Betula kweichowensis Hu

Betula kweichowensisis is an extremely handsome tree with ‘curiously metallic-looking’ bark (Grimshaw & Bayton 2009) and large leaves that may develop vivid autumnal shades of yellow-orange, particularly in sheltered situations – it is one of the best autumn-colouring species at Wakehurst Place, Sussex (McAllister 2005). The elongated male catkins are the showiest of all the birches – up to 16 cm long when fully expanded in April – but the large fruiting catkins are also highly ornamental. Betula kweichowensis is one of a handful of species, among them B. fargesii, that tolerate or even thrive in heavy wet soils, a characteristic that deserves wider exploitation. The more southerly subspecies fansipanensis was described in 2013 by Ashburner & McAll. from south Yunnan and North Vietnam, treated here in a separate entry.

Betula kweichowensis most closely resembles B. alleghaniensis and B. lenta, two other species in section Lentae, but its handsome leaves are much larger than in either of those species and its fruiting catkins are bulkier with a more elongated central lobe to each scale. The leaves of B. kweichowensis are strikingly similar to those of the distantly-related B. cylindrostachya, but lack attenuate tips to the leaf teeth, and differ in their semi-craspedodromous leaf venation (i.e. the ends of one or two basal pairs of secondary veins are barely distinguishable from the tertiary venation, curling back at the tooth base and sending a branch into the tooth). By contrast, the venation in B. cylindrostachya is craspedodromous, the secondary veins significantly thicker than the third-order venation and running straight into the teeth as in almost all other birch species. The fruiting catkins are slow to break up, and the scales remain attached for up to a year, hanging down on a thread, a feature that distinguishes them from those of Betula globispica and B. fargesii in which the scales separate from one another but remain firmly attached to the axis and release the seeds in a similar way to a pine cone.

Although discovered as long ago as 1899, and collected by E. H. Wilson in 1900 (Sargent 1916), Betula kweichowensis never gained a foothold in cultivation and appears not to have been introduced until 1985 when seed was introduced from Fanjing Shan in northeastern Guizhou province. This collection (initially distributed as Betula austrosinensis) was made by the Simmons, Fleigner and Russell expedition (SFRX or GUIZ 82) at 1200 m in mixed forest dominated by Fagus longipetolata, in association with Tsuga and Stewartia – a habitat typical of several of the birches in section Lentae, which tend to grow scattered in mixed forest instead of forming pure stands. One of the finders, Jim Russell, described the species as a ‘fairly common […] rough-barked tree of some seventy to eighty feet. The strobiles [fruiting catkins] are upright, in clusters on ascending twigs and, from below, look curiously like beech mast’ (Russell, quoted in McAllister & Ashburner 2013). Other introductions include SICH 2456 collected near Bao Ding Guo, Maoxian County in 2003, grown at Kew (Acc. 2004–755*1).

Betula kweichowensis is very similar to a fossil species, B. leopoldae, described from the Eocene in British Columbia, c. 50 Ma – the earliest known fossil certainly referable to Betula, comprising many well-preserved leaves, fruiting catkins, catkin scales and seeds (Crane & Stockey 1987; McAllister 2005). Many of the characteristics of these two species are those predicted for the ancestral form of the genus Betula, with leaves not unlike those of some species of Alnus or Carpinus, though the high chromosome number (decaploid, 2n = 140) indicates that B. kweichowensis itself is a far more recently derived species.

Unusually for a birch, the roots of Betula kweichowensis appear highly tolerant of poor aeration. In fact, although trees will survive in drier soils, they grow much more slowly and reach a lesser height than in wetter conditions (Albertson & Weaver 1945). This response to edaphic conditions is typical of other early-evolved angiosperm trees such as planes (Platanus), alders (Alnus), Liquidambar and wingnuts (Pterocarya), which are widely used in urban contexts with poorly aerated and heavy soils. Like them, Betula kweichowensis might be a suitable subject for use with difficult substrates. The species is self-incompatible, and more than one clone must be grown to obtain viable seed. Cuttings from young trees root relatively easily, but those from older and fruiting trees are more difficult.

Material from SABE 183 identified in catalogues as B. insignis is correctly referred to B. fargesii (see that entry).