Picea sitchensis (Bong.) Carrière

Picea sitchensis is sometimes called the king of the spruces. Comfortably larger than any other species, it can reach 100 m tall and develop massive trunks in native forests in the Pacific Northwest of North America, where the oldest trees are thought to be in the region of 1000 years old (Debreczy & Rácz 2011). It is distributed along a relatively narrow strip of land parallel to the coast, from Kodiak Island in southern Alaska south to northern California. Its entire distribution is within the influence of the Pacific Ocean, never more than about 80 km away. The climate is characterised by cool, humid summers, mild to cold winters, and abundant year-round precipitation (Farjon 2017; Van Pelt 2001). This narrow distribution within relatively uniform climatic conditions renders P. sitchensis particularly vulnerable to climate change: modelling of three different climate change scenarios all predict a gradual deline within its native range (Ma et al. 2019).

The largest trees typically occur in British Columbia and Washington, but the tallest, 100.2 m × 2.73 m dbh, was found in California’s Redwood National Park in 2021, presumably having been drawn up by the Coast Redwood (Sequoia sempervirens) it grows with (Monumental Trees 2024). Other very large and/or tall trees occur in Carmanah Creek, British Columbia, where a tree known as ‘Carmanah Giant’ was measured at 96 m × 4.45 m dbh in 2023; the largest recorded girth belongs to a tree at Lake Quinault, Washington, 58.2 m × 5.38 m dbh in 2001 (Monumental Trees 2024; Van Pelt 2001). Like many other conifers it associates with, such as Abies grandis, Pseudotsuga menziesii, and Tsuga heterophylla, Picea sitchensis is ideally suited to cultivation in other maritime climates, and nowhere more so than in maritime districts of northwest Europe, and in particular the UK and Ireland where, for many decades now, it has been among the most commonly planted trees.

Indeed, Sitka Spruce (to many simply Sitka) is the mainstay of the commercial forestry industry in these islands, most particularly in Scotland. In the relatively recent past it accounted for as much as 80% of all annual tree planting here; concerted efforts to ‘diversify’ have brought this figure down to about 45% in recent years (Forest Research 2023). The inverted commas are employed because diversity has mostly been achieved via a huge uplift in native woodland planting, woodlands which are usually managed for nature conservation purposes rather than timber production, for which Sitka remains the overwhelming favourite of industry professionals despite the considerable effort that has been made to promote alternatives.

The ongoing global focus on tree pest and disease issues is, gradually, beginning to bear fruit as the UK forestry industry accepts, however reluctantly, that change must come. A recent study by the UK government’s Forest Research body examined data from 87 forest experiments planted in Britain between 1929 and 1995 to compare long-term performance of 52 species with Sitka Spruce. They concluded: ‘No species had significantly higher General Yield Class (GYC) estimates than Sitka…but [Abies grandis, Abies procera, Chamaecyparis lawsoniana, Picea × lutzii, Pinus pinaster, Pinus sylvestris, Pseudotsuga menziesii, Thuja plicata and Tsuga heterophylla] all had GYC estimates which were not significantly lower than that of Sitka spruce under certain soil and climate conditions’ (Stokes, Jinks & Kerr 2023).

In other words, the economic models that guide the UK forestry industry will need to be modified in tandem with diversification away from Sitka, but for an industry that contributes several billion pounds per annum to the UK’s GDP, this overwhelming reliance on a single species should have rung alarm bells among policy makers and forced change far sooner. The astute reader will notice that several of the nine alternatives listed are already experiencing worrying pest and disease issues in various parts of the world, including the UK, and that not all of them share the same climatic preferences as Sitka to begin with.

Sitka’s rise to dominance in Britain can be traced back to the mid-19th century. Archibald Menzies observed it at Puget Sound in May 1792 whilst with the Vancouver expedition, but it would not be introduced until David Douglas sent seed to the Horticultural Society in London in 1831 (Elwes & Henry 1906–1913). Only very few plants were raised from this sending, and by 1972 the great British dendrologist Alan Mitchell considered only two trees in Ireland to be extant originals. Thomas Jeffries’ 1851 introduction for the Oregon Association – a consortium of largely Scottish landowners who clubbed together to re-introduce many of Douglas’s Pacific Coast discoveries – was far larger and more successful. Elwes & Henry observed that it was Jeffries’s collection ‘from which the pineta of Scotland and England have been stocked, and it has now become a common tree’ (Elwes & Henry 1906–1913). The oldest extant trees in UK cultivation are derived from this sending, including four spectacular trees in the pinetum at Scone Palace, Perth, planted in 1852.

The rampant growth rates of the newcomer soon captured the attentions of foresters. It was not long before trees were producing viable home-grown seed (which at that time was cheaper to procure than imports of wild seed) and landowners like Sir James Stirling Maxwell at Corrour began establishing experimental plantations to test Sitka as a timber crop from the late 19th century onward. Not only was Sitka fast, but it would grow larger, faster and better than any other species on exposed upland sites, and on nutrient poor and poorly drained soils, too (Elwes & Henry 1906–1913). Particularly after the first world war, when Britain’s timber stocks were at an all-time low, planting increased in earnest, largely under the aegis of the newly formed Forestry Commission, but it was from the 1960s and the 1970s in particular that the UK timber industry doubled down on Sitka, and during the latter decades of the 20th century vast swathes of upland Britain were planted with huge monocultures. It is worth highlighting the very narrow gene pool of most commercial Sitka grown in Britain since the early 20th century, from when foresters have tended to favour material from Haida Gwaii (Queen Charlotte Islands) off the British Columbia coast ‘because of their climatic and latitudinal match with Britain and the excellent growth and high quality timber of the indigenous stands’ (Samuel, Fletcher & Lines 2007).

Many groups and individuals tried to raise awareness at the time of the often very serious negative impacts of these plantations on sensitive upland sites, particularly the expansive blanket bogs in northern Scotland known as Flow Country, but economics won. The rest, as the saying goes, is history, and has been the subject of much study and controversy such that entire works have been dedicated to it, notably Shades Of Green: an environmental and cultural history of Sitka Spruce (Tittensor 2016). In modern Britain Sitka remains the favourite species of the forestry industry but is much-maligned by a portion of the public who consider themselves well informed, but perhaps do not appreciate the complexities involved, not least that the UK remains the world’s third largest importer of forest products (Forestry Journal 2024).

The many benefits of hindsight, together with better and stricter environmental regulations, now guard against the mistakes of the past. There is little doubt that Sitka could and should remain a considerable component of UK productive forests for generations to come, but its dominance, and the associated risks, must be mitigated. These include Sitka’s ability to freely regenerate in appropriate climatic zones, leading to its designation as an invasive species in some jurisdictions where it is less economically significant, such as Norway (Nygaard & Øyen 2017). Were it not such a key economic crop in the UK it would surely be similarly designated here, too.

After nearly 200 years in cultivation many of the tallest trees in the UK, Ireland, and adjacent parts of northwestern Europe are no longer in Victorian pineta but are those trees which have grown up in closely planted forests originally intended as a timber crop, or else have self-seeded into favourable positions. This certainly seems to be the case of the tallest known Sitka in Britain, which grows in a gorge of the River Findhorn just upriver of Randolph’s Leap in Morayshire; in 2021 it was 65.8 m × 2.06 cm dbh (Tree Register 2024). One of the tallest specimen trees not in woodsy conditions grows by the west drive at Murthly Castle, Perthshire; it has made a remarkably tall tree in what is essentially a parkland situation with a handful of nearby broadleaves little more than half its height of 55.5 m × 2.09 m dbh in 2017 (Tree Register 2024). The largest recorded girths in Britain do belong to specimen rather than forest trees, which mostly tend to have markedly flared boles; the biggest of all are to 2.77 m dbh at Fairburn and 2.58 m dbh at Kilvarock, both in the Scottish Highlands and measured in 2013 and 2017 respectively, while the largest of the 1852 trees at Scone, which all have flared boles, was 2.56 m dbh in 2017 (Tree Register 2024).

Beyond the UK and Ireland, there are few parts of our study area where Picea sitchensis has been so widely planted. Michael Dirr considers it to be functionally absent from the eastern United States (Dirr 2009) but there are examples scattered in major collections, such as the Arnold Arboretum where the oldest extant plants were accessioned in 1980 (Arnold Arboretum 2024); certainly its performance here, and in continental climate zones in general, pales in comparison to native trees and those cultivated in oceanic climate zones.

On Europe’s western seaboard there are good examples in Norway, including several over 50 m tall and to 1.55 m dbh around Bergen, and another 37 m × 2 m dbh in Dydland in the far south of the country (Monumental Trees 2024). Particularly in western Norway Picea sitchensis and several other Pacific Northwest conifers are considered invasive (Nygaard & Øyen 2017). In Sweden the largest examples on record are in the southwest of the country, south of Gothenburg, including one to 37 m tall and to 1.92 m dbh (Monumental Trees 2024).

An interesting record from Kirkjubæjarklaustur in southern Iceland notes a tree 28.4 m × 0.47 m in 2023; further south in Europe, the tallest record readily available from France belongs to a tree growing in Arboretum Domanial de la Jonchère Saint Maurice, Haute-Vienne, 43.6 m × 0.76 m dbh in 2023, while the largest is in the park at Lieu-dit La Villeraud, Creuse, 38 m × 1.47m dbh in 2024 (Monumental Trees 2024).