Jökull - 01.01.2004, Síða 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