We tend to think of our forests as unchanging. The old trees have always been there and will always be. In Scotland, we take for granted the towering giants of redwoods, Douglas fir and Scots pine, and gnarled ancient oaks of the unique temperate rain forest.
And yet forests have constantly been changing. Land clearances and the industrial revolution of the 18th and 19th Centuries reduced the forest cover in Scotland to about 5%, nearly a third of what it is now. Since the early 20th Century, Scotland has been increasing its tree cover, but at the cost of mainly concentrating on timber production. Planted with Sitka spruce trees, the forests are now under threat of a combined onslaught by climate change and invasive pests.
The forests are changing now globally, probably at the fastest pace ever, with deforestation caused by logging, land use and climate change, and pests and diseases. Satellite imaging, provided by Landsat, can be used to estimate forest cover and, subsequently, forest loss over large areas. Looking at the red points on the map below – marking high forest loss – we can identify the main areas of global concern – tropical forests of South America, South-East Asia, North-East Canada and Siberia.
The loss of trees is also happening on a smaller scale in many regions, including Europe. Ash dieback has ravaged ash trees, starting in Poland and Lithuania and getting as far as Scotland in the mid-2010s. Emerald Ash Borer has spread through the United States and is now approaching the European Union countries from its new base in Russia and Ukraine.
Spruce trees are fast disappearing in Central Europe, Germany, Poland, Czechia and Slovakia. Although Scotland has so far been spared, there are lessons that we can learn from the recent outbreaks of the eight toothed bark beetle, Ips typographus, in the Carpathian region.
A story of Norway spruce in Southern Poland
As a family, we spent summer holidays in that region during our annual visits to the country of my birth. The landscape has constantly reminded me of Scottish Borders – with rolling hills and deep valleys, but with a much denser wood cover, mainly Norway – or European – spruce (Picea abies), and with mountain pine and grass meadows higher up interspersed with stone outcrops. Walking underneath the tall, majestic spruce trees has always given me a very special feeling. There are also mixed forest areas with old larch, beech and oak trees, and rich understory and plenty of opportunities for mushroom picking.
I remember very well when in 2009 we went for a hike in the Barania Góra (Ram’s Head) mountain region. Very quickly we noticed that there is something not right about the forest health. As we climbed higher, more extensive areas were covered in dead trees, their stumps growing out of grass and shrubs. A noticeboard explained that hot summers had caused the trees to weaken, and the bark beetle attacks followed.
A comparison of Google earth pictures from 1985 and 2013 (shown below) clearly indicates dark areas of a dense spruce forest becoming light green, illustrating the scale of the damage. Unfortunately, there are no Google Earth pictures of this area between 1985 and 2013; the primary beetle infestation occurred between 2006 and 2010.
History of spruce forests
But why were there so many spruce trees there in the first place? Not too dissimilar to Scotland, this region of Poland experienced rapid industrial growth in 19C. Original tree cover, a mixed forest of pine, beech and spruce, gave way to spruce plantations. The beechwood was valued for charcoal, used in iron and steel production – with a large iron smelting factory nearby.
The forests were then replanted with spruce – highly valued as timber for protecting coal mines from collapsing. A low-quality seed was used to keep the costs low, producing forests that looked impressive but hid a weakness of monoculture (68-97% of spruce even as late as early 21C) and susceptibility to drought.
Due to different political factors, the forests were left to grow long past their optimal rotation length, some parts reaching well over 100 years of age. Then came the droughts. The early 2000s were characterised by low rainfall, rarely reaching the long-term average. And then, the bark beetle delivered the death knoll, with large outbreaks in 2007-2010 and then again from 2015.
The death of the spruce forest was rapid compared to the length of life of a tree; it really took a couple of years from 2006 to devastate the whole area. The decisions taken 100-150 years ago did not consider the possibility of climate change and increased pest risk.
What now for Scottish spruce forests
We are facing similar decisions in the 21st Century. In 2017, 94% of the Scottish timber production was softwood, primarily sourced from spruce plantations. The plantations of Sitka spruce are covering over 500 thousand hectares, nearly 7% of the total area of Scotland, support Scottish forestry employing over 25,000 people and contributing almost £1 billion Gross Value Added. With a high and growing price of timber, a commercially sensible decision is to continue expanding what suits best. It is easy to criticise people living 100-200 years ago, but the key question for us is, can we do better?
It is not yet clear how much Ips typographus is of an immediate risk to Scottish spruce forests. Plant Health Risk Register assigns the highest possible risk rating (125), although includes a lower likelihood of arrival and impact when mitigation – quarantine and monitoring – is implemented. In addition, it is not yet clear how Sitka spruce (as opposed to Norway spruce) is affected by Ips.
Scottish weather might (paradoxically) be the most significant mitigating factor – the pest only develops at temperatures exceeding 10°C; flights take place above 15 °C, but mass swarming has only been recorded when the daily maximum temperature exceeds 20°C. However, with climate change, this protection might not be applied anymore in the nearest future and even more in the long run.
As the Plant Health Risk Register acknowledges, with over 40% of all forest in Scotland occupied by Sitka spruce, the risks are enormous.
The key message from the story I described above is that choices we make in planting trees do have long-term consequences and need to consider long-term uncertainties. Moreover, the decisions need to be taken against a background of accelerating climate change and an unprecedented increase in pest and pathogen arrivals into the UK. As the diagram below clearly demonstrates, any tree planted 50 years ago – to be harvested now – will have experienced many more threats than the ones planted a long time ago, as seen in the diagram below, including the first sightings of Ips typographus.
As the recent Woodland Trust report clearly states,
Just 7% of Britain’s native woodlands are currently in good ecological condition
State of the UK’s Woods and Trees 2021 provides clear evidence that there is an urgent need to act.State of UK’s Woods and Trees, Woodland Trust, 2021
we need to take the warning seriously and consider the future of our forests very carefully.
I am grateful to Prof. Jacek Kozak (UJ, Poland), Prof. Chris Quine (Forest Research) and Dr Glyn Jones (FERA) for the ongoing collaboration which helped in writing this article. The blog post was commissioned by the Plant Health Centre, Scotland.