Vancouver, British Columbia, Canada, V6T 2Bl; Biospherics Environmental Inc., 16971 12 Ave., Surrey, British Columbia, Canada V4P 2P4. 158 J. Hattori Bot. Lab. No. 82 I 9 9 7

Taxonomic considerations regarding the Pottiaceae follow Zander (1993; more famil­ iar names follow some of Zander's taxonomic decisions). Delgadillo (1996) has expressed uncertainty regarding Crossidium rosei reported by Mcintosh (1989) from the British Co­ lumbia steppe. This taxon as well another Crossidium-like species that was collected from the same area (Mcintosh 1986) need more careful taxonomic clarification before a final de­ termination can be made, and, therefore, they are not included in the following discussions. A detailed flora and keys of the bryophytes of the semi-arid regions of south-central British Columbia is in preparation by the author.

DISCUSSION The Widespread Elements 1. Global or Cosmopolitan Taxa These taxa are widely distributed in all major land masses, and many are found on oceanic islands. Bryum argenteum Hedw. Leptobryum pyriforme (Hedw.) Wils. B. caespiticium Hedw. Polytrichum juniperinum Hedw. Ceratodon purpureus (Hedw. )Brid. P piliferum Hedw. Funaria hygrometrica Hedw. Schistidium apocarpum (Hedw.) B.S.G. 2. Circumpolar Taxa This element contains species that are more or less continuous in their distributions across the Northern Hemisphere. Five of the taxa (denoted by*) have more northemly dis­ tributions than the others, and tend to penetrate temperate latitudes only along some moun­ tain chains. *Alaina brevirostris (Hook. & Grev.) Ditrichumfiexicaule var. densum (B.S.G.) Kindb. Braithw. *Athalamia hyalina (Sommerf.) Hatt. Encalypta rhaptocarpa Schwaegr. Barbu/a convoluta Hedw. Eurhynchium pulchellum (Hedw.) Jenn. Brachythecium albicans (Hedw.) Grimmia anodon B.S.G. B.S.G. Hennediel/a heimii (Hedw.) Zand. Bryoerythrophyllum recurvirostrum [=Pottia heimii (Hedw.) Fum. ex Hampe] (Hedw.) Chen *Hypnum revolutum (Mitt.) Lindb. Cephaloziella divaricata (Sm.) Schiff. *H. vaucheri Lesq. Conardia compacta (C. Muell.) Orthotrichum anomalum Hedw. H. Robins. Pohlia cruda (Hedw.) Lindb. Coscinodon cribrosus (Hook.) C. Jens. * Pseudoleskeella tectorum (Brid.) Broth. Didymodon rigidulus var. gracilis Syntrichia ruralis (Hedw.) Web. & Mohr (Schleich. ex Hook. & Grev.) Zander [ = Tortu/a ruralis (Hedw.) Gaertn.] In general, taxa with global distributions have broad ecological tolerances. Most of these taxa grow in both mesic and xeric sites, although they are usually stunted and sterile in the driest sites. In addition, most are opportunistic in that they grow quickly in available open areas, in particular those resulting from anthropogenic activities (During 1979). In contrast, many of the circumpolar taxa, as well as the species in the other elements T. T. McINTOSH: Biogeography of the bryophytes of the semi-arid steppe 159 discussed later, show less generalized ecological tolerances, and many of them are found only in open sites. Restriction to these warmer and drier sites may be necessary in order for these taxa to complete certain aspects of their life cycles. Johnsen ( 1969) found that a peri­ od of complete drought was essential for capsule maturation and the subsequent shedding of the operculum in Orthotrichum anomalum. Although moisture was necessary for all other aspects of its life cycle, when water was applied to the gametophores during the drought period, the plants died. Van der Linden and Farrar (1983) found that the common prairie moss, Weissia controversa Hedw., attained its peak rate of photosynthesis at high temperatures. In contrast, nearby forest taxa attained maximum photosynthesis at much lower temperatures. Also, only a few of the more restricted taxa appear opportunistic.

The Endemic Elements Nine bryophytes from the study area are endemic to western North America. Bryoerythrophyllum columbianum Entosthodon rubiginosus (Williams) (Herm. & Lawt.) Zand. Grout Coscinodon ca/yptratus (Hook.) Homa/othecium aeneum (Mitt.) Lawt. C. Jens. Schistidium heterophyllum (Kindb.) Didymodon occidenta/is Zander Mcintosh Didymodon brachyphyllus (Sull. Tortu/a brevipes (Lesq.) Broth. in Whipp!.) Zand. [ =Didymodon Tortu/a nevadensis (Card. & Ther.) Zand. vinea/is var. brachyphyllus (Sul I. in [ = Pottia nevadensis Card. & Ther.] Whipp!.) Zander] All of the taxa listed in this group are more or less restricted to portions of the Cordilleran steppe, desert, and associated dry mountain regions southwards to central Mex­ ico. This area has been available for plant habitation for long periods of time and has had a diverse orogenic and climatic history (Stebbins and Major 1965; Axelrod and Raven 1985). It remains highly diverse, both topographically and climatologically. As a result, many plant taxa have evolved in situ and have remained in the area (Stebbins and Major 1965). Whether the bryophyte endemics listed here evolved in this region or represent reductions in range of taxa that evolved elsewhere, may be impossible to determine. It is likely that at least some of the taxa have evolved in or near the area (e.g., Coscinodon ca/yptratus and Homalothecium aeneum).

The Disjunctive Elements 1. Circumtemperate Disjunctive Taxa Even though some circumtemperate species are widespread in the northern continents, their continental ranges are separated by wide expanses of ocean, and are thus disjunctive. Some taxa in this group show a western North American - Eurasian/north African distirbu­ tion (*), and others show an even more restricted western North American - western Eurasian and, in some cases, north African distribution ( + ). A/oina bifrons (De Not.) Delgad. *Brachythecium co//inum (C. Muell.) A/oina rigida (Hedw.) Limpr. B.S.G. Barbu/a unguiculata Hedw. + Crossidium aberrans Holz. & Bartr. 160 J. Hattori Bot. Lab. No. 82 I 9 9 7

+Didymodon australasiae (Hook. & Grev.) *Pseudocrossidium revolutum (Brid. Zand. [ = Trichostomopsis in Schrad.) Zand. australasiae (Hook. & Grev.) Robins.] *Pterygoneurum kozlovii Laz. ex Laz. Didymodon fa/lax (Hedw.) Zand. + Pterygoneurum lamellatum (Lindb.) Jur. *Didymodon luridus Homsch. in *Pterygoneurum ovatum (Hedw.) Dix. Spreng. [ = Didymodon vinealis var. Pterygoneurum subsessile (Brid.) Jur. luridus (Homsch. in Spreng.) Zand.] R. sorocarpa Bisch. Didymodon vinealis (Brid.) Zand. Riccia crystallina L. Encalypta vulgaris Hedw. *Syntrichia caninervis Mitt. + Funaria muhlenbergii Tum. [=Tortu la caninervis (Mitt.) Broth.] *Grimmia alpestris (Web. & Mohr) Syntrichia intermedia Brid. Schleich. ex Nees [=Tortu/a intermedia (Brid.) Berk.] Grimmia laevigata (Brid.) Brid. Tortu/a acaulon (L. ex With.) Zand. [not *Grimmia ova/is (Hedw.) Lindb. T. atherodes; see page 378 in Zander *Grimmia plagiopodia Hedw. (1993) =Phascum cuspidatum Hedw.] Grimmia pulvinata (Hedw.) Sm. Tortu/a atrovirens (Sm.) Lindb. Grimmia trichophylla Grev. [ = Desmatodon convolutus (Brid.) Grout] *Hilpertia velenovskyi (Schiffn.) Zand. +Tortu/a guepinii (B.S.G.) Broth. [=Tortu/a scotteri Zand. & Steere] [=Desmatodon guepinii B.S.G.] *Manniafragrans (Balbis) Frye & Clark Tortu/a obtusifolia (Schwaegr.) Math. + Microbryum vlassovii (Laz.) Zand. [ = Desmatodon obtusifolius (Schwaegr.) [=Phascum vlassovii Laz.] Schimp.] + Orthotrichum cupulatum Brid. +Tortu/a protobryoides Zand. [ = Pottia *Orthotrichum hallii Sull. & Lesq. bryoides (Dicks.) Mitt.] *Orthotrichum laevigatum Zett. +Tortu/a wilsonii (Hook.) Zand. [ = Pottia *Orthotrichum rupestre Schleich. & wilsonii (Hook.) B.S.G.] Schwaegr. Weissia brachycarpa (Nees. & Homsch.) Jur. The discussion of disjunctive patterns is a major focus of contemporary biogeographic research. It is also the center of the greatest controversy over the two potential causes of disjunctions: long distance dispersal (movement over or through wide areas of inhospitable habitat and subsequent establishment in a new area), or vicariance (the fragmentation of a formerly continuous range). With respect to rather immobile and poorly dispersible organ­ isms, such as maple trees (Wolfe 1981 ), oak trees (Axelrod 1983), or reptiles (Savage 1982), long range dispersal appears extremely unlikely and vicariance theories are com­ monly invoked. In contrast, with respect to bryophytes, highly dispersible organisms in many cases, causes are less evident.

The Long Distance Dispersal Hypothesis Van Zanten and P6cs (1981) listed a series of intrinsic (biotic) and extrinsic (abiotic) factors that are important to the success of a long range dispersal event. Intrinsic factors in­ cluded the production of large quantities of small spores, species opportunism, inhabiting open habitats, and an acrocarpous growth form. Extrinsic factors that they considered im- T. T. McINTOSH : Biogeography of the bryophytes of the semi-arid steppe 161 portant were if the giving and receiving areas of the dispersal event are in the same climatic belt, if the receiving area is large and climatically diverse, and if an unsaturated receiving area is available. Schuster (1984) and van Zanten and P6cs (1981) agreed that this latter factor is of great importance; the former (Schuster 1984) considered this of greater impor­ tance than intrinsic and other extrinsic factors Regarding the intrinsic criteria, 28 of the above listed disjunct taxa produce sporo­ phytes frequently throughout their ranges. The large majority of the taxa can complete their life cycle in exposed sites, and most are acrocarpous. Also, at least some appear to be op­ portunistic (Barbu/a unguiculata and Pterygoneurum ovatum, for example). Most of the taxa have small spores. Whether or not small spore size is advantageous to dispersal is questionable. The hepatics, Athalamia hyalina, Mannia fragrans, and the two Riccia species, have large spores, and are all relatively widespread across the Northern Hemi­ sphere. Large spores may prove advantageous to spore dispersal and establishment in that they tend to live longer and are more resistant to environmental stress (Schuster 1984). Schuster (1984) also pointed out that, in general, hepatics with large spores have wider ranges than those with smaller spores. Van Zanten and P6cs (1981) considered that smaller spores are more easily lifted into the air than larger spores. However, there is no evidence for this assumption. Also, winds can be strong enough to pick up particles much larger and heavier than spores and carry them long distances. Dust clouds have been blown across the Atlantic Ocean from north Africa to the central Caribbean (Schuster 1984; van Zanten and Pocs 1981 ). One possible advantage of smaller spores is that the smaller the spore, the more that are produced per capsule, which may prove an important dispersal consideration (Schuster 1984 ). Schuster ( 1984) also considered autoicous sexuality an important character in suc­ cessful long distance dispersal. Autoicous taxa comprise over half of this disjunct group. Theoretically, once an autoicous taxon becomes established in a new site it can reproduce and spread more quickly than a dioicous taxon as both sexes are included within the same plant. Conversely, dioicous taxa, unless a plant of the opposite sex grows nearby, cannot produce sporophytes and spread as effectively locally, unless the taxon has effective asexual reproduction. Van Zanten and P6cs (1981), however, do not consider sexuality important in long range dispersal. They noted that bryophyte spores probably travel in clouds or in clus­ ters and several plants of each sex may travel together and land adjacent to one another. Extrinsic factors appear to favor long distance dispersal in this group. Semi-arid, open habitats are widespread in the Northern Hemisphere. They are within the same climatic zone. Both east and west hemispheres have extensive and ecologically diverse receiving areas that, in some cases, may be unsaturated, especially if disturbance factors are consid­ ered (although, if the study area in south-central British Columbia is representative, very little may be able to grow in these unsaturated areas; the trampling of domestic grazing herds is clearly detrimental to bryophyte establishment). However, whether or not most of these taxa are capable of crossing the extensive ocean barriers is uncertain. Some of the disjunct taxa, Barbu/a unguiculata, Didymodon fa/lax, D. vinealis, Grimmia laevigata, G. pulvinata, G. trichophylla, and Tortu/a obtusifo­ /ia, are found in Hawaii (Hoe 1974). This suggests that these taxa are capable of dispersal 162 J. Hattori Bot. Lab. No. 82 I 9 9 7 over long distances, probably by air currents in this case. Also, some of the species, Brachythecium collinum, Tortu/a obtusifolia, Didymodon fal/ax, Grimmia alpestris, G. pla­ giopodia, G. pulvinata, G. trichophylla and Orthotrichum rupestre, are rare disjuncts in Greenland (Mogensen 1982), further supporting an hypothesis of long distance dispersal. However, to explain the latitudinal disjunctions of these taxa strictly on the basis of long distance dispersal is questionable. The bryoftoras in the two disjunct continental areas are strikingly similar (over 70% at the species level, applying S0rensen's Index; Mcintosh, 1986). It would be difficult to account for this based on long distance dispersal alone, espe­ cially considering that some of the taxa are usually sterile throughout their range, although the ability to reproduce sexually may have been lost (Schuster 1984). Furthermore, even though some species are present in suitable intervening areas, most of the taxa are absent from these areas.

The Vicariance Hypothesis Regarding arid land bryophyte disjunctions in Northern Hemisphere, two explanations based on vicariance are possible. At least some of the taxa may be representatives of an open, temperate, arid vegeta­ tion zone that may have been present across part or all of the southern portions of Laurasia before the Eurasian and North American continents completely separated (Axelrod 1975; Karnefelt 1980; Looman 1964). This arid region would have been widespread across Laurasia from the late Cretaceous into the Eocene, intermittently replaced by tropical or wet temperate forests (Wolfe 1969). The subsequent widening of the north Atlantic Ocean reduced and eventually eliminated much of the potential for direct migration between the areas (Schuster 1984), thus producing two disjunctive temperate regions. Representative bryophytes of a trans-Laurasian temperate belt, therefore, would have to be extremely an­ cient (>40 million years old) and genetically conservative, since disjunct populations of most species are morphologically nearly identical. However, at least a few of the taxa ex­ press morphological differences between the two continents. These include Tortu/a guepinii (with smaller gametophores in North America than in Europe), Didymodon luridus (with North American plants smaller and with more rounded leaves), Tortu/a wilsonii (with North American plants generally larger, and having more toothed leaves and longer more hyaline awns), Pseudocrossidium revolutum (with North American plants smaller and lacking leaf gemmae in most cases), and Pterygoneurum kozlovii (with North American populations having larger and rougher spores, and larger upper leaf cells than the European plants). Speciation in many bryophyte groups is extremely slow. Fossils of Ter­ tiary age, at least those from mesic and wetter habitats, support this view, with many fossils appearing identical to extant bryophytes (Miller 1984). However, there is a complete lack of dry land bryophyte fossil records. A second vicariance scenario is more recent. Following the dissection of the Laurasian landmass and the concurrent development of the north Atlantic Ocean, it was no longer possible for a belt of temperate vegetation to be connected laterally between the two conti­ nental regions. Instead, connections would have had to exist over higher latitudes, across Greenland or, more likely, across the Beringian (Alaskan and Siberian) region (Pielou T. T. McINTOSH: Biogeography of the bryophytes of the semi-arid steppe 163

1979). This region has been connected frequently in the past since the separation of the northern continents (Young 1982; Yurtsev 1982). There is evidence that widespread peri­ ods of aridity occurred at least during the late Eocene and Oligocene (Axelrod 1975; Sol­ brig 1976). During this time the Eocene subtropical forests that previously stretched around the North Pacific were replaced by much drier forest types, accompanied by open vegeta­ tion (Axelrod 1975). Dry climatic phases occurred through the Pliocene and into the Pleis­ tocene (Daubenmire 1975; Solbrig 1976; Wolfe 1969), particularly during the numerous interglacial periods (Young 1982). Steppe vegetation was present in the Arctic during the hypsithermal (xerothermic) interval that occurred in the Holocene, sometime between 10,000 and 6,000 years ago (Alley 1976; Daubenmire 1975; Hebda 1982). At this time, North American steppe and grasslands were more extensive than at present (Hebda 1982), and open steppe-like vegetation was probably contiguous from the temperate regions into the Arctic, both east of the Rocky Mountains and through the low inter-mountain valleys in the interior portions of British Columbia (Murray et al. 1983 ). Direct communication with the Asiatic steppe province, more extensive during this period as well (Yurtsev 1982), would not have been possible, since a land connection did not exist (Hopkins l 982; Young 1982). However, dispersal distances would have been greatly diminished and some migra­ tion could have occurred across the narrow strip of open water. Remnants of disjunct temperate steppes have been found on unglaciated but usually steep and dry, south-facing terrain in interior Alaska and in the Yukon (Murray et al. 1983), as well as in north-eastern sections of Asia (Young 1982; Yurtsev 1982). Along with lichens, and to a lesser degree, vascular plants, Murray et al. (1983) reported a series of characteristically temperate bryophyte taxa from these sites, including Alaina rigida, Ptery­ goneurum lamellatum, P ovatum, P subsessile, Syntrichia caninervis, and Tortu/a acaulon. Earlier, Steere (1979) found temperate disjunct bryophytes in Arctic Alaska. His list in­ cluded Coscinodon calyptratus, Grimmia plagiopodia, Pterygoneurum lamellatum, P sub­ sessile (cf. Steere and lwatsuki 1974), Syntrichia caninervis and Tortu/a obtusifolia. Other disjunct temperate taxa found in or near the Arctic include Pseudocrossidium revolutum (Janssens and Zander 1980) and Bryoerythrophyllum columbianum (as Didymodon columbianus Herm. & Lawt. in Steere and Scotter, 1978). In addition, Murray (1984) found Indusiella thianschanica, a bryophyte previously known only from the steppe or desert regions of Eurasia and absent from temperate North America. As pointed out by Murray et al. (1983 ), some of these taxa could have reached their northern stations by long distance dispersal. For example, a few species have been collect­ ed in previously glaciated sites in northern Alberta and northern British Columbia. Includ­ ed are Grimmia plagiopodia, Pterygoneurum ovatum, P subsessile, Tortu/a acaulon, and T. obtusifolia, although these may also be relicts of Hypsithermal age. Combined with the high potential for dispersal of most of the taxa in the disjunct group, it is possible that their migration was independent of an accompanying steppe vegetation. Nonetheless, the major­ ity of the taxa are most frequent in or even restricted to unglaciated areas (Murray et al. 1983 ), sites inhabited by similarly disjunct lichens and vascular plants. 164 J. Hattori Bot. Lab. No. 82 I 9 9 7

2. Bipolar Taxa Included in this group are circumpolar and circumtemperate taxa of Northern Hemi­ sphere which have spotty distributions in the Southern Hemisphere. Several of the taxa mentioned under previous categories, circumpolar and circumtemperate disjunctive taxa, are listed again below. The reason for this is that causes for the disjunctive distribution of these taxa in the Northern Hemisphere are possibly different from the causes that resulted in their disjunction between the Northern and Southern Hemispheres. Aloina bifrons [South Africa, South Antarctica] America, Australia, New Zealand] Grimmia pulvinata [South America, Alaina rigida [South America, South Africa, Australia, Australia] New Zealand] Barbu/a convoluta [New Zealand] Grimmia trichophyl/a [South America, Barbu/a unguiculata [New Zealand, Australia, New Zealand] South America] Hennediel/a heimii [South America, Brachythecium albicans [Australia, Antarctica, Australia, New Zealand] New Zealand] Hypnum revo/utum [Antarctica] Bryoerythrophyl/um recurvirostrum Pohlia cruda [Australia, New Zealand] [New Zealand, Australia, South Pterygoneurum ovatum [Australia, New Africa] Zealand] Cephaloziel/a divaricata [Australia] Pterygoneurum subsessile [South Didymodon austra/asiae [South Africa, America] South America, Australia, Riccia crystallina [South America, New Zealand] South Africa] Didymodon vinea/is [South America] Riccia sorocarpa [South Africa] Enca/ypta vulgaris [Australia, New Syntrichia ruralis [South America, South Zealand] Africa, Australia] Grimmia anodon [South America) Syntrichia iniermedia [South Africa] Grimmia laevigata [South America, Tortu/a acaulon [New Zealand] South Africa, Australia, New Tortu/a atrovirens [South Africa, South Zealand] America, Australia, New Zealand] Grimmia plagiopodia [South America, Weissia brachycarpa [Australia] Most of the taxa that exhibit bipolar disjunctive distributions are fairly widespread in the Northern Hemisphere. Only Didymodon australasiae has a relatively narrow distribu­ tion. Hypnum revolutum is the only member of the more northern circumpolar group that has a strictly bipolar disjunction. The poor representation ofboreal and arctic-alpine taxa in the bipolar element probably reflects both the greater distances between suitable habitats in both hemispheres, as compared with the taxa distributed in temperate zones, as well as the lower frequency of such habitats in the Southern Hemisphere. In addition, temperate taxa may be more tolerant of peri-tropical environmental conditions, and thus more able to cross this barrier.

The Long Distance Dispersal Hypothesis Long distance dispersal is often used to explain bipolar disjunctive patterns of T. T. McINTOSH: Biogeography of the bryophytes of the semi-arid steppe 165 bryophytes (Schuster 1984; Schofield 1974, 1980), as well as vascular plants (Raven 1972; Raven and Axelrod 1974), and lichens (Rogers 1977), even though distances are great and trans-equatorial dispersal is considered very difficult (van Zanten and P6cs 1981; Solbrig 1972). This hypothesis is supported by the generally high potential for dispersal of the bipo­ lar taxa. Most of them commonly produce sporophytes throughout their ranges. The usual­ ly sterile Cephaloziella divaricata and Grimmia trichophylla produce abundant gemmae. Most are able to complete their life cycle in open habitats, and the majority are acrocar­ pous. Furthermore, some of the bipolar species are opportunistic, and about half on them are autoicous. Extrinsic factors noted earlier that may favor the Northern Hemisphere disjunctions are not as favorable with respect to the bipolar taxa. Receiving areas are both smaller and more isolated in the Southern Hemisphere. In some cases, even though they are often wide­ spread in the Northern Hemisphere, their rare occurrence in the south suggests a single and long distance dispersal event (e.g., Tortu/a acau/on, Barbu/a unguicu/ata), which may be human-influenced (Schofield 1974). Schofield (1974) and Schuster (1984) noted that in cases where long range dispersal is likely to be the cause, then it has probably occurred relatively recently, as few morphologi­ cal or ecological differences have been noted between populations from both hemispheres. As noted earlier, however, rates of evolution may be very slow in these groups, and the lack of differences between distant populations does not necessarily indicate a recent dispersal event. They suggested that most natural long distance events would have occurred during or just following the Pleistocene glaciations, in periods when a cooler and drier climate compressed the world's vegetation zones towards the equator, producing relatively shorter dispersal routes between the two hemispheres, as well as more open, less colonized sub­ stratum.

The Vicariance Hypothesis Trans-tropical migrations of arid-land plant taxa between North and South America, and the subsequent splitting of their ranges, could have occurred in the past. When the Isth­ mus of Panama was formed during the Pliocene (Coney 1982; Gentry 1982), a continuous or near-continuous dryland link may have been available. Not only were the continents con­ nected and xeric climatic periods present (Solbrig 1976), it was during the Pliocene that most of the Andes uplift occurred, favoring arid conditions both in the lowlands and in alpine regions as are present today (Sarmiento 1976; Solbrig 1976). Trans-tropical dispersal might have occurred in the Pleistocene, during temperate plu­ vial periods. In contrast to the temperate regions that were wetter than normal during these periods, the tropical regions were probably considerably drier (Gentry 1982; Prance 1982). The tropical forests were apparently greatly reduced in size, probably present only as scat­ tered remnants (Gentry 1982), and arid regions were extensive. Also, migrations across tropical regions could have occurred in the Hypsithermal Period of the Holocene (Hebda 1982), during an expansion of the world's steppe and grassland systems, and concurrent contraction of the tropical zone. 166 1. Hattori Bot. Lab. No. 82 I 9 9 7

There is some doubt as to whether a continuous trans-equatorial belt of open arid veg­ etation could ever have existed. Raven (1972) considered such a direct pathway "impossi­ ble to imagine", as the dry areas of both hemispheres have so few organisms in common, at least at the species level. Presumably, a link of this calibre would have allowed the migra­ tion of many more taxa. An alternative to a direct or continuous link across the tropics is hop-dispersal (Schofield 1974; Raven 1972), in which taxa have dispersed across the tropics by migrating short distances over unfavorable vegetation or water from one small area of open arid habi­ tat to another. Schofield (1974) suggested that mountain hopping during the Pleistocene may be responsible for the bipolar distribution of some taxa. The possibility of any of these disjunct ranges resulting from the divergence of an an­ cient Permo-Triassic arid zone postulated by Miller (1976, 1982), Schuster (1984) and Frey and Kurschner (1988) is questionable. Most disjunctions representing this epoch are proba­ bly at the generic or familial levels (Crum 1981 , for the genus Eccremidium; Lindsay 1977, with respect to lichens; Schuster 1984). Few, if any, species are likely to have survived to the present, even if slow evolution rates are considered. Nonetheless, trans-tropical migra­ tion, by whatever means, is difficult. This is reflected by the low number of taxa that are common to both hemispheres.

The Human Introduction Hypothesis Some long distance-related bipolar disjunctions in bryophytes have probably resulted from human introduction, as many of the taxa are found in their southern stations in or near areas of anthropogenic disturbance. Schofield (1974) considered Brachythecium albicans, Barbu/a unguiculata, B. convoluta, Tortu/a acaulon, and Pterygoneurum ovatum to be pos­ sibly accidental introductions by man. Catcheside (1980) suspected Alaina rigida to have been introduced into Australia.

ACKNOWLEDGEMENTS Many thanks to Wilf Schofield for his energy, encouragement, and patience during my PhD years and today. Thanks to Mike Hawkes and Kevin Giles for their comments, and to my friend, Benito Tan, many kudos.

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