Plant of the Week – 15th March 2021 – Scots Pine (Pinus sylvestris)

Scots pine Pinus sylvestris is one of only three conifers native to the British Isles and the national tree of Scotland. Pinewoods in the Highlands are the home of iconic animal species including Red Squirrel, Capercaillie, Scottish Crossbills, European Crested Tits and Wildcat, with many distinctive plants, lichens and fungi.

Fig. 1. The natural distribution of Scots pine. Credit: the map was developed by the European Commission Joint Research Centre based on the EUFORGEN mapand released under Creative Commons Attribution 4.0 International (CC-BY 4.0).

The trees are tall (20 metres at 100 years) and thrive on poor acidic soils and gravels, often forming open stands with a well-developed shrub layer of Vaccinium, Calluna and Juniperus, and often accompanied by Birch, Aspen and Rowan. The oldest trees in Scotland exceed 400 years. Its name `Scots pine’ is perhaps surprising as the natural distribution is much wider than the name suggests, going well beyond the Arctic Circle in Scandinavia and extending south to northern Spain. It occurs to about 141 oE, almost touching the Pacific Ocean in the easterly direction (Fig 1).

The species is valued for its timber and is often used in plantations. The reddish timber is easy to work and has many uses in the construction and furniture industries; in the past it was used for ships’ masts and as a source of turpentine and tar. Being one of the hardiest trees, it is also planted as a windbreak around farms and to provide shelter for stock on hill farms.

It does not regenerate under its own shade but appears when the ground is cleared by fire or wind and plenty of seed is present. This is an important consideration in silviculture and key to any attempts to conserve ancient pinewoods. In plantations, seedlings are found at the edges of tracks but seldom under the canopy. Deer browse the foliage of seedlings and saplings and may limit its spread; thus, any afforestation or re-forestation must be accompanied by control of deer and other herbivores.

Fig. 3. A single tree at the tree line in the Cairngorm Mountains (600 m a.s.l.). Images of the same tree, taken from 1980 to 2006, slowly recovering from extreme damage. I hope to find it again in 2021. Photos: John Grace.

At the highest altitudes and latitudes of northern Europe Pinus sylvestris is often a main component of the tree line, where it survives damage by wind, snow and ice (Fig. 3). Scotland has only one location where a more or less natural tree line occurs, at Creag Fhiachlach in the Cairngorm Mountains at about 600 m above sea level, overlooking the Spey Valley. There, trees are contorted and often dwarfed (the crooked-wood or ‘krummholz’ condition found in tree lines of `Europe and North America).

Fig. 4. Värriö Subarctic Research Station belongs to the Institute for Atmospheric Research at the University of Helsinki, constructed from pine logs, deep within in a Scots pine woodland near to the latitudinal limit of the species. Note the straight stems, essential in building with logs. Photo: John Grace.

When I first discovered the Cairngorm tree line I was inspired to apply for a Grant. The application succeeded and I was able to employ a small team. We found that dwarf pine trees (like dwarf shrubs everywhere) are sheltered and when the sun shines their growing tissues can be considerably warmer than the air above; that’s what enables them to survive in locations with a low air temperature (Wilson et al. 1987). Although many of those trees are dwarfed by snow and wind damage there are some individuals which appear truly plagiotropic – they appear to be genetically dwarfed. The species is well known for occurring in many forms across its range, from spire-like to bushy (cf Fig. 4 and Fig. 2), but so far I have found no description of true dwarf forms.

Probably, more is known about the physiology and ecology of Scots pine than of any other tree species, thanks to the strenuous efforts of researchers in Finland and Sweden. However, in a British context there are two inter-related stories to tell: (i) how it got to these islands and (ii) the Caledonian Forest.

Fig. 5. Finnish Lapland (67.7 degrees N), showing the spire-like form of trees, unlike those in Scotland. Photo: John Grace.

How it got here

Pine pollen is freely produced and the grains are lightweight and extremely tough; it is dispersed over large areas and preserved as a distinct signature in bogs; by radio-carbon dating and counting the hundreds of thousands of pollen grains extracted from cores of peat, the history of the species can be constructed (Huntley and Birks 1983, Birks 1989, Bennett et al 1991).  

Fig. 6. Male cones of Scots pine produce copious pollen which may be carried large distances by the wind. Photo: John Grace.

During the Late Glacial Maximum (23,000-18,000 years ago) most of Europe was too cold for pine; however, pollen analysis shows that pine and many other tree species survived continuously in somewhat milder locations within southern and central Europe, known as refugia (Bennett et al 1991, Birks and Willis 2008). As warming occurred, the species began to spread north.

Even before all the ice had gone,  some 13,000 years ago, pine pollen was frequent in what today is northern Italy and Romania. Shortly after, it occurred further north, forming 75 % of the pollen in Alpine regions and it was in southern England 9,000 years ago. Over the following 1,500 years it appears at sites from southern to northern England, spreading at the astonishing rate of 300 -700 metres per year. It is hard to imagine how the spread could have been so fast, but rates based on radiocarbon dating of pine stumps tell a similar story. Around 8000 years ago pine reached Scotland and appears to have been widespread 6000 years ago, but not in southern Scotland (Tipping 1994). This was its maximum extent and since then it has declined. Today, relict populations survive at about 80 locations in Scotland. Those in England are missing, presumably due to over-exploitation and changes in land use over the last few thousand years. The Scots pines often seen in England have all been planted and often the seed has been brought from elsewhere. In the rather dry East of England it thrives as huge plantations (eg Thetford Forest).

The above narrative is called into question by observations of the pattern of genetic variability, first alerted by Ian Forrest at the Forestry Commission’s Northern Research Station near Penicuik. His analysis of mono-terpenes extracted from Scottish populations demonstrated considerable geographic variability. Data from Wester Ross showed substantial deviations from the rest (Forrest 1980). Later, using molecular genetic approaches at these locations and others, Kinloch, Westfall and Forrest (1986) went so far as to say “variability in the relict populations is almost the highest of any plant species studied”. The Discussion at the end of their paper is a ‘must-read’. The authors re-visited the older ideas that the species could have survived in the British Isles during glaciation; they also proposed multiple pathways of colonisation, including possibly one from the west (Ireland or the Western Isles).

The work of Richard Ennos’s group at Edinburgh University is further evidence of the species entering Scotland from the west as well as the south  (Sinclair, Morman and Ennos 1999).  Mitochondrial DNA was extracted and examined from seeds and buds of Scots pine collected from 38 Western European populations (20 from native pinewoods in Scotland). Three main types were found but the Scottish samples were mostly of two types, called a and b which have different distributions in Europe: a occurs in Central Europe whilst b is found in Scandinavia, Spain and northern Italy. In Scotland, type b was found in the West.

Other researchers have since pursued the enquiry, deploying molecular techniques to reveal movement of the species in the whole of Europe. In Hungary, To´th  et al (2017) found evidence of nine glacial refugia in southern Europe, with post-glacial migration routes spreading north and frequently diverging. They acknowledge both a southern and western route to Britain, but it is quite unclear where the western introduction came from, and we may never know. It is tempting to say that the populations in Western Scotland came from the Iberian peninsula.

Fig. 7. Likely northerly spread of Pinus sylvestris from southern refugia, based on several accounts in the literature.

The Wood of Caledon

The decline of Scots pine from its maximum extent thousands of years ago, to the time of the Roman invasion, is a controversial subject which is much debated. In Neolithic times there may have been local concentrations of humans, but the whole of Scotland is unlikely to have had more than a few hundred thousand people. They were hunter-gatherers, eeking out an existence from hazelnuts, fish and molluscs. The decline of pine may have been aided by these folk, but more likely the main cause was the high rainfall and the consequent formation of peat. Scots pine doesn’t mind drought, but it hates wet feet!  

When the Romans came to Scotland and built Hadrian’s Wall and the Antonine Wall it is unlikely that the surrounding lands were wooded; if they were, the walls would have been pointless (soldiers patrolling the wall must have been able to scan the distant horizon for trouble). Shortly afterwards, Ptolemy made a map of Scotland from accounts by the invaders. It is a child-like map and not to be relied upon too heavily. However, it does show that only a small part of the country, principally the Highlands, is wooded and labelled ‘Wood of Caledon’. From here comes the notion of The Caledonian Forest, much-romanticised by authors, film-makers and TV presenters.

Nearly a thousand years later, the Doomsday Book (1086) asserted that Scotland was 15 % forested i.e. slightly less than today. Pont’s survey and maps (1583-1596) have been scrutinised by scholars: woodlands are generally sparse. Later came William Roy’s Military Survey of Scotland (1747-1755). The picture is bleak: lowlands were 2-3 % forested, a little higher in the Highlands, but overall the country was only 4 % forested.

Sir Walter Scott (1771-1832) referred to ‘the waste lands’ of Scotland and was a strong advocate of tree planting. He did some himself at his Abbotsford house, Melrose. He advised people to use seed from the Highlands rather than imported seed from Canada (at that time all conifer trees were casually called ‘firr’ by landowners, and perhaps Douglas Fir Pseudotsuga menziesii was the Canadian tree he had in mind).

When well-meaning people say they want to ‘restore’ the native Caledonian pine forest they perpetuate myth. Today’s environment is quite different from that of pre-Roman times and so planting trees does not ‘restore’; at best it only mimics. Nevertheless, to walk in the pinewood, for most of us, is uplifting; it has spiritual appeal. Pineforests and the wildlife therein are ‘jewels in the crown’ of Scotland’s biodiversity, and you can even enjoyment for the kids (see Fig 7).

Fig. 7. Landmark Forest Adventure Park, Carrbridge near Aviemore, Scotland. A good use for mature Pine forest. Photo: John Grace.

Acknowledgement of sources

Before writing this, I re-read Stephen and Carlisle’s The Native Pinewoods of Scotland (1959) as well as A History of the Native Woodlands of Scotland 1500-1920 (Smout, MacDonald and Watson, 2005). I dipped into Richard Tipping’s work on peatlands, and even found something useful in Walter Scott’s Prose Works: Periodical Criticism.

Link to our YouTube channel

Two of our Botanical Society lectures are relevant to this article. Find them at:

https://www.youtube.com/channel/UC8vz4IELFMp70aOxM6uo2oQ

Look for these titles: (i) The Scottish Uplands: how to revive a degraded landscape – Dr Helen Armstrong; (ii) Trees, Climate and the end of Scottish Independence. Prof Rob Wilson, University of St Andrews.

References

Bennett KD, Tzedakis PC and Willis KJ (1991) Quaternary refugia of North European trees. Journal of Biogeography 18, 103–115.

Birks HJB (1989) Holocene Isochrone maps and patterns of tree-spreading in the British Isles. Journal of Biogeography 16, 503-540.

Birks HJB and Willis KJ (2008) Alpines, trees, and refugia in Europe. Plant Ecology and Diversity 1, 147-160.

Forrest GI (1980) Genotypic Variation among Native Scots Pine Populations in Scotland based on Monoterpene Analysis Forestry: An International Journal of Forest Research, Volume 53, 101–128.https://doi.org/10.1093/forestry/53.2.101

Huntley B and Birks HJB (1983) An atlas of past and present pollen maps for Europe: 0–13000 years ago. Cambridge: University Press.

Kinloch BB, Westfall RD and Forrest GI (1986), Caledonian Scots  Pine: origins and genetic structure. New Phytologist 104, 703-729. https://doi.org/10.1111/j.1469-8137.1986.tb00671.x

Sinclair WT, Morman JD and Ennos RA (1999) Scots pine (Pinus sylvestris L.) in western Europe: evidence from mitochondrial DNA variation. Molecular Ecology 8, 83-88.

Tipping R (2008) Blanket peat in the Scottish Highlands: timing, cause, spread and the myth of environmental determinism. Biodiversity and Conservation 17, 2097–2113.

Toth EG, Kobolkuti ZA, Pedryc A and Hohn M (2017) Evolutionary history and phylogeography of Scots pine (Pinus sylvestris L.) in Europe based on molecular markers. Journal of Forestry Research 28, 637-651.

Wilson C, Grace J, Allen S and Slack F (1987) Temperature and stature: A study of temperatures in montane vegetation. Functional Ecology  1,  405-413.

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