A relatively stable, true-breeding hybrid between Abies alba and A. cephalonica, intermediate between its parents. Tree 20–40(–60) m tall, to 1.5 m dbh. Crown narrowly conical in young trees, broadening with age, turning broad-columnar, with shallowly ascending branches (horizontal or downswept in the lowest, or densely shaded parts of the crown). Vegetative branchlets yellowish grey, pubescent, especially in the lower parts of the crown; branchlets in the upper crown and on cone-bearing shoots yellowish white or shining milky white, nearly glabrous. Leaves narrower than in A. alba, pectinately arranged (assurgent on cone-bearing branches), 1.5–3(–3.5) cm long, tips rounded, blunt, occasionally tapered and acute on young and vigorous trees, rarely just emarginate on shaded shoots of older trees, glossy dark green above, with two greyish white stomatal bands below, occasionally with some flecks of stomata near the tip on the upper surface. Pollen cones crowded, 2 cm long, greenish-yellow with purple to red microsporophylls. Pollen cones crowded, 2 cm long, greenish-yellow with purple to red microsporophylls. Female cones 9–15(–20) cm long, 3–5(–6) cm across, cylindrical, usually abruptly rounded at the tips, maturing from yellow-green to greenish brown; seed scales cyanthiform, 2.5–3 × 2.5–3.5 cm at midcone; bracts exserted and somewhat reflexed at maturity. (Farjon 2017; Debreczy & Rácz 2011).
Distribution Albania Bulgaria Greece North Macedonia
Habitat A mid-elevation species found at 700–1500 m asl, usually in the company of Picea abies, but occasionally in pure stands and locally with other associates including Pinus heldreichii, Pinus peuce and Pinus sylvestris. At lower elevations it occurs in mixed forests with Fagus.
USDA Hardiness Zone 5-7
RHS Hardiness Rating H7
Conservation status Not evaluated (NE)
Taxonomic note Farjon (1990) treated the King Boris Fir as a true species and this position was followed by Grimshaw & Bayton (2009) in New Trees. By 2017, with its hybrid origin proved beyond reasonable doubt, Farjon had determined that it ‘seems appropriate to recognize this taxon as of hybrid origin in the way we write its name’ and he adopted the nothospecies rendering A. × borisii-regis (Farjon 2017), but this is an atypical approach for a taxon Farjon himself calls ‘a stable and truebreeding hybrid’. We will continue to write it simply as A. borisii-regis.
Even when the enigmatic King Boris Fir was first described to science – named for Boris III, King of Bulgaria (1894–1943) – Mattfeld speculated that it was a Tertiary hybrid between Abies alba and A. cephalonica (Bean 1976). A handful of alternative theories emerged during the 20th century, but Mattfeld’s early suspicion became more or less the accepted view especially after the publication of Liu’s monograph (Liu 1971). The research of Mitsopoulos & Panetsos (1987) showed southern migrations of A. alba into populations of A. cephalonica during recent glacials to be the likely source of A. borisii-regis, since supported by Bella et al. (2014) who demonstrated molecular evidence for hybrid origin. Nevertheless, it remains a weakly characterised taxon in literature and numerous studies have highlighted the highly polymorphic nature of Abies populations in the southern Balkans, with Mattfeld himself characterising A. borisii-regis as a hybrid swarm (Bella et al. 2014). This highly variable nature is likely the result of multiple hybridisation events, rather than a single origin. This may also explain controversy surrounding its lectotypification: Michael Frankis considers Farjon’s lectotype to result in ‘a different concept to that which Mattfeld intended: the lectotype should be Mattfeld 833 from the Rhodope Mts in Bulgaria (Liu 1971), but Farjon instead says a Sintenis specimen from Mt Olympus in Greece. The result of this is that Farjon’s lectotype actually results in a different taxon (a hybrid between the Bulgarian taxon and A. cephalonica, with a narrow range primarily in northern Greece) being given this name, while Mattfeld’s specimen is of a taxon with a wide range in the Balkans north to Romania, southern Ukraine and Croatia’ (M. Frankis pers. comm. 2020).
Bella et al. (2014) note that ‘individuals…show either intermediate or a combination of A. alba and A. cephalonica characteristics, but they lack diagnostic characters that could facilitate distinction from the parental species’. Perhaps because A. borisii-regis is represented in cultivation by a relatively narrow sample, cultivated trees can generally be distinguished in the following ways: from A. alba in the more densely set and narrower leaves with entire (cf. emarginate) tips; from A. cephalonica in the yellowish grey, pubescent branchlets (cf. shining pale brown or reddish brown and glabrous) (Farjon 2017).
A. borisii-regis has a discontinuous distribution in the southern Balkans, from Albania into North Macedonia and northern Greece, from the Greek region of Macedonia south to the northern Peloponnese, and also through the Pirin and Rhodope Mountains of southern Bulgaria (Grimshaw & Bayton 2009). It is an important tree species on Mount Olympos and perhaps it was from here, a popular and important destination for visitors to Greece for hundreds of years, that the species was introduced to cultivation. Alan Mitchell gives the date of introduction to Britain as 1883 but goes on to state there is ‘a complete lack of dated trees’ (Mitchell 1972). It is telling that others, including Bean (1976), do not speculate as to the date of introduction; a wise ploy considering the name A. borisii-regis was not published by Mattfeld until 1925, meaning the early introductions would have been assumed to be either A. alba (and its long-defunct var. acutifolia) or A. cephalonica. This is probably one reason why Mitchell suggested in 1972 that the species was rare, generally confined to major collections, and ‘usually unrecognised’ (Mitchell 1972).
As of spring 2020 the Tree Register keeps records of about 38 individuals in the UK and Ireland. The largest trees tend to occur in areas favoured by A. alba, namely the cooler and wetter north and west, but the influence of A. cephalonica has endowed the King Boris Fir with greater heat and drought tolerance than its northern parent (Grimshaw & Bayton 2009). The tree at the Royal Botanic Garden Edinburgh that attracted so much praise in New Trees is apparently unusual for coning most years (pers. obs.) (Debreczy & Rácz (2011) suggest the King Boris Fir only cones occasionally). The tree at Stonefield Castle, Argyll, Scotland, was 36 m in 1992 (Grimshaw & Bayton 2009) and c. 43 m × 1.75 m dbh in 2012. Another growing near Dunkeld Cathedral, Perthshire, Scotland, was 35 m × 1.66 m dbh in 2017 (Tree Register 2020). It is cultivated in several continental European collections (Farjon 2017; Grimshaw & Bayton 2009) and in North America too, though it is scarcer there. In their 1988 survey of firs growing at the Arnold Arboretum Warren and Johnson reported ‘3 magnificent 60 year old specimens…Acquired as seeds from trees growing wild in Greece, they are 16–20 m tall and 60 cm in diameter’ (Warren & Johnson 1988).
The King Boris Fir is still occasionally collected. Four young trees at Murthly Castle, Perthshire, Scotland, are traceable to collections made in Bulgaria in 2006 by Martin Gardner and Sabina Knees (GAK s.n.). Further seed collections made by the Royal Botanic Garden Edinburgh in Albania in 2009, under the numbers CGS 32, 46, 48, 51 and 53, have been distributed by the International Conifer Conservation Programme to multiple collections in the UK; most are thriving (BG-BASE data 2020; pers. obs.).
A few King Boris Fir cultivars have been named, but given the difficulties in identifying even typical trees, their parental identities can only be taken on trust.