FLORA OF ICELAND elements: Betula hybrid (pubescens x nana), Hybrid Birch, Birki?

  Betula hybrid (pubescens x nana); the Hybrid birch is a hybrid offspring from the downy birch and the dwarf birch. As a hybrid, many characteristics are in between those of the parent species. One has to bear in mind that the Dwarf Birch is diploïd, where the Downy Birch (Mountain Birch) is a tetraploïd. The hybrid is, logically, a triploïd. Research (see articles mentioned in the page of Betula pubescens) has shown that the hybrids are diminished fertile (triploïds cannot form half-triploïd gametes), but not completely infertile. It can form both haploïd gametes (normally compatible with the "nana" gametes) or diploïd gametes (normally compatible with "pubescens" gametes). Thus genes can flow from one true species through the hybrid to the other. To make things more complicated 3 possible products can be formed from the hybrid gametes fertilizing other hybrids. The combination of two diploid gametes lead to a tetraploid (pubescens?) while the combination of two haploid gametes lead to a diploid result (nana?). But also a product can be formed from a haploid and diploid gamete (both from the triploid hybrid) leading again to a triploid. Among these though, there will be a larger variation in characteristics like leaf-shape than in hybrids from pure nana and pubescens species where the charateristics hold the middle. This makes it very difficult to define the differences between the three taxa. Add to that the two intergrading forms of the pubescens ("true" type to the "tortuosa subspecies" type) and a very complicated picture arises, confronting the casual visitor to Iceland, trying to identify a birch.
  It is a member of the Birch family (Betulaceae).