Picea koyamae Shiras.

Picea koyamae is one of four spruces endemic to Japan (the others are P. alcoquiana, P. maximowiczii and P. polita). Both P. koyamae and P. maximowiczii have very narrow distributions in the Japanese Alps on the Pacific side of central Honshu, but P. koyamae might have the smallest range of all, with an extent of occurrence estimated at less than 100 km², among which are scattered fewer than 1000 mature trees (Katsuki & Gardner 2019). With this tiny population size, its fragmented nature, and taking into account historic logging and habitat loss, climate change, and acute vulnerability to climate change and major disturbance events such as fires and typhoons, P. koyamae is one of just two spruces worldwide currently categorised as Critically Endangered (IUCN 2024).

It was found on Yatsugatake by Japanese botanist Mitsua Koyama, in October 1911, and named in his honour by Shirasawa in 1913 (Wilson 1916). From the beginning there was speculation that Picea koyamae might also occur on mainland Asia; Wilson considered material collected by Purdom in northeast Hebei ‘very similar in every way’ to his own collections of P. koyamae from Yatsugatake (Wilson 1916) and within a few years major works would be stating with authority that P. koyamae also grew in Korea (e.g. Bean 1976). We now know that it is a Japanese endemic, but this speculation led to all manner of confusion in early literature and in collections. For example, Dallimore & Jackson (1966) give P. koraiensis as a synonym of P. koyamae, and for many years trees in collections labelled P. koyamae were actually P. koraiensis (Jacobson 1996). After seeing it on Yatsugtake Wilson admired it greatly calling it ‘shapely and decidedly ornamental’ (Wilson 1916); those who formed a dim view of it (like Jacobson (1996)) were probably meeting P. koraiensis.

Wilson saw Picea koyamae in 1914, in the company of its namesake Mitsua Koyama, and at the same time obtained seed for the Arnold Arboretum (W 7523). Their requirements satisfied, the Arnold shared material widely with other botanical institutions the following year, which is when Picea koyamae arrived in collections such as Kew and Edinburgh in the UK, and further seed would reach collections on both sides of the Atlantic in the early 1920s. P. koyamae is well suited to the British climate and young trees can grow rapidly here, although it seems always to start slowly; once into their stride young trees can put on up to 60 cm of vertical growth per year, although 30–40 cm is perhaps more typical (Mitchell 1972; pers. obs.). Of the many mature trees growing in the UK only those with impeccable records can be considered Wilson originals; some of the largest were planted much later than his introduction, for example the best of several comporable specimens at the Frank Best Memorial Arboretum, Denbigshire, 26 m tall × 67 cm dbh in 2023, from a 1953 planting (Tree Register 2024).

Alan Mitchell knew of just two originals in 1972, one at Dawyck in the Scottish Borders and another at Stanage, Wales. The Dawyck tree was planted in 1918; it was 14.6 m × 26 cm in 1966, and 20 m × 45 cm in 2014. The Stanage tree was planted in 1920; it was 20.1 m × 38 cm in 1970, and 26.7 m × 57 cm in 2023 (Tree Register 2024; Mitchell 1972). We can add to this two originals at the Royal Botanic Garden Edinburgh: one was badly damaged in a storm in January 2012 and had to be removed; in Edinburgh’s very sandy soil it was not as large as others cited, but it had been larger than the remaining tree which, having been suppressed for most of its life by a large beech tree, was a mere 9 m × 21 cm in 2004 (Tree Register 2024). The tallest of eleven original trees from W 7523 at the Arnold Arboretum was 17.4 m in 1982; the largest is now 60 cm dbh. Many of these genotypes have been grafted in recent years (Arnold Arboretum 2024; Warren 1982). Curiously, the website Monumental Trees lists only a single example of P. koyamae from mainland Europe, growing in the Späth Arboretum, Berlin, 22.5 m × 53 cm in 2019 (Monumental Trees 2024). It is certainly not as common across mainland Europe as it is in Britain.

More recent, wild-source introductions to the UK include those made by the joint Edinburgh-Kew Japan expedition of 2013. This expedition, conceived by RBG Edinburgh’s International Conifer Conservation Programme (ICCP), targeted a range of threatened conifer species in central Japan, including Koyama Spruce, P. maximowiczii, Pseudotsuga japonica and Sciadopitys verticillata. Large numbers of P. koyamae were raised from EIKJE 160, 162, 163, 182, 183 and 193. In addition to those planted at Kew, Edinburgh and its regional gardens Benmore and Dawyck, and Bedgebury National Pinetum, the ICCP distributed plants widely among its network of ‘safe sites’. Of particular note are large plantings at Drumlanrig, Dumfriesshire, and Drummond Castle, Perthshire (Royal Botanic Garden Edinburgh 2024; pers. obs.).

These young trees are establishing well almost everywhere. Still in their juvenile phase they all exhibit thin, rather green leaves pressed forwards above the shoot, and level branch tips. This contrasts with the rather thick, distinctly glaucous, assurgent leaves and upward curving branch tips of trees and young grafts traceable to W 7523. They have in common more or less glabrous, deeply grooved, yellowish- to brownish-orange shoots, usually more darkly coloured on the upper side, with rather large buds obscured by a concentration of leaves (pers. obs.; Mitchell 1972). The stout, distinctively coloured shoot distinguishes P. koyamae from P. abies, as does the bark. Mature trees have bark exfoliating in irregular large plates in a manner very like P. asperata, but with shorter leaves with an acute rather than pungent apex. P. koraiensis differs in its less exaggeratedly flaking bark, the leaves usually green and always somewhat more prickly, and the larger terminal buds less obscured by leaves (pers. obs.).