Betula raddeana Trautv.

Native to the Transcaucasus, the rare Betula raddeana is cultivated in a few specialist arboreta, though political instability in its native region means that it remains an enigmatic and still little understood species. It was thought initially (on the basis of herbarium specimens) to be most closely related to the Himalayan Betula utilis, as a possible Caucasian outlier in the Caucasus (McAllister & Ashburner 2013). However, familiarity with living material suggests that Betula raddeana belongs with the rest of the shaggy-barked species in section Dahuricae. It resembles Betula nigra in the dense creamy-white hairiness of its young shoots and leaf undersides, and in its rhomboidal leaf shape, with only five or six pairs of veins. Its chromosome number (2n=84, hexaploid) is the same as that of Japanese material of B. davurica, to which it appears close in other characters, notably its production of hardly any short spur shoots in their early years, all shoots showing some degree of internodal extension so that even the weakest shoots are elongated. The close relationship of Betula raddeana and B. davurica is another example of pairs of similar species found in the Caucasus and Eastern Asia, also seen in Pterocarya, Parrotia, and Zelkova. It is thought that originally continuous populations were split by the raising of the Himalaya, which began in the Miocene (c. 30 Ma) and caused the drying out of much of central Asia which had formerly been the northern shore of the Tethys Sea.

Bean (1976) noted the velvety pubescence on the young shoots of Betula raddeana, and indeed the species could be mistaken for B. pubescens, though individuals in fruit can be distinguished by the immature fruiting catkins, held upright to horizontally and with a ‘spiky’ appearance from the recurved (not appressed) spreading scales with longer (not equally-sized) central lobes; they ripen and disperse their fruits earlier than most other birches. The male catkins of B. raddeana are borne in larger groups of (1–)3–5, rather than 1–3, and the elliptic-ovate leaf shape also differs from the ovate-deltoid-rhombic shape of B. pubescens, the teeth are of uniform size, and the abaxial surface is obviously greener (easily seen in comparison).

Curiously, the most compelling evidence of the species’ taxonomic affinity is inferred less from botanical characters than from the observed horticultural characteristics of plants in cultivation, notably their ease of transplantation and the remarkable facility with which cuttings can be struck. Unlike most white-barked birches, which do not transplant easily and require particularly careful lifting and manoeuvring of the shallow root-plate, Betula raddeana – like other species in section Dahuricae – develops strong roots that grow vertically downwards and firmly anchor the tree, facilitating successful transplantation even when considerable damage is done to the roots and little soil remains attached (McAllister 1999). White-barked birches are also difficult to propagate from cuttings, whereas species in section Dahuricae, notably B. nigra, root with relative ease. In the case of B. raddeana, rooting is extraordinarily straightforward, almost every cutting in early trials rooted within three weeks and grew on vigorously, with buds of most of the cuttings breaking to produce new shoots.

Betula raddeana was described in 1887 by Ernst Rudolph Trautvetter and named after Gustav Ferdinand Richard von Radde (1831–1903), plant collector and beetle expert who spent the last four decades of his life living in Tbilisi, studying the birds, insects and flowers of the Caucasus, the Black sea coast and the shores of the Caspian Sea as far east as Turkmenia. Little appears to have been written on the species’ ecology. Seifriz (1931) describes the species as generally occurring below B. pubescens and co-occurring with the enigmatic B. pubescens var. raddeana, which might be the hybrid between these species posited by Kuzeneva (1936, trans. 1970). With a distribution that extends across the Caucasus, Betula raddeana is listed by the IUCN as of Least Concern (Hadjiev, Shetekauri & Litvinskaya 2014), though it was included in the Red Data Book of the Russian Federation (Iliashenko & Iliashenko 2000). Since the species consists of scattered populations, many of them small, it may well be rarer than its IUCN conservation assessment implies. For a taxon with a restricted range it is surprisingly variable – apparently even within a population. However, the species is able to produce stool shoots, and it is likely that individual trees are long lived (great variability has been reported in other tree species with this capacity for long life (Rogers 2000)).

Provenances in cultivation

Bean (1976) thought the species first introduced to this country from Budapest in 1924; it was introduced to the University of Washington Arboretum in 1953 (O’Mulligan 1953). A plant accessioned to RBG Edinburgh in 1962 has made an attractive tree with a substantial trunk (Acc. 1962–1696). In 1980 an expedition from the Cary Arboretum, New York State, collected seed of Betula raddeana from Karachayevsk in the Karachai-Cherkesk Autonomous region of the northern Caucasus and distributed some via the Botanic Gardens seed exchange. The single seedling raised at Ness Botanic Gardens grew into a rather thick-twigged, fastigiate young tree.

Trees at Ness raised from seed obtained in 1988 from Gunib, Dagestan, eastern Caucasus have formed single-stemmed trees, in contrast to the multi-stemmed, basal sprouting habit of the other two collections in cultivation (see below), and develop the somewhat shaggy bark typical of other species in section Dahuricae. The two seedlings raised from this collection differed from one another in degree of hairiness, and had a spreading branching pattern unlike the fastigiate earlier introduction. They both root easily from cuttings, are deep rooted, easy to transplant, vigorous, and drought tolerant.

At Ness, the Karachayevsk introduction forms an attractive, broadly fastigiate, rather thick-twigged tree with a cluster of white-barked trunks – the clone circulates in the trade as the clone B. raddeana ‘Hugh McAllister’. The Gunib trees are beginning to develop a slightly shaggy bark. Both these accessions are drought tolerant and deep rooted, allowing other plants to be grown beneath them – an arrangement not normally possible under shallow-rooted species such as B. pendula. These characteristics make it likely that the species, and perhaps particularly fastigiate forms, could be useful in horticulture, perhaps even as a street tree where the shallow-rooted silver birches grow poorly. In this connection, the species’ ability to root from cuttings is critical. Since many of the desirable characteristics of B. raddeana relate to its root-system, the species might well be of use as an understock to produce grafted birches suitable for street planting.

A further introduction from Chewi, Georgia at 1800–2000 m asl was raised at Ness from seed obtained in 1993 through the Bergen Botanic Gardens seed list (labelled B. litwinowii). This accession is more slender in habit, less vigorous in growth rate, and multistemmed from a very early age, perhaps related to its montane provenance (higher-altitude trees often tend towards a multi-stemmed or shrubby habit). The four surviving trees of this provenance are developing attractive habits, three spreading and one fastigiate, and with whiter bark than the other two collections. Their eventual size in cultivation is uncertain, but they may prove to be attractive, small, multi-stemmed white-barked birches suitable for small gardens and confined spaces.