FLORA OF ICELAND elements: Betula pubescens, Downy Birch, Birki

  Betula pubescens; the downy birch is the major tree species of the north- Atlantic. It can be found from the North- America's, southern Greenland to the northern and western Europe's. On Iceland it is the only tree species in the wild that manages to survive. Small numbers of Rowan however spread along within the birch stands. There are many speculations on the originality of the present birch stands. Today they form only a very small part (1%) of the coverage of the land of Iceland. At the time of the first Norse settlement it is estimated that at the time 30% of Iceland was covered by birch stands. However, from historical data it is not known what the vigour of the birches were. To date almost all birches on Iceland are often lower than 2 meters tall, very much in contrast to continental downy birches which easily grow up to 20 meters. A possibility is that, as a result of tree-harvesting (wood material) for clearings and fuel-materials and the low-level natural occurring hybridization between B. pubescens and B. nana (the dwarf birch) the modern Iceland B. pubescens population has become far less vital than the stands that existed before the Norse invasions. This however, is contradicted by the fact that there are locally very well developed birch forests. They can be found in the north near Akureyri and Ásbyrgi, in the east near Egilsstadir and also in the Thörsmörk region. These forests thrive in protected valleys. Birch water, a juice that supposedly stimulates hair growth, can be obtained from this tree by tapping the juice from the tree.
  The leaves are ovate, about 3-5cm long, broad-based tapering to a more or less pointed apex. Young twigs are downy pubescent.
  The trees have both male and female catkins. The male catkins are longer and hang down from the young branchlets while the female catkins are shorter (2cm) and stand erect. The male catkins have yellow anthers, the female usually with red stigmas.
  Polycormic forms (see below) lower than two meters often align coastal mountains like fjords, where they can grow up to 5m in more interior regions. Tall (up to 20m) monocormic forms are restricted to a few sheltered relict locations.
  It is a member of the Birch family (Betulaceae). The Icelandic name of this species is Birki as well as Ilmbjörk.

A note on the effect of hybridization on Betula pubescens. Through the hybrid of "pubescens" x "nana" genes flow from nana into pubescens. It is believed that this is the reason that most Icelandic birch stands are made up of polycormic shrub-like trees. Polycormic means that multiple stems rise from the ground level. They are crooked showing stunted growth. Also the leaves of these trees are more rounded shaped compared to the rather pointed apex' of the standard (monocormic) tree morf. These polycormic birches are regarded to belong to the subspecies "tortuosa", which, in English, is named Mountain Birch. This subspecies is by far dominant on Iceland over the standard monocormic (true tree) birches. The latter are limited on Iceland to more or less sheltered places and possibly remnant vegetations which have not been chopped down by the Viking settlers and their offspring. A point I still need to establish is if the Viking settlers have - through their practises - stimulated the gene introgression between the species.
You can read more on this interesting topic in the following scientific papers:

Morphological Variation among Betula nana (diploid), B. pubescens (tetraploid) and their Triploid Hybrids in Iceland (Æ. Th. Thórsson, S. Pálsson, A. Sigurgeirsson and K. Anamthawat-Jónsson) - this paper deals with the differences between the taxa (see below on the 4 birch types).

Birch hybridization in Thistilfjördur, North-east Iceland during the Holocene (Lilja Karlsdóttir, Margrét Hallsdóttir, Ólafur Eggertsson, Ægir Th Thorssón, Kesara Anamthawat-Jónsson) - this paper deals with the historic data on hybrids.

Population structure and genetic variation of fragmented mountain birch forests in Iceland - (Snæbjörn Pálsson, Pawel Wasowicz, Starri Heiðmarsson, Kristinn Pétur Magnússon) - A study on the variation within the "Mountain Birch".

Concluding one could state that there are four main birch types on Iceland:
1) The B. nana species (dwarf birch)
2) The hybrid species (no English name)
3) The B. pubescens species: ssp tortuosa (downy birch / mountain birch)
4) The B. pubescens species: nominate subspecies (downy birch)
Because of different amount of genetic introgression from nana into pubescens there is no strict morphological divide between these four groups.

More about the hybridization can be read on page of the Betula hybrid.