Native Scots Pines - colonisation, refugia, genes and paradigms

The Scots Pine reaches its most north-west oceanic limit in the Scotland and scientific investigation has attempted to answer some key questions on the native Pinewoods, notably the former extent, colonisation history from the last glaciation, climatic impacts, human impacts, regeneration and more beside. Radiocarbon dating of organic remains, palynology, dendrochronology and DNA analysis are just some of the techniques that science has applied to native Scots Pine populations. There have been some surprising insights, not least on scientific paradigms, methodology and assumptions.

Native Scots Pinewood

Those who follow Quaternary research, will be aware that there are concerns that some paradigms on glaciation, climate models and palaeoecology, might require revision or some caveats applied. I'll highlight some of the 'discussion' points.

Pollen analysis is apparently a time consuming activity and palynologists have devised some guidelines to reduce the tedium and speed things along, notably sampling frequencies and critical pollen thresholds. Scots Pine produces abundant pollen and less than 20% in a pollen assemblage was treated by pollen analysts as a cut off point for the presence of pine at a site. From the mid 1990's it was proposed that the presence of pine stomata in a sample was considered a fairly good indication of the physical presence of a tree near to the sample site, pine needles aren't carried so far on the wind compared to pollen. In 2005 Cindy Froyd had the wit to look more carefully for the presence of pine stomata in loch sediments in Scotland and published a paper Fossil stomata reveal early pine presence in Scotland: Implications for postglacial colonization analyses. Despite the low % pollen counts for Scots pine, the presence of stomata placed its appearance 1,600 years earlier than the woodland history generated by standard pollen analysis at one study site.

Black Mount ~ 2010

1,360 m above sea level on Mt Åreskutan in Sweden, tree remains (macro fossils) were sampled by Leif Kullman and radiocarbon dated. A Scots Pine was dated at 11,700 years before present (BP), Norway Spruce 11,400 years BP and Birch 14,000 years BP.  Leif Kullman published the findings which raised some inconvenient points. In a reply to the paper it was pointed out that that climate models didn't support the dates, pollen analysis detected no presence for the trees and glaciologists had modelled the area covered by glacial ice when trees were growing there. Leif Kullman provided an insightful and entertaining reply in the Journal of Biogeography.

Refugium from grazing herbivores, Assynt ~ 2008

In 1986 a Forestry research paper titled Caledonian Scots Pine : Origin and genetic structure  stated that Scotland's Scots Pines were genetically distinct from the European population and showed a high degree of genetic heterogeneity within the native pinewoods. Whilst acknowledging the climate models and glacial iee models, a compelling case was made against colonisation from the continent and instead for glacial refugia on mountain tops and/or near to Scotland. Of interest are the results of the forestry trials conducted between WWI and WWII and at Glen Trool.

A paper published in Nature in 1998 determined that the genetic variation for maternally inherited mitochondrial DNA (mtDNA) in populations of native Scots Pines showed a distinct geographical genetic diversity amongst the pinewoods. The mtDNA in Scots Pines of Wester Ross point at colonisation after the last Ice age from the west, indicating a refugia or refugium on the continental shelf or a nunatak from glacial ice.

In 2013 another paper was published titled Among population differentiation at nuclear genes in native Scots pine (Pinus sylvestris L.) in Scotland that again reported that the latest colonisation of native pines in Scotland from Europe was unlikely.

Holocene pine stumps, Kintail ~ 2013

The following text is taken from Quaternary of Scotland Geoconservation Review Site - Lochan an Druim.

Highlights 

The pollen and plant macro-fossils preserved in the sediments which infill the basin at Lochan an Druim provide an important record of vegetational history and environmental changes during the Lateglacial and Holocene in the extreme north-west of Scotland. 

 Description 

The vegetational history of the site has been studied in detail by Birks (1984 and unpublished data; pollen diagram in Birks, 1980).

Interpretation  

There is no evidence from the pollen stratigraphy at this site to suggest that pine or oak ever grew this far north (Birks, 1977, 1989), even though pine stumps occur locally in the Eriboll area.

The site is of national importance because of its detailed and well-dated Lateglacial and Holocene pollen stratigraphy and for the co-ordinated study of pollen and plant macrofossils in the Lateglacial. These palaeoecological data provide important insights into the Lateglacial environment at low altitudes in the extreme north-west of Scotland; there is no other site that has been studied in such detail from this part of Scotland. These insights are as follows: ... (4) The Holocene vegetational history from Lochan an Druim is particularly important because of the apparent lack of pine and oak. Its pollen record is thus intermediate between sites further south in western Scotland (Birks, 1980) ... Lochan an Druim thus represents a uniquely important site for the reconstruction of Quaternary vegetational history and past environments. 

Conclusions 

Pollen and plant remains in the sediments from Lochan an Druim provide a record of the environmental history of the far north-west of Scotland during the Lateglacial and Holocene (approximately the last 12,500 years). They show that conditions remained extreme even during the relative climatic warming in the Lateglacial Interstadial. Later, during the Holocene, birch and hazel woodland developed, but oak did not extend this far north and pine was probably only locally present. Lochan an Druim provides valuable comparisons with other areas and is important as part of the network of sites that show the wider geographical variations in the patterns of vegetation development since the end of the last ice age.

Part of the the Loch an Druim Pollen Profile illustrating the low Pinus Sylvestris pollen percentage 

I have visited this area of the Highlands on a few occasions and have seen enough stumps and roots exposed in peat cuttings, eroding blanket peat and stream sides, to form an opinion that at some stage in the past Scots Pine seemed more widespread than just local to Loch Eriboll.

Scots Pine, Mar Forest ~ 2014

In 2008, Andrew Moir, a PhD research student, published a thesis 'The dendroclimatology of Modern and Neolithic Scots pine (Pinus sylvestris L.) in the peatlands of northern Scotland' samples were obtained from pine stumps exposed in blanket peat, the sites appeared to have been visible from the car as they are all close to the roadside.

The sites were described thus

An Dubh-loch lies just off the A838, 5 km north-west of Tongue, 6 km south of the northern coast of Scotland. This lowland site (140m) is very exposed and treeless today. During site reconnaissance in September 2001, the valley side mire found at the southern end of the loch was found to contain some pine stumps. These appear to have become exposed through peat cutting. The majority of stumps contained insufficient rings, but three suitable samples were collected and the site assigned the code DUBH

Loch Crocach lies 5 km east of Tongue, 5 km south of the northern coast of Scotland. During site reconnaissance in September 2001, a cutting was located through a valleyside mire at this site. A dense layer of pine stumps had been apparently excavated and subsequently used to form a sculpture. A number of pine stump sections had been discarded during the operation and four suitable samples were collected and the site assigned the code CROC. The site is well exposed and treeless today

Polla on Loch Eriboll - This sample site lies 12 km south of the town on Durness which lies on the north coast of Scotland. This treeless vallyside mire spreads just south-west of the A838, at the southern end of Loch Eribol. Two levels of pine stumps were observed. Peat cutting was clearly evident in the area.

Loch na Thull lies 25 km south-west of Durness, 5 km inland from the west coast of Scotland This treeless valleyside mire site is located between the road to Skerricha and Loch na Thull end. A scattered single layer of pine-stumps appear to have become exposed through peat cutting

Skerricha lies 25 km south-west of Durness, 5 km inland from the west coast of Scotland. This valleyside mire site is located at the north end of the loch. The site is treeless. A single layer of pine-stumps exposed through peat cutting was identified at this site.

Loch na Druim is about 8 km south of Durness on the edge of a sea loch at Eriboll. The Polla sample site is local to Loch na Druim, the other sites sampled for Holocene pine are further north and east or further south and west of, but in the same region. There were other sites too further south in Wester Ross and also further inland to the east of Sutherland, where Holocene pine was exposed in peat cuttings and sampled as part of the research.

The thesis reported on a number of interesting findings, one of which was that about 3,200 BC the climate was optimal for a large area of blanket peat in NW Sutherland to be rapidly colonised by Scots Pines. Tree ring growth rates indicate growing conditions deteriorated leading to episodes of Scots Pine mortality, although one of the trees lived for over 350 years. There was also good correlation of tree rings and tree ring chronological sequences proposed. The evidence for two layers of stumps suggests that on two occasions the climate was optimal at some sites for Scots Pines to colonise blanket. Although many sampling sites had minerogenic soils nearby, the thesis paid heed to the low pollen profile for Scots Pine in the region and to overcome the low distance for seed dispersal from Scot's Pines, typically 10's of metres and a maximum of 2 Km. An imaginative scenario was envisaged in the thesis of seed dispersal from a high mountain during an extreme wind gust. The radial growth on the in situ stumps indicated that the prevailing wind direction whilst the Scots Pines were growing on the blanket peat, was from the North.

To my mind the research into holocene pine stumps in blanket peat has identified a significant ecological event(s) and climate change, it raises some profound questions on how the nationally important Loch an Druim pollen stratigraphy actually is for Quaternary studies in the North of Scotalnd.

Holocene Scots Pine stumps, Rannoch Moor - 2008

The history of Scotland's native Pines, their genes and physical presence of mega and macro fossils does present problems to some paradigms of the Quaternary, notably the extent and coverage of glacial ice, colonisation history, palaeoclimate models and palaeoecology. The dating of Scots Pine and other tree remains over 11,000 years ago, high on a Swedish mountain, also challenges some Quaternary paradigms.

It is a fundamental problem with Science, the reputations, research funds, careers, legacy, egos and hubris, that have so often, stifled progress. The field of Archaeology is a perfect example of the need for accurate indicators of past climates and environments, to interpret and place sites and artefacts into context.