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.