Jökull


Jökull - 01.01.2004, Side 8

Jökull - 01.01.2004, Side 8
Hafdís Hanna Ægisdóttir and Þóra Ellen Þórhallsdóttir 2) The Alpine Endemic Element As mentioned earlier, a relatively high proportion of endemic species is found among the Scandina- vian arctic-alpine flora compared to the lowland flora. Supporters of the theory of glacial survival believed that these species survived the last glaciation. Accord- ing to Nordal (1987), glacial survival is not necessary to explain their existence in the Scandinavian flora. Five of these endemic taxa (Antennaria nordhageni- ana, Arnica alpina, Poa stricta, Saxifraga opdalen- sis, Taraxacum dovrense and T. norvegicum) belong to apomictic groups within which “speciation” can be rapid, e.g. is a single mutation with phenotypic ef- fect distinct enough to give rise to a new “species”. Five other taxa: Dactylorhiza pseudocordigera, Pa- paver laestadianum, Pyrola norvegica, Euphrasia hy- perborea and E. lapponica belong to groups with complex and often unresolved taxonomy where jus- tification for specific delimitation may be questioned (Nordal, 1987). Some of the endemics are high poly- ploids e.g. Draba cacuminum, D. dovrensis and Prim- ula scandinavia. The high polyploids may have been established by hybridization followed by allopoly- ploidation, but that kind of speciation could certainly occur during postglacial time (Nordal, 1987). She concluded that the endemic species may well be no more than postglacial in age (e.g. no older than about 10,000–15,000 years). 3) Disjunction and Centricity Nordal (1987) pointed out that glacial survival is not necessary to explain the centricity of the alpine flora of Scandinavia. She argued that in the late glacial period, many of the disjunct species may have had a wide and more or less continuous distribution in Scan- dinavia but later had their range contracted by com- petition and/or the ice expansion 11,000–10,000 BP (Nordal, 1987). Long-Distance Dispersal Excluding migration along land bridges, could long- distance dispersal explain the present distribution of the west-arctic element? Nordal (1987) ques- tioned whether the lack of special adaptation pre- sented serious obstacles to long-distance dispersal as Dahl (1963) had argued. Nordal’s arguments were i.a. based on the fact that three of the west-arctic species/species complexes, all of which lack special adaptation to dispersal, are in fact represented in southern South America. If migratory birds brought these species all the way across the American conti- nent, why could they not be brought across the At- lantic Ocean in the same fashion (Nordal, 1987)? If arctic long distance dispersal actually took place, how did it occur? Most likely, plants were car- ried by wind across sea-ice in winter, across glaciers and snow in the treeless arctic environment, by ice- bergs, or by birds. Seeds and fruit lacking hair, wings or other morphological adaptations may occasionally be dispersed by wind over large distances (Bennike, 1999; Brochmann and Steen, 1999). For example, Cerastium arcticum has no apparent adaptations for long-distance dispersal. Recent molecular analyses show that populations of C. arcticum on both sides of the Atlantic share identical multilocus phenotypes, most probably caused by postglacial dispersal (Hagen et al., 2001). Dispersal by Ice and Wood The idea of biota dispersal by icebergs or drift ice is far from new. Darwin (1859) introduced it in his fa- mous book “The Origin of Species”. A few decades later Blytt suggested this as a possible explanation of the west-arctic element in Scandinavia (Nordal, 1987). Hultén (1962) and Nordal (1987) also con- sidered drift ice or even driftwood as means of long- distance dispersal in the Northern Hemisphere (Jo- hansen and Hytteborn, 2001). Drift ice and driftwood have also been considered important dispersal vec- tors for the immigration of the Icelandic flora, with the flora transported from northern Eurasia via the Transpolar Drift and East Greenland Current (Rund- gren and Ingólfsson, 1999). During the late Weichselian and early Holocene, both drift ice and driftwood may have been important for the dispersal of diaspores from Siberia and north- west Russia to parts of the North Atlantic region. The basins of the great Siberian rivers, draining areas far to the south, are believed to have been sources of seeds or other biota. Species like Draba sibirica, Oxytropis deflexa ssp. norvegica, Potentilla stipularis and Trise- tum subalpestre, all with highly disjunct distributions 8 JÖKULL No. 54
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