Jökull - 01.01.2004, Blaðsíða 4

Hafdís Hanna Ægisdóttir and Þóra Ellen Þórhallsdóttir
flora. Although Scandinavia only has a very small
endemic element, there is a difference between the
lowland and the alpine flora, the latter having more
endemic species. If the tabula rasa theory is correct,
the arctic-alpine plant species and the lowland plant
species are approximately of the same age in Scandi-
navia, assuming that their ancestors migrated from the
south and east to Scandinavia in late-glacial and early
postglacial times. If so, the relatively large number
of endemic taxa in the mountain flora was considered
to be a problem. However, their relatively large num-
ber can be explained if the mountain flora survived
the last glacial age in refuges along the Scandinavian
coast (Dahl, 1955).
3. Disjunction and Centricity in the Alpine Flora of
Scandinavia
The Swedish botanist Fries (1913) was the first to
divide the Scandinavian mountain flora into groups
according to distribution patterns. Fries showed that
many species in the Scandinavian mountain flora had
a centric distribution. About 25 species showed a bi-
centric distribution, occupying an area in the moun-
tains of southern Norway and another in northern
Scandinavia. The uni-centric plant species were only
found in one of these areas (Gjærevoll, 1963). The
fact that 80% of the west-arctic element are “cen-
tric” (Nordal, 1987) was explained by e.g. Nordha-
gen (1936, 1963) by the glacial survival theory. The
species must have survived in two separate refugia
in Scandinavia; one in the Dovre/Jotunheimen moun-
tains in south Scandinavia and the other in north Scan-
dinavia from the Arctic Circle northward to Troms
and Finmark (Dahl, 1955).
Land bridge or Long-Distance Dispersal
The close phytogeographic relationship within the
flora of Scandinavia, the Scottish mountains, the
Faeroes, Iceland and Greenland, made scientists won-
der how the species originally dispersed. Dahl (1958;
1961; 1963) proposed the following alternatives:
1. Beringian land bridge,
2. Land bridge connecting North America with
Europe and,
3. Long-distance dispersal.
1. Beringian land bridge:
The flora was originally circumpolar through migra-
tion across the Bering Strait. Subsequently, some
species became extinct in Siberia and western North
America but survived in areas on both sides of the At-
lantic Ocean (Dahl, 1958).
2. North American – European land bridge:
Plants dispersed from northeast America/Greenland
to northwest Europe via an earlier land bridge con-
necting Europe and North America.
According to Lindroth (1963), R. F. Scharff was
the most ardent early supporter of the land bridge
idea and he introduced it in papers from the beginning
of the 20th century. The Norwegian botanist Dahl
(1958) later supported the idea. Several others (e.g.
Löve and Löve, 1956; Einarsson, 1961) assumed that
the land bridge remained into Late Tertiary with sub-
sequent survival of the biota through all Pleistocene
glaciations.
3. Long distance dispersal:
Plants dispersed from northeast America/Greenland
to northwest Europe by long distance transport. They
were classified according to their mode of transport
into: 1) Wind dispersed, 2) Dispersed with animals,
3) Dispersed by the sea, 4) Limnic species and 5)
Species with no adaptation to long-distance dispersal
(Dahl, 1958).
Dahl (1963) concluded that the second alternative,
dispersal via an earlier North Atlantic land bridge,
was the most probable one. Dahl rejected alternative
1 (original circumpolar distribution becoming extinct)
as a general explanation, because several polymorphic
taxa on both sides of the Atlantic are closer to each
other than to related forms in the Bering region. Dahl
also considered long distance dispersal unlikely, due
to lack of special adaptations in the west-arctic ele-
ment. Steindórsson (1963) agreed with Dahl when he
discussed the likelihood of plant species immigrating
to Iceland. Steindórsson (1963) considered three pos-
sibilities for immigration; ocean currents, air, and mi-
grating birds, and found them all unlikely.
4 JÖKULL No. 54