Dwarf Birches Betula Nana And B Glandulosa

The dwarf birches Betula nana and B. glandulosa share the same propensity for hybridization as the mountain birches. Betula nana is circumpolar in distribution and represented by two subspecies: ssp. nana in Europe and western Asia and ssp. exilis in North America and central and eastern Asia. Betula glandulosa is also a closely related shrub found across North America and Greenland, and where the two species overlap there is much hybridization as well as taxonomic confusion (DeGroot et al., 1997; Fig. 9.16). Both of these dwarf birch species have chromosome counts of 2n = 28 with triploids and tetraploids reported only in hybrids where both euploid and aneuploid chromosome counts are found ranging from 28 to 56 (Anamthawat-Jonsson & Thorsson, 2003).

The various dwarf birches differ in their climatic tolerances. Betula nana extends further north than B. glandulosa, reaching 79° N in Spitsbergen and 80° N in north-west Greenland where both subspecies of B. nana occur. By contrast, B. glandulosa is restricted to more oceanic conditions with greater winter snow cover, as in the maritime provinces of Canada where it is typically associated with bryophyte communities in areas of lichen-spruce (Picea mariana) forest (see Fig. 5.19). The most northerly location for B. glandulosa is in Baffin Island at approximately 68° N. The interior of southern Baffin Island between 64 and 68° N contains a locally rich and diverse vegetation, which is indicative of the low arctic bioclimatic zone and marks the present northern limit of that zone in the eastern Canadian Arctic.

9.5.1 Biogeographical history of dwarf birch

Palynological studies of lake sediments in the eastern Canadian Arctic have revealed a presence of Betula since 4750 BP. Elements of a low arctic vegetation association have been present in the area since this time, indicating a local bioclimatic system that has been relatively stable (Jacobs et al., 1997). Given this long-term persistence of B. glandulosa at this locality it is of interest to note that at this northern limit less than 0.5% of the seeds (samaras) are viable and these marginal populations are maintained by asexual reproduction (Weis & Hermanutz, 1993). The most probable cause of this sterility, the failure of pollination, was discussed in Section 4.2.

Both species (B. nana and B. glandulosa) are successful at colonizing a wide range of habitats, although within any one region the ecological preferences of the species may be restricted. In the Colorado Rockies B. glandulosa occurs in the southern boreal region at altitudes up to about 3000 m a.s.l. in the subarctic and alpine tundra, while B. nana, although restricted to blanket peat in Britain, nevertheless survives on xeric and rocky sites in the Arctic. In Greenland where B. nana became a dominant plant in the early Holocene, a climatic change c. 5000 BP almost exterminated the species at more oceanic sites. It lost only little ground inland and has retained its position there since in many kinds of heath and other vegetation types (Fredskild, 1991). This is one more example of the widespread tendency for woody species to react adversely to the warmer wetter winter conditions that accompany increases in oceanicity (see below).

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