Pseudotsuga Carrière

The genus Pseudotsuga – an unusual combination of Latin (pseudo – false) and Japanese words (tsuga – hemlock) – was described by Carrière in 1867 and based on the Douglas Fir, Pseudotsuga menzesii, which had previously been placed in both Abies and Pinus (Farjon 2017). The Douglas Fir, P. menziesii, has become one of the most important timber trees in the temperate world, besides which it is one of the noblest of conifers that may be grown in our area for pleasure or for profit. It is no surprise, then, that the colloquial name for P. menziesii has come to be applied, quite inappropriately some would argue, to the genus at large.

There are several arguments to be made against adapting the colloquial name for Pseudotsuga menziesii for other species, chief among them is perhaps the diversity that exists within P. menziesii itself. ‘Douglas Fir’ applies to P. menziesii as a whole, but there are two varieties: the nominate var. menziesii is one of the giant conifers of the temperate rainforests of the Pacific Northwest, while var. glauca is a slower-growing and smaller tree distributed in the interior of western North America in a truly continental climate. Thus there are two Douglas Firs, one ideally suited to oceanic climate zones but not to a continental regime, and the other vice versa. This discrepancy is explored in greater detail in the P. menziesii article.

Even allowing for such a disparity, Douglas Fir has little in common with its sister species. Pseudotsuga is distributed in western North America and eastern Asia. In North America it has an enormous natural range covering some 38° of latitude and is commonly the dominant species throughout its distribution, while in Asia its representatives are much more scattered, occurring in mountainous areas of southern Japan, southern China, northern Vietnam, and Taiwan where they occur as a scattered component of mixed forests, barely ever dominating the ecosystems it occurs in (Farjon 2017). These distribution patterns reflect the more or less north-south orientation of major mountain ranges in North America, enabling relatively rapid north-south migration in response to the cycle of glacial and inter-glacial periods, and the more complex topography and therefore more complex migrations in Asia (Debreczy & Rácz 2011). P. menziesii has been successfully introduced throughout temperate areas and has become a culturally and economically significant species wherever it may be successfully grown; all the others have little more than a toe-hold in cultivation and seem unlikely ever to excite even a fraction of the interest enjoyed by the American giant, except perhaps the American relict P. macrocarpa, which has potential as a drought-tolerant alternative conifer species in many parts of the world. Indeed, in writing about these trees, the authors’ task would be made much easier if P. menziesii could be artificially siphoned off and treated as a monospecific genus, but it is not so, and the principle of vox populi, not to mention a lack of helpful alternatives, makes the continued use of such names as Japanese Douglas Fir and Taiwan Douglas Fir an inevitability.

This combination, of extensive and scattered distributions on two continents, has resulted in considerable variation and a consensus taxonomy has never quite been achieved, with most modern treatments recognising either four (Farjon 2017) five (Lipscomb 1993) or six (Debreczy & Rácz 2011; Fu, Li & Mill 1999) species. As is often the case with conifers there is broad (but not total) agreement on the distinctiveness of taxa, but not on their relationships, placement and rank.

Modern scientific methods have only recently been brought to bear on this problem. Wei et al. investigated mitochondrial and chloroplast DNA regions and one nuclear gene and concluded that the North American and Asian taxa each consituted a monophyletic clade; their results provided further evidence for a North American origin for Pseudotsuga, which probably migrated to Asia by way of the Bering land bridge during the early Miocene, and that the last common ancestor for Asian taxa was extant 15–25 MYA (Wei et al. 2010). Their results further support an emerging consensus that North American populations comprise two species, Pseudotsuga macrocarpa with a restricted distribution in southern California, and P. menziesii with an extensive distribution from Canada to Mexico, the latter including one infraspecific taxon, usually var. glauca (Adams et al. 2013) but see that article for a discussion of other approaches.

The situation in Asia is more complex. Japan has its own uncontentious endemic species, Pseudotsuga japonica, but in China, Vietnam and Taiwan treatments vary considerably. The most conservative treatments have been those of Farjon (1990; 2001; 2017; and a co-author of this work) who considers all ‘mainland’ Asian populations, together with those on Taiwan, to belong to a single widespread species, P. sinensis, with three varieties, var. brevifolia, var. gaussenii, and var. sinensis. Within var. sinensis Farjon includes P. forrestii from Yunnan and P. wilsoniana from Taiwan. Flora of China (Fu, Li & Mill 1999) recognise P. brevifolia and P. forrestii at species rank, sink var. gaussenii into var. sinensis, and treat Taiwanese populations as P. sinensis var. wilsoniana. Debreczy & Rácz (2011) adopt the same position as Flora of China but continue to recognise P. s. var. gaussenii.

Nearly all these entities have found their way into cultivation, with the notable exception of Pseudotsuga sinensis var. gaussenii, and Taiwanese populations in particular have been the source of many modern introductions to collections in our area (Grimshaw & Bayton 2009). Mindful of Trees and Shrubs Online’s predominantly horticultural audience, the authors of this account have sought to balance taxonomic and horticultural interests and so treat the ‘mainland plus Taiwan’ populations of Asian Pseudotsuga as one widespread species, P. sinensis, but with five varieties: var. brevifolia, var. forrestii, var. gaussenii, var. sinensis, and var. wilsoniana. Doing so enables us to discretely frame our discussions of introductions from Taiwan, and acknowledges the results of recent molecular investigations that support P. forrestii as a distinct entity (Wei et al. 2010). Further discussion may be found in each article.

Horticulturally, American and Asian Pseudotsuga are again very disparate in their performance. The American taxa – even the rare Pseudotsuga macrocarpa – are all capable of forming large trees across our area, none more so than P. menziesii var. menziesii, which within a century of its introduction could claim to be among the tallest and fastest growing trees in several European countries (Johnson 2010; Mitchell 1996). Many of the most magnificent designed landscapes in Europe are punctuated by its characteristic silhouette, and it has been planted over vast areas of appropriate climatic zones across the world for its strong, versatile timber. By contrast, the Asian taxa remain modest, even unremarkable and at times quite gaunt trees, while the inevitable and unfair comparison with P. menziesii does them no favours. They are found only in a few specialist collections where their rarity, in the absence of merit, ensures their survival.

Pseudotsuga menziesii var. menziesii in particular is host to a bewildering diversity of fungi, and susceptible to a range of pests and pathogens, though only a few are a significant threat, and then mostly only in plantation settings (Lavender & Hermann 2014; Wilson 2011). Emerging diseases affecting P. menziesii include Phytophthora pluvialis, first discovered in Oregon in 2013 and since found affecting trees in cultivation especially in western Britain, symptoms include needle cast, shoot dieback and lesions (forestry.gov.scot). In horticultural situations the main enemies of P. menziesii are honey fungus (Armillaria spp.) and other wood decay fungi such as Phaeolus schweinitzii. In cultivation the other American and Asian taxa are more sensitive; not being endowed with P. menziesii’s ability to outgrow many would-be problems they can suffer from adelgid aphids and fungal conditions, notably Rhabdocline pseudotsugae (Rushforth 1987). Many of the Asian taxa often look chlorotic in cultivation but this is rarely a significant problem: on seeing the UK’s finest, if rather yellowish-green P. sinensis var. forrestii at Werrington, Cornwall, Tom Christian remarked that he wished he could have seen it in full health, whence Keith Rushforth replied ‘it’s looked like that since before you were born’!

Propagation is by seed or grafting, the latter being the necessary method for the various cultivars of Pseudotsuga menziesii. With such a large natural range there is considerable merit in selecting suitable provenances of P. menziesii for ex-situ cultivation, but in many areas it has been so widely cultivated for so long that copious seed production is guaranteed, and when seed is sourced from sufficiently diverse ex-situ plantings home-grown P. menziesii will perform extremely well. For this reason it is a popular species in continuous cover forestry models in Europe and beyond (Wilson 2011). Unless wild-sourced seed is available the Asian taxa must also be grafted; while these will graft on to P. menziesii growing-on such grafts can be very difficult, and in the unlikely event Asian rootstocks are available these should be used in preference over P. menziesii (K. Rushforth pers. comm. 2023).