Jökull - 01.01.2004, Blaðsíða 1

Reviewed research article
Theories on migration and history of the North-Atlantic flora:
a review
Hafdís Hanna Ægisdóttir1,2 and Þóra Ellen Þórhallsdóttir1
1 Institute of Biology, Askja, University of Iceland, Sturlugata 7, IS-101 Reykjavík, Iceland
2Current address: Institute of Botany, University of Basel, Schönbeinstrasse 6, CH-4056 Basel, Switzerland.
email: Hafdis-Hanna.Aegisdottir@unibas.ch; theth@hi.is
Abstract — Two theories on migration and the history of the North Atlantic flora have been the subject of
a popular debate for over a century. In late 19th century when signs of glaciations had been recognized in
Scandinavia, it was concluded that the entire flora and fauna had been destroyed during glacial periods and
that all plants immigrated after the last glacial period. This so called tabula rasa theory was soon opposed by
the glacial survival theory which stated that plants survived the last or several of the Pleistocene glaciations
in ice-free refugia within North Europe. The glacial survival theory was very popular, in the mid-20th century,
notably because of three botanical arguments that were forwarded in its support: 1) it alone could explain the
distribution of the so-called west arctic element e.g. species found in Scandinavia, Iceland and North America
but missing in the Alps, Ural Mountains and Asia, 2) the alpine endemic element e.g. the relatively higher
proportion of endemic species in the alpine flora of Scandinavia compared to the lowland, and 3) the special
disjunction of the alpine flora. Later, those arguments were reconsidered for the Scandinavian flora and it
was concluded that they could be explained without glacial survival. In the last decade, new techniques e.g.
molecular methods, results from ice core projects and pollen analyses have offered fascinating possibilities to
re-formulate the questions asked by research pioneers. The debate on the plant migration and origin of the
flora in the North Atlantic region thus continues.
INTRODUCTION
It is generally believed that the present-day distribu-
tion of plants and animals in the North Atlantic re-
gions is largely a product of Quaternary climate and
environmental change (e.g. Bennike, 1999; Tremblay
and Schoen, 1999; Tiffney and Manchester, 2001).
Contrasting ideas on plant migration and the origin
of the flora in the North Atlantic regions have been
the subject of popular debate for over a century, with
two contrasting theories emerging by the close of the
19th century: a) The tabula rasa (clean slate) theory,
according to which all plants immigrated after the last
glacial period (Nathorst, 1892; Nordal, 1987) and b)
The glacial survival theory of plant survival during the
last or several of the Pleistocene glaciations in ice-free
refugia (e.g. Dahl, 1963; Löve and Löve, 1963).
The first ideas
The Tabula rasa Theory
It was not until the mid-19th century that scientists
first began accepting the fact that major parts of north-
ern Europe, North America and Siberia had once been
covered with ice. Besides striated and polished rocks,
European scientists found extensive unsorted sedi-
ments that could only be explained by the existence
of a large ice sheet (Dahl, 1946; 1955).
Before the end of the 19th century, signs of glacia-
tion were recognized on the outermost islands along
the coast of Scandinavia. This led to the conclusion
that the entire Scandinavian peninsula had been cov-
ered with ice during the “ice age” with the consequent
destruction of the flora and fauna. This argument sub-
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