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The taxonomic revision of Liu (1971) treated A. borisii-regis as a natural hybrid between A. alba Mill. and A. cephalonica Loudon, and this was also suggested by the analysis of Mitsopoulos & Panetsos (1987). Morphology is intermediate between the two purported parent taxa, although unlike in A. cephalonica the young shoots are pubescent, and the leaves differ from A. alba in that the leaf apices are entire and there are a few stomata on the upper leaf surface. The bands of stomata on the underside of the leaf are grey-green, rather than white as in A. alba (Warren & Johnson 1988), and cones are very rarely produced (Anon. 1964). Rushforth (1987a) suggested that it is not a recent hybrid, and should be considered as a member of a group of species including A. alba, A. cephalonica and A. nordmanniana. Farjon (2001) treats it as a full species. Anon. 1964, Warren & Johnson 1988, Farjon 1990. Distribution ALBANIA; BULGARIA: Pirin and Rhodope Mts.; GREECE: Macedonia; MACEDONIA (Former Yugoslav Republic). Habitat Between 700 and 1800 m asl in mountainous areas. USDA Hardiness Zone 6. Conservation status Lower Risk. Illustration Liu 1971; NT44, NT46. Cross-reference K31.
The King Boris Fir is abundant in the mountains of Macedonia, and in places forms quite a dense forest of vigorous young trees. It is not easily distinguishable from other European silver firs, and like A. alba it can make a big tree in the right conditions, but it is in general much more adaptable and useful for horticulture than that species (Rushforth 1987a). The Tree Register of the British Isles (TROBI) has 14 records of specimens over 30 m tall, and 26 over 20 m, the tallest noted being 38.5 m (109 cm dbh), at Rossylongan, Co. Donegal, in 2000 (Tree Register of Ireland, TROI) – although the identity of this individual is not certain. On the British mainland the largest recorded was 36 m (dbh 155 cm), at Stonefield Castle Hotel, Argyll, measured in 1992 by Alan Mitchell (TROBI). Most of these big trees are in areas particularly known to favour A. alba, namely the damper, cooler north and west, but not all of them are, and A. borisii-regis would seem to be a good candidate to attempt where a silver fir is desired in drier areas. At the Royal Botanic Garden Edinburgh there is a very fine specimen planted in 1968 (22 m in 2004, TROBI) that is the best-looking fir in the collection, well furnished with densely leafy branches. Rather slower-growing, having been planted in 1911, is a large example at Wakehurst Place, measured at 27 m in 1991 (Kew records). It is also in cultivation in many European collections and generally thrives, with specimens at Rogów Arboretum being described as ‘fast-growing, tall and healthy’ (P. Banaszczak, pers. comm. 2007).