The Pleistocene is the world’s recent period of repeated glaciations between 2.5 million to 11700 years ago. More than 20 cycles of glaciation occurred during the Pleistocene, punctuated by warmer interglacial periods. The last glaciation lasted from approximately 110 000 to 12000 years ago, reaching its maximum around 20 000 years ago. The last ice sheets removed most vegetation, except for some survivors on nunataks above the great ice sheet and in some ice-free locations within the great ice sheet’s cover, known as glacial refugia. A major glacial refugia was Beringia, covering the northern slopes of Alaska and the adjacent regions of northwestern Canada and northeastern Russia (see further below under “amphi-Beringian distribution” and here). From there, the bryophyte colonisation took place as a continuous process while the big ice sheet shrunk. Simultaneously, bryophytes spread northwards into the Arctic from southern ice-free areas.
Animation of the Beringia Land Bridge from 21,000 ago to today. The change in sea level was caused by glaciation during the pleistocene and following deglaciation. Animation by NOAA [Public domain], via Wikimedia Commons.
Bryophytes disperse easily and efficiently by wind, transporting their spores, gemmae and gametophore fragments over long distances. This leads to large distributional ranges and facilitates a fast adaptation to changing climates (Frahm 2012). It is also a likely reason why only few bryophytes are considered true Arctic endemic species, e.g. Scapania matveyevae, Gymnocolea fascinifera, Schistochilopsis hyperarctica. Sea ice can also act as a transport mechanism, where dispersal is powered by the circumpolar wind systems.
Distribution patterns of arctic bryophytes parallel those of flowering plants to some extent. However, fewer species are restricted to high latitudes and the majorities are also widely distributed in boreal and temperate regions (Longton 1988). Besides the cosmopolitan species such as Bryum argenteum, Ceratodon purpureus, Aneura pinguis and Cephalozia bicuspidata, three other general distribution types are of special interest in a broader, circumpolar Arctic perspective (CAFF 2013b):
The circumboreal species are commonly distributed in temperate, boreal areas and arctic climatic areas of the Northern Hemisphere. They originate from the boreal zone, but dispersed into the Arctic. As much as 80% of arctic bryophytes are considered to have a circumboreal distribution (Steere 1978, CAFF 2013b). Typical species belonging to this category are Hylocomium splendens, Catoscopium nigritum and Meesia ulignosa.
Arctic bryophyte species are cold-adapted and characteristically have a circumpolar distribution. Some species occur only north of the Arctic Circle whereas others, although more frequent in arctic regions, extend southward where high altitude or climatic factors provide appropriate environmental conditions (Steere 1978). Typical representatives of the arctic category are e.g. Cyrtomnium hymenophyllum, Hygrohypnum polare, Kiaria glacialis, and Haplodon wormskjoeldii. Typical arctic species are assumed to have occupied southern refugia (mountain areas south of glaciated areas) during the last glacial maximum, and recolonised northward following the retreating ice sheet (Stewart et al. 2010).
The amphi-Beringian distribution pattern is explained by the existence of an ice-free land bridge between north-eastern Asia and north-western North America during the last glaciation (Beringia). Species with this distribution pattern, such as Drepanocladus latinervis, Pohlia beringiensis and Racomitrium afoninae, are likely remnants of a Tertiary bryoflora (Steere 1969). There is evidence that large unglaciated areas covered the northern slopes of Alaska and adjacent regions of north-western Canada and north-eastern Russia during the Pleistocene, acting as major glacial refugia for arctic plants (Abbott and Brochmann 2003, read also here). Those glacial refugia allowed the exchange of plants between Asia and America and the diversification of plants. They also provided a source of tundra plants for post-glacial expansion into deglaciated areas throughout the Arctic when glaciers retreated (Ickert-Bond et al. 2009).