Betula occidentalis Hook.

Once seen, the peculiar sea-green of the gummy, glutinous young leaves of Betula occidentalis is unmistakable, as are the very resinous young shoots. The species forms a shrub or small tree up to 10 m in height, though in cultivation in the British Isles the shrubs rarely develop to this size. In wild specimens that develop thick stems, the shining polished brown of the bark is another good diagnostic character, the effect almost rivalling that of Prunus serrula, though in a slightly duller shade of black-brown. The colour of the autumnal foliage can also be very attractive. The species can resemble smaller plants of B. papyrifera, though it can be distinguished from that species by the dark, non-peeling bark, by the lack of tufts in the vein axils tufts typical of B. papyrifera, and by the fact that the leaf apices are never acuminate.

Betula occidentalis occupies a vast range from Alaska and the Northern Territories of Canada southwards in the mountains to Arizona, occurring as tall, upright-growing, brown- or brownish-black-barked bushes, often marking out watercourses and damp patches, as for example in the low-rainfall foothill country of southern Alberta east of the Rocky Mountains. It also occurs commonly along rivers further westwards into the mountains in higher-rainfall areas, but is less conspicuous there amongst the richer vegetation except when its foliage colours golden-orange in the autumn.

Although confined mostly to the North American Cordillera, populations of Betula occidentalis extend north-eastwards into Manitoba and even north-western Ontario, the distribution and range mirroring fairly closely that of Alnus tenuifolia, though that species does not extend as far east (Hosie 1963; Furlow 1997). Towards the northern end of the distribution, and especially in Alaska, hybridisation of Betula occidentalis with other birches has been recorded. Where it comes into contact with B. pendula subsp. mandshurica the two species hybridise, for example along the Elbow River in Calgary, Alberta (McAllister & Ashburner 2013). Hybrids also occur with the Shrub Birch, B. glandulosa, to give the stout-twigged shrub B. × eastwoodiae. Dugle (1966) reports that this hybrid is found from Alberta northwards, but a population near Babb in Montana is evidence of its presence further south. Hultén (1968, cited in Furlow 1997) suggested that what is referred to as Betula occidentalis in Alaska is in fact an extensive hybrid swarm between B. pendula subsp. mandshurica (as B. resinifera) and B. glandulosa. As all these species are diploid, knowledge of chromosome numbers will not resolve the question, but careful field observations combined with molecular work ought to enable a view to be taken. On the other hand, it cannot be excluded that ‘true’ B. occidentalis might also occur and be hybridising with the supposed hybrid swarm.

Betula occidentalis was first described by William Hooker from specimens collected by Scouler, Douglas and Drummond in the mountains of western Canada. The name Betula fontinalis Sarg. was widely used for this species during the twentieth century, but Hooker’s earlier name is now considered correct (Furlow 1997).

Bean (1976) reported the species’ having been introduced to Kew in 1897, where it made ‘a graceful small tree’. In favourable sites the species can indeed make a very attractive specimen. Disappointingly, however, it fails to grow well in most sites in the British Isles and also (strangely) in the Arnold Arboretum in Boston, Massachussetts. An exception is in the Cambridge University Botanic Garden where the tree appears to benefit from the colder climate of eastern England. More often, however, cultivated Betula occidentalis does little more than form an untidy tangle of dead twigs, producing fresh growth every summer that inevitably dies back during the following winter. In such situations it is better to plant B. × utahensis, its hybrid with B. papyrifera, especially the form from the Lochsa Valley, Idaho, which has most of the attractive features of B. occidentalis but can be paler in bark. The hybrid is always more vigorous, with larger leaves which are often sticky and frequently very shiny. Plants also sprout vigorous shoots from the base in the same way as the B. occidentalis parent.