Bog wood: the Caledonian forest and climate change

Exposed bogwood on the shores of the channel between Loch Bad an Sgalaig and Dubh Loch, near Gairloch

Anyone who has wandered the Scottish hills must have at some point been perplexed by the presence of bog wood, preserved tree stumps with radiating roots that protrude from the peat in what appear nowadays to be the most unlikely of places to find a tree. They can most often be observed where flowing water has eroded banks of peat, revealing the bog wood, the wizened, bleached tentacles of giant fossilised star fish. Some of the best examples are to be found on the edges of dammed lochs, as in the photo above, where the rise and fall of the water level gradually washes away the peat, leaving the bog wood trunks clearly exposed and undamaged, poised, spider-like on the gravel, a memorial to the great Caledonian Forest.


I recently read Emma Wood's 'Peatbogs, Plague and Potatoes', an excellent book that traces the evolution of the landscape we see around us today and provides a particularly good account of the chronology of afforestation and deforestation since the end of the last ice age. She concludes that climate change rather than human activity was responsible for the decline of the great forests that once cloaked the Scottish landscape.


Over the years I have heard many theories regarding about how Scotland transformed from forest to bog: that the trees had been chopped down by farmers; eaten by over stocked sheep and deer; consumed by wood hungry industrial activity such as shipbuilding or iron smelting. Doubtless all of those causes have played a part at various points in history and in the present day, but when evidence from pollen samples preserved in loch sediments are considered alongside indicators of past climate such as Greenland ice cores, the evidence points to the main culprit being the worsening climate. The loss of tree cover from areas with little or no human intervention, such as Rannoch Moor, supports this version of events.


The age of the landscape that we see around us today is measured not, like the age of the underlying rocks, in millions or even billions of years, but in mere thousands.  The individual landforms that make up this landscape, the glacier sculpted hills, the mounds of meltwater transported rock and gravel, did not exist until the ice receded at the end of the last ice age around 11,000 years ago.

Though the landforms themselves have maintained broadly the same shape since the ice melted, they have appeared in various guises. For a couple of hundred years after the ice melted the landscape would have been of bare tundra, home to mammoths, woolly rhinoceros, elk, wild horses, reindeer, Arctic fox and lemmings. Once soil formed from the glacial till, around 11,000 years ago, trees began to colonise the land, obscuring the landforms. At first there were tundra plants: willow, juniper and rowan. Then followed birch and hazel. Finally, 8 - 9 thousand years ago came elm, oak and Scots pine. As the climate continued to warm, this mixed forest cover reached up the flanks of the mountains. Of the tundra animals only the elk adapted, but forest animals walked over the land bridge that connected Britain to continental Europe to colonise this newly formed habitat;  red and roe deer, a massive bovine called an auroch, wild boar, badgers, squirrels, otter, beaver, lynx, bear and wolves. In short, Scotland was at least as diverse an environment as the contemporary north American wilderness, literally teeming with animal life.

The animals shared the landscape with a small population of Mesolithic hunter gatherers and it is tempting for me at least, to think of this as a semi idyllic period in which there was nothing but satisfying outdoor activity, food would have been plentiful, though the risk of predation by large animals would have been very real.

Idyllic or not, this period of a small population living in harmony with the environment was relatively short lived. By five thousand years ago Scotland had moved into the Neolithic, most of the population were at least part time farmers and had started to exert their influence over the environment, clearing trees for fields. There were perhaps only a few thousand of them, so the cleared trees were able to regenerate, until the climate took
a turn for the worse. Conditions had been getting warmer, reaching an optimum around six thousand years ago. This warming was also melting the polar ice caps and around this time the sea levels rose sufficiently to make Britain was an island, and by five thousand years or so ago the conditions were warm and dry enough to be unfavourable to large stands of trees. Meanwhile agriculture was becoming more established and Neolithic monument building was nearing its peak.

Between 4 and 2.5 thousand years ago, perhaps abetted by the cooling sulphate particles from an eruption of Iceland's Hekla volcano, the climate deteriorated. Temperatures dropped and rainfall increased, possibly an increased maritime influence on the climate as sea levels rose,  making an island and reducing the size of the landmass. The rain leached nutrients from the soil, resulting in a loss of fertility. The treeline fell as heather moorland and blanket bog displaced the forests. In 'The Scots: A Genetic Journey' Moffat and Wilson go so far as to suggest, based on evidence from studies of population genetics, that the harsh environmental conditions following the Hekla eruption in 1159 BC may have resulted in a near complete, though temporary, depopulation of western Scotland.

The climate did warm in the 1st century, but by then the damage was done. Trees couldn't recolonise the moor and bog, so forest cover was restricted to favourable pockets. There were further cold snaps between the 6th and 8th centuries and again in the 9th, just before the start of the Medieval Warm Period. The Medieval Warm Period saw a return to upland cereal growing, with areas that had not been cultivated since the peak of the Neolithic falling under the blades of iron ploughs. This same warm period saw the Norse expansion into Iceland, Greenland and beyond into North America.

It was the Norse colonies that suffered first when the medieval warm period drew to a close in the late 13th century and by 1310 the Little Ice Age was really biting in Scotland. The climate was somewhat warmer in the first half of the 16th century, but harsh, Little Ice Age, conditions, continued to affect life in Scotland intermittently until 1900. As was the case in 1159 BC, cold spikes were compounded by the cooling effect of sulphate particles from volcanic eruptions - Iceland's Laki in 1783 and Indonesia's Tambora in 1815 - with disastrous consequences for the human population.

It is often the case that gaining a little knowledge merely serves to highlight vast swathes of previously unrecognised ignorance. Learning how a few thousand years of climate history have exerted such profound changes in my immediate environment has awakened in me a great desire to find out more about the preceding billions of years of climate and the diverse landscapes and environments of the past.

Assisting me on this journey will be two recent discoveries, 'The Goldilocks Planet' by Jan Zalasiewicz and Mark Williams, subtitled 'the four billion year story of Earth's climate' and the thoroughly excellent Bits of Science blog, a great source for summaries and interpretation of recently published climate science.  Knowledge of past climates and of the factors affecting them is evolving so rapidly that books inevitably struggle to keep pace. In my opinion such studies of past climate are the most important science of our time, for it is only through understanding the past that we may predict and prepare for the climates of the future.

A brief history of Scotland up to 1000 BC

 Ice age Earth at glacial maximum. Based on: "Ice age terrestrial carbon changes revisited" by Thomas J. Crowley (Global Biogeochemical Cycles, Vol. 9, 1995, pp. 377-389

I spent part of the Christmas break reading 'The Scots: A Genetic Journey' by Alistair Moffat and James F. Wilson. In this excellent book Moffat and Wilson review the recent work of population geneticists in light of the fragments of archeological evidence and, from the ancient period onwards, in terms of historical and literary evidence. What follows is a distillation of some of the notes I took while reading the book and a few snippets that I have gathered from elsewhere. If you see anything you disagree with please leave a comment and set me straight. 

The peopling of the world in general and of Scotland in particular is a subject that has long fascinated me. As a result I have previously been disappointed with history books. They  tend to focus almost exclusively on comparatively recent history, on battles and kings, running out of steam well before they start to discuss the period that I am most interested in, the mesolithic. This is entirely understandable for the hunter gatherers of the mesolithic did not many durable artefacts for us to discover. All has changed in recent years, for not only have several concrete traces of the passing of the shadowy peoples of the mesolithic been uncovered, their genetic legacy has been revealed using modern DNA analysis techniques. 

The earliest human remains that I am aware of having been discovered in the British Isles are bone and teeth from the dizzyingly early date of around 500,000 BC (Boxgrove, Sussex).  These were not modern humans like us, homo sapiens, rather they were left by earlier homonins of the species homo heidelbergensis. Flint fragments have been discovered from even earlier, 700,000 BC (Pakefield, Suffolk), close to three quarters of a million years ago. The occupants immediately prior to ourselves, homo neanderthalis, are believed to have reached the British Isles, or the northern tip of continental Europe as it was then, around  100,000 years ago, leaving behind evidence for archeologists to uncover in the south of England. In recent years the popular image of neanderthals has greatly improved; they were formerly believed to be unthinking brutes, now we credit them with culture and technology as sophisticated as that of our own distant ancestors, and by inference with minds as creative as our own. Indeed until recently the mainstream view was that the people of prehistoric times were little more advanced than apes. I would not be surprised if in the coming years we also revise our views of earlier homonins and begin to ascribe to them at least some of the traits that we used to jealously reserve for our own kind. 

Many years ago I visited Olduvai Gorge in Tanzania (now re-named Oldupai Gorge), a site that has been associated with a number of significant archaeological finds. What struck me most as I walked round the small museum on the site was a stone chopping tool dated to around two million years old, reputed to be the oldest human artefact in the world. Even more spectacular was the discovery of the fossilised footprints of a band of early humans which have been dated to 3.75 million years old. As I considered these finds I realised that we are at leading edge of literally millions of years of human history, and that we are in the privileged position of being among the first generations to have any real concept just how much has gone before.

Against this backdrop of millions of years - thousands of millennia - the history of our own species is but the blinking of an eye. The earliest modern human of our own species, homo sapiens, appears in the fossil record 150,000 years ago, and all humans currently alive can trace their ancestry to a small population who lived in Africa around 70,000 BC. Around this time it is believed that a catastrophic event almost wiped homo sapiens from the surface of the planet.  The massive amount of ash ejected by the eruption of the Toba super volcano in Indonesia led to between six and ten harsh volcanic winters.   A small number of our ancestors, perhaps only a few thousand, managed to cling on to life in the Rift Valley and it is from these people that all  modern humans are descended. A small band of these survivors embarked on a journey which would eventually result in the world being fully populated by modern humans. They walked north and sailed across the Red Sea to arrive in Arabia around 60,000 BC. There is some evidence that they mated with neanderthals prior to their dispersal along the Indian Ocean coast. Their eastwards migration was extremely rapid, taking only around 2,000 years to reach New Guinea and Australia. Hence the culture of the Australian Aboriginals, of which the early white settlers were so contemptuous, was 58,000 years old, ten times the biblical age of the earth.

In light of the very rapid eastwards migration it seems surprising that it took these early people 20,000 years, until around 40,000 BC, to get from Arabia to Europe. Particularly so when some evidence suggests that they were also established in North America more than 50,000 years ago. The reasons for these contrasting rates of colonisation are likely climatic. From around 120,000 years ago the world was in the grip of an ice age, with ice sheets covering most of what is now the UK and extending down into northern Europe. So much of the earth's water was locked up in the great ice sheets that sea levels were much lower than they are today and it was possible for early people to walk over a land bridge from Siberia to Alaska from where they spread southwards to colonise the Americas. 

A similar land bridge connected what is now Britain with continental Europe and was used by neanderthals to access the southern margins of Britain. The neanderthals were hardier than us, adapted to a cold climate. Homo sapiens spent the last ice age sheltering in what are termed the Western Refuges in southern France, where they left striking evidence of their occupation in the renowned cave paintings of Lascaux (15,000 BC) and Chauvet  (25,000 BC). 

The climate warmed and the ice began to retreat around 15,000 BC. As soon as vegetation established itself animal herds would have seek out the fresh grazing and people would have followed them. By 12,000 BC the southern Hebrides were clear of ice and we can infer from the discovery on the Island of Islay of Ahrensburgian arrowheads dated to 10,800 BC that people journeyed there by boat, most likely for the purpose of hunting, perhaps reindeer were their quarry. This was no isolated expedition; similar artefacts have been found on Tiree, Jura and Orkney.  

I was particularly impressed by this finding, for it shows that humans visited Scotland prior to the final part of the Ice Age, the Loch Lomond Readvance. The Loch Lomond Stadial started around 9,400 BC when the collapse of massive ice dams in Canada resulted in a discharge of fresh water sufficiently large to stop the Atlantic Conveyor in its tracks, shutting off the warming influence of the Gulf Stream and plunging northern Europe back into the grip of the ice. When the conveyor re-established itself the warming was rapid, occurring over perhaps only over a single decade. To a large extent the glaciers of the Loch Lomond Readvance and the catastrophic floods resulting from their melting defined the shape of the landscape we see around us today, carving the coires and steep-sided glens of the west coast mountains and transporting vast quantities of material - sand, gravel and boulders - and depositing them in glacial landforms.

The first evidence of post-glacial habitation are flint tools and hazlenuts found at Cramond in Edinburgh, dated to 8,600 - 8,200 BC, co-incident with the beginning of the current ice-free period, the Holocene interglacial. Moffat and Wilson speculate that sufficient resources may have existed at this site to sustain year-round living, though the people lived in relatively portable shelters of wood and animal hide. Not all shelters from this period were like this, some were much more substantial such as that discovered at  East Barns near Dunbar and dated to 8,000 BC. These people may have lived primarily through hunting and gathering but there is some evidence from this period that hints at the existane of the existence of a mature trading economy, such as the antler processing facility at Starr Carr in Derbyshire (8700 BC) where the sawn-off antler masks, often sported by the presenters of television programmes about prehistory, were found. These early people left their mark on the nation's DNA, bringing with them from the Western Refuges three male Y chromosome markers that together are today found in around around 5 % of the modern population of Scotland.

While the inhabitants of Scotland were enjoying what may well have been an idyllic egalitarian life of boating, bothying and plentiful leisure time in a bountiful environment from which all their needs may have been satisfied, elsewhere the technology that was to change the world was being developed. The accepted date for the development of agriculture in the Fertile Crescent (the present day Middle East) is around 9,000 BC. This marked the start of the neolithic period and the beginning of the end for the mesolithic. The first trace of the arrival of such methods in Scotland is from 3,900 BC in Balbridie in Aberdeenshire, where the remains of great halls designed to house both humans and their livestock were uncovered. It is believed that agricultural methods were brought wholesale rather than piecemeal; stock, tools and seeds were all brought by skilled agriculturalists across the North Sea at the same time, rising sea levels having made Britain an island by this time.  DNA analysis reveals that around 70 % of Scots men are descended from these early farmers, from immigrants whose DNA markers originated in the Near East less than 10,000 years ago. The reason that the farmers came to dominate over hunter gatherers is likely a consequence of the new foods available to them. Dairy products and cereal crops would have decreased weaning time and therefore increased birthrate - lactating women find it harder to conceive.  Some studies have shown that early agriculturalists were smaller than hunter gatherers of the same period, suggesting that they may have been disadvantaged nutritionally. Even so their higher birthrate would still have allowed them to outbreed mesolithic peoples.

We saw above how the climatic deterioration that followed the eruption of the Toba super volcano was so decisive in shaping human history. A similar though lesser event occurred in 1159 BC when Iceland's Hekla volcano erupted. This is the big brother of Eyafjallajokul, the volcano whose eruption which caused so much travel disruption in 2010. This eruption occurred in a climate that had already begun to deteriorate from a climatic optimum between 2,000 and 1,500 BC when summer temperatures were around 3 C higher than today. The land had been deforested for agriculture by humans and this, combined with the wetter and cooler climate, led to the spread of blanket peat. The volcanic winter that followed the Hekla eruption is likely to have been the final straw that led to depopulation of Western Scotland apart from a small number of more favourable refuges. Moffat and Wilson propose that the entire western seaboard was depopulated and once the volcanic winter ended, it was repopulated by peoples from Ireland. Hence from the 1st millennium BC, both landscape and language - peat and Gaelic - were set and similar to that which persisted up to Culloden.


Edit 05/05/12. This week's Nature has a lot to say on the topic of the peopling of the globe. Most of the earlier assertions quoted above are the subject of active debate between the archeologists and the population geneticists. 

Prehistoric bothy trips

Kiloran Bay, Colonsay. Archaeologists have discovered evidence of large scale hazlenut processing by mesolithic hunter-gatherers

I've always been more interested in prehistory than in history. Up until quite recently there has been very little reading matter on the topic, however  recent advances in science are allowing us to build up a far more complete picture of our ancestors and the world they inhabited.

The good old BBC is currently dishing up two excellent programmes in this area. Long-haired historian Neil Oliver is half way through his BBC2 series 'A History of Ancient Britain', while on Radio Scotland Alistair Moffat is fronting  'The Scots: A Genetic Journey'.

In the year and a half since I started this blog I've been getting my head around the geology of Scotland, so the timescales of human interaction with our landscape are staggeringly short in comparison. 

70,000 years ago: modern humans leave Africa

50,000 years ago: modern humans colonise Europe

11,000 years ago: end of ice age allows Mesolithic humans to colonise Britain

6,000 years ago: the New Stone Age (Neolithic period) reaches Northern Europe

4,000 years ago: Bronze Age

3,000 years ago: Iron Age, arrival of Gaelic culture in Scotland

I love the way that considering such long periods of time emphasises our position at the leading edge of the human journey. We are the first generation able to accurately assess our position in the scheme of things, able to consider what lies ahead in the context of what has already passed.

Neil Oliver visited the Hebrides; Col and Tiree; Colonsay; Islay and Jura. He painted a picture of mesolithic hunter gatherers sailing between these islands, harvesting the seasonal bounties of the land.  Alistair Moffat visited Scotland's oldest house, a few postholes on a site near Torness dating from 9,500 years ago. So the experts are telling us that Scotland's earliest inhabitants  were people who migrated regularly between sites on which they would have had either natural shelters or bothies.

Perhaps prehistoric bothies looked something like this reconstruction at Abriachan

I'm not really one for romanticising the past, but this mesolithic lifestyle doesn't look at all bad. Indeed it appears to have consisted wholly of the elements that us modern men use to counterbalance our stress-filled lifestyles of indoor work and consumerism: boating, bothying, fishing, fires and eating.