Given its northern geographic location it is obvious that Iceland has been covered by thick ice-masses during the glacial periods. Another feature of Iceland is its distant position to the main land masses (Europe/America). Revegetation after a glacial period is thus extra difficult as plant species have to return from far over the ocean. This under the assumption species became extinct during the glacial periods. This however, is not (entirely) true. So let us review the possibilities and likeliness of revegetation following the last glacial era.
There are a number of theories how plant species revegetated Iceland following the end of the last glacial period (the younger Dryas: ending about 11 000 thousand years ago). These are:
First, what is a nunatak: it is an ice-free area in a glacier. Usually it is a mountain rising above the glacier or an ice cap but it could be a more or less flat ground under specific circumstances. The expression is from the Greenland Inuit.
The last major glacial period is called the Weichselian glacial period which lasted from about 110-thousand years ago to about 11,7-thousand years ago. Gradual warming started already some 25-thousand years ago. However, the last 15-thousand years ago were marked by three short severe cold spells with warmer interstadials. The cold spells are named the Oldest Dryas, the Old Dryas and the young Dryas. The interstadial warmer periods are called the Bølling and the Allerød. The last cold glacial period - the Younger Dryas - last from 12,900 to 11,700. Then temperatures rose very quickly to reach present values.
In the 1990's interesting research was done in the Skagi region of north-western Iceland. Here a region around a lake was identified that had supposedly been a nunatak during the Younger Dryas period (and the immediate period thereafter). An important scientific article reporting on the findings of this research as well as a general discussion was written by Rundgren & Ingólfsson (1). They report that pollen of quite a few number of species were found in drill-samples of the lake bottom soil. They conclude that quite a few species locally survived the Younger Dryas period and even make a point that species could have survived throughout the complete Weichsel period.
There are two snags though:
1) In the first background page it has been stated that a peculiar feature of the Icelandic flora is the near lack of endemic species. If species were to survive through glacial periods the harsh - very selective - nunatak refuges over the last million years or so, one would expect accelerated speciation. But that does not seem to be the case!
2) A problem I have concerns their dating. They study the period 11,2-thousand to 9-thousand years ago. But the younger Dryas ended at least 11,6-thousand years ago, that is 500 years before their oldest samples. Now it is very well possible that large parts of Iceland were still covered by Ice, but the climate must have become warmer already. So it is very well possible that their findings were not glacial-survivors but rather early settlers from the south!
Does this mean that the Nunatak theory has to be abandoned. Certainly not!. Plants growing on nunataks exist today, even on Iceland. On the Esjufjöll mountain, towering high above the southern Vatnajökull, many species of plants grow.
Migratory birds settling on Iceland (or migrating to even more northern regions) would have accidentally brought in seeds mainly from Europe (the present migration patterns follow the route Africa/mainland Europe/British Isles/Iceland/Eastern Canada). It is believed (or speculated) that mainly water plants have been reintroduced to Iceland by birds.
Light airborne seeds can travel large distances with little effort. As such one could speculate that seeds for example of birch-species could easily have reached Iceland and subsequently settled as soon as the ice sheets retreated. It is known that the Dwarf Birch (Betula nana - Fjalldrapi) was present throughout the Younger Dryas in Scotland and southern Scandinavia. By 10-thousand years ago it had resettled on all the northern Atlantic islands as well as Greenland. The Rundgren & Ingólfsson study show it was on Iceland already 11-thousand years ago. As stated above this could be on a nunatak but could also be a resettlement. A problem with this theory is once again a characteristic feature of Iceland's flora: it is dominated by European species, not by American species. Given that the prevailing winds over the Atlantic is south-westerly, one would expect the opposite if wind was a major factor.
It is known that many seeds of land plants can stay in the sea for years and can still germinate when conditions are right again. This theory also suffers the same improbability as the "wind spread theory". The ocean currents of the Atlantic move north-west from the western Atlantic over the gap between Scotland and Iceland on towards Norway where it bends straight north. This would be to the disadvantage of European species too.
In the past it has been postulated that icebergs breaking off from southern Scandinavia at the initial warming of the planet may have reached Iceland carrying plant material. This theory has been dismissed as it has become clear that the Atlantic currents were the same throughout the glacial/interstadial periods. Icebergs could not have drifted to Iceland from southern Scandinavia directly to Iceland. However, recently it has been suggested that icebergs from northern Norway could have done so later on. A traveler to northern Iceland will discover that many bays and coves along the north coast have many washed-up tree trunks. It is known that these trees have grown in Siberia and have made their way to Iceland by the arctic currents. Icebergs from northern Norway could also reach the arctic currents and as such flow back to Iceland carrying seeds from the continent.
Most likely the vegetation of Iceland recovered from the last glacial era by different means. The nunatak theory is popular, but probably not as important as often suggested. The fact that Iceland has so few endemic species prove that many species again and again found their way to Iceland over the last few million years of glaciation and interstadials, and thus probably also did so 11-thousand years ago at the end of the Young Dryas (sometimes referred to as the "Small Ice Age").
2) Is the flora American or European?
3) The main vegetation types
4) Regional differences in the flora
5) Influences of the glacial era's and the revegetation after 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.