Jökull - 01.01.2004, Blaðsíða 3

Theories on migration and history of the North-Atlantic flora: a review
last century, Thoroddsen (1905–1906) introduced the
idea that the highest parts of the coastal mountains
in Iceland had remained ice-free during the maxi-
mum extent of the one major glaciation that he rec-
ognized. Nearly three decades later, Lindroth also
supported the idea, based on entomological studies
in southeast Iceland (Norðdahl, 1991 and references
therein). Based on geomorphological evidence, the
Icelandic geologist Sigurdur Thorarinsson (1937) ar-
gued that large parts of northwest and north Iceland
were unglaciated during the last glacial period.
Scandinavia
The Norwegian geologist Hansen (1904) proposed a
theory of a broad ice-free margin along the Norwegian
Atlantic coast, where a large number of plant species
could have survived the last glacial period. Hansen
also believed that he could trace a more or less con-
tinuous line of moraines in the fjord-districts of Nor-
way, which marked the outer border of the last large
ice-sheet along the Norwegian Atlantic coast (Dahl,
1955). Later it was shown that this line marks a cer-
tain stage during the deglaciation of Norway and not
the outer boundary of the last ice-sheet (Nordhagen,
1963). In 1912, the geologist Vogt claimed that the
outermost Norwegian islands of Lofoten, Værö and
Röst, had been ice-free during the last glacial pe-
riod (Nordhagen, 1963). Although some early geol-
ogists believed that ice-free areas existed during the
last glacial period, the glacial survival theory also re-
ceived strong criticism from several Norwegian and
Swedish Quaternary geologists who demanded proof
for the existence of ice-free areas. Other geologists
did not totally denounce the “refugium theory” but
preferred to remain uncommitted (Nordhagen, 1963).
A MATURATION OF IDEAS - IS
GLACIAL SURVIVAL THE ANSWER?
The North Atlantic Biota and their History was the ti-
tle of a symposium held at the University of Iceland,
Reykjavík in 1962 (Löve and Löve, 1963). Biologists
participating in the symposium almost unanimously
agreed that the observed distribution of certain Scan-
dinavian plant species could not be explained unless
they had survived in ice-free refugia at least during the
last glacial period. The tabula rasa theory, claiming
that all plants immigrated after the last glacial period,
had few proponents.
Three botanical arguments were primarily for-
warded in support of the glacial survival theory:
1. The west-arctic element,
2. The alpine endemic element and
3. The special disjunction of the alpine flora.
1. The West-Arctic Element
Dahl (1955) discussed some plant/geographic patterns
which were difficult to reconcile with the tabula rasa
theory. The west-arctic element is of special interest.
This comprises plant species found in Iceland, Scan-
dinavia, Greenland and North America (a few occur-
ing also in Novaya Zemlya and Svalbard) but miss-
ing in the Alps, Ural Mountains, and Asia. Dahl felt
it was difficult to explain the west-arctic element in
the Scandinavian mountain flora by migration from
the south or east after the last glacial age. No similar
west-arctic elements have been found in the Alps, but
why did these plants not migrate to the Alps if they
lived along the margins of the ice in central Europe
through the ice age (Dahl, 1955)? As Blytt (1881) and
others pointed out, the plant/geographic connections
within Greenland, Iceland, Scotland and Scandinavia
are so close that direct migration across the Atlantic
Ocean was very likely (Dahl, 1955).
In Scandinavia, the west-arctic element includes
about 30 species. About half of these species are
represented in Iceland and about 40% in Spitzber-
gen (Dahl, 1955). They show several distinct distri-
bution patterns, e.g. species which have their western
limits in Greenland (e.g. Arenaria pseudofrigida) and
in eastern North America (e.g. Pedicularis flammea).
Other species reach as far as the Alaska/Beringia re-
gion (e.g. Campanula uniflora) and some have a high
arctic distribution, connecting Svalbard with northern
Greenland (e.g. Minuartia rossii) (Dahl, 1955, 1998).
2. The Alpine Endemic Element
According to Dahl (1955), the glacial survival theory
is important to explain endemism in the Scandinavian
JÖKULL No. 54 3