On the vegetation of Iceland   surf tip

Introduction

Given its northern geographic location, it is obvious that Iceland has been covered by thick ice sheets during the glacial periods. Another characteristic of Iceland is its great distance to thenearest continent (Europe and North America). Revegetation after a glacial period was therefore particularly difficult, as plant species had to recolonise the island by crossing vast stretches of ocean. This assumes that all species became extinct during the glacial periods. However, this is probably not entirely true. Let us therefore review the possible ways in which Iceland's vegetation recovered after the last glacial period.

Several theories have been proposed to explain how plant species recolonised Iceland following the end of the last glacial period (the younger Dryas: ending about 11,000 years ago). These include:

• The nunatak theory: plants survived the glacial period
• Birds spread seeds
• Wind spread seeds
• Icebergs/Sea currents spread seeds

The nunatak theory

First, what is a nunatak? It is an ice-free area surrounded by a glacier. Usually it is a mountain peak rising above a glacier or ice cap, although under specific circumstances it may also consist of relatively flat ground. The term originates from the Greenland Inuit.
The last major glacial period is known as the Weichselian Glaciation, which lasted from about 110,000 years ago until approximately 11,700 years ago. Gradual warming had already begun some 25,000 years earlier. However, the final 15,000 years were characterised by three relatively short but severe cold periods separated by warmer interstadials. These cold periods are known as the Oldest Dryas, the Older Dryas and the Younger Dryas, while the warmer intervals are called the Bølling and Allerød interstadials. The final cold phase, the Younger Dryas, lasted from approximately 12,900 to 11,700 years ago. Temperatures then rose rapidly to values comparable with those of today.
In the 1990s, interesting research was carried out in the Skagi region of north-western Iceland. A lake basin was identified that may have remained ice-free as a nunatak during the Younger Dryas and possibly for some time afterwards. An important scientific paper discussing these findings, together with a broader interpretation, was published by Rundgren & Ingólfsson ⁽¹⁾. They reported pollen from a considerable number of plant species in sediment cores taken from the lake bottom. They concluded that several species survived locally throughout the Younger Dryas and even suggested that some species may have persisted throughout the entire Weichselian Glaciation.
There are, however, two important objections:
1) In the first background page it was pointed out that one remarkable feature of the Icelandic flora is the near absence of endemic species. If plants had repeatedly survived successive glacial periods in the harsh and highly selective nunatak refuges over the past million years, one would expect an increased rate of speciation. This does not appear to have happened.
2) Another concern is the dating of the samples. The study focuses on the period from 11,200 to 9,000 years ago, whereas the Younger Dryas had ended at least 11,700 years ago, approximately 500 years before their oldest samples. Large parts of Iceland may still have been ice-covered, but the climate was already becoming warmer. Consequently, some of the plants identified may not have been glacial survivors but rather early colonists arriving from the south.
Does this mean that the nunatak theory should be abandoned? Certainly not. Plants growing on nunataks exist today, including in Iceland. On Esjufjöll, rising high above southern Vatnajökull, numerous plant species can still be found.

Birds spread seeds

Migratory birds settling on Iceland, or migrating to even more northern regions, may accidentally have transported seeds, primarily from Europe. Present migration routes generally follow the pattern Africa – mainland Europe – the British Isles – Iceland – eastern Canada and/or Greenland. It is widely believed, although not conclusively proven, that birds were particularly important in reintroducing aquatic plant species to Iceland.

Wind spread seeds;
Sea spreads seeds;
Icebergs spread seeds

Light windborne seeds can travel great distances. It is therefore conceivable that seeds for example of birch species could easily have reached Iceland as soon as the ice retreated. Dwarf Birch (Betula nana – Fjalldrapi) is known to have survived throughout the Younger Dryas in Scotland and southern Scandinavia. By 10,000 years ago it had resettled on all the northern Atlantic islands as well as Greenland. The Rundgren & Ingólfsson study indicates that it was already present in Iceland around 11,000 years ago. As discussed above, this could indicate survival on a nunatak, but it might equally represent early recolonisation. A weakness of the wind-dispersal theory is another characteristic feature of the Icelandic flora: it is dominated by European rather than North American species. Given that the prevailing winds over the North Atlantic are south-westerly, one might expect the opposite if wind had been the dominant means of dispersal.

Many seeds of terrestrial plants are known to remain viable after floating in seawater for extended periods. However, this hypothesis faces a similar problem. The main North Atlantic currents flow north-eastward from the western Atlantic through the gap between Scotland and Iceland before continuing towards Norway. These currents would not particularly favour the transport of European plant species to Iceland.

It was once proposed that icebergs calving from southern Scandinavia during the initial warming at the end of the glacial period carried plant material to Iceland. This hypothesis was later rejected because evidence indicates that the main Atlantic current system remained broadly similar throughout the glacial and interstadial periods. Icebergs from southern Scandinavia would therefore not have drifted directly to Iceland. More recently, however, it has been suggested that icebergs originating from northern Norway may have reached Iceland via Arctic currents. Travellers in northern Iceland will notice the large numbers of driftwood logs washed ashore in many bays and coves. These trees originated in Siberia and were transported to Iceland by Arctic currents. Icebergs from northern Norway could likewise have entered these currents and eventually reached Iceland while carrying plant material and seeds from the European continent.

Conclusion

The vegetation of Iceland most likely recovered after the last glacial period through a combination of different dispersal mechanisms. The nunatak theory remains attractive, but it was probably less important than is often suggested. The very small number of endemic species indicates that many plants repeatedly reached Iceland during successive glacial and interstadial periods over the past few million years. The same processes probably played a major role around 11,000 years ago, following the end of the Younger Dryas.(sometimes referred to as the "Small Ice Age").

Continue with:
1)  introduction
2)  Is the flora North American or European?
3)  The main vegetation types
4)  Regional differences in the flora
5) The influence of the glacial periods and the revegetation following the last glacial period (this page)

⁽¹⁾ Plant Survival in Iceland during periods of glaciation? - M. Rundgren & Ó. Ingólfsson, 1999, Journal of Biogeography, 26, 2,387-396.