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Annual mean temperature in the Scottish Highlands has increased by about 0.7°C since 1800.

Caledonian forests threatened by climatic change

Efforts to conserve the remaining fragments of the great Forest of Caledon in Scotland may be doomed to fail unless a new strategy is rapidly adopted, new research suggests.

These forest fragments are characterised by the dominance of Scots pine and are a unique feature of the Scottish Highlands.

As a result they are categorised as a “priority habitat” under the EU Habitats and Species Directive, and ten of the remaining areas have been designated by Scottish Natural Heritage as Special Areas of Conservation.

Using a computer modelling approach, researchers showed there is an imbalance between the present, warmer, climate and the dominance of Scots pine in these forest fragments.

The present climate is more favourable for the growth of oak trees at these sites, so if the forests were to be disturbed or destroyed – perhaps by fire or disease – they would be replaced by oak woodlands, the researchers found.

“Trees grow slowly, so ancient forests can be slow to adapt to rapidly changing conditions,” said co-author Dr Jon Bennie, of the University of Exeter.

“This research shows that centuries of forest clearance and a warming climate since the start of the industrial revolution has already left some of the last remaining patches of Caledonian pine forest stranded out of kilter with their environment in the Scottish Highlands.

“As groves of Scots pine come to the end of their lives we expect them to be replaced by oak woodland even in places where pine has dominated in the past – with implications for the many plant and animal species that are associated with old-growth pine forest. 

“Conservationists concerned with preserving the unique biodiversity of Caledonian pine forest are going to need to take a landscape-scale view in the future, encouraging new Scots pine forest to establish in new areas in cooler parts of the landscape as existing pine forests change towards a mixed oak woodland.

“This is an important example of the challenges and dilemmas that conservationists face in conserving biodiversity and valuable landscapes for future generations under a rapidly changing climate.”

The research team was led by Professor Brian Huntley of Durham University, with colleagues from the universities of Exeter, York and Lund.

Scots pine trees came to dominate these forests under the cooler conditions that predated the Industrial Revolution, with many having become established more than 200 years ago.

Once established the Scots pine was able to hold off the threat from other trees, such as the oak, which are better adapted to warmer conditions.

As humans have rapidly added carbon dioxide to the atmosphere through industrialisation, the annual mean temperature in the Scottish Highlands has increased by about 0.7°C since 1800, with larger increases during the winter months.

According to the researchers this means a “climatic debt” has been accumulated – which means that if the Scots pines die or are destroyed, they will be replaced by oak trees more suited to today’s higher temperatures.

Professor Huntley, in Durham University’s Department of Biosciences, said: “The climatic debt will be repaid when the pines forming the present forest canopy die or are killed, when oaks and other trees better adapted to the climatic conditions now prevailing will replace them.

“Such an outcome is inevitable; the only question is how soon it will come about, and that is mainly a matter of chance, and hence unpredictable. A forest fire could happen this year, or may not occur for decades.

“Pathogen outbreaks are similarly unpredictable, as past experience with Dutch Elm Disease and the recent outbreak of Ash Dieback Disease have shown.”

Dr Andrew Suggitt, formerly of the University of Exeter but now of the University of York, said: “Our study suggests that our much-loved Caledonian pine forests are threatened by climate warming, and that the current protection they receive may not be enough in a warmer world.

“We will therefore need to be much more proactive about where we designate nature reserves to ensure that priority species can cope with warming.

“This will feed in to the wider, ongoing debate on the future of the Scottish Uplands which I hope the whole country engages with.”

The researchers recommend that sites that are now, and will be in future, more favourable for the growth of Scots pine than for oaks need to be identified and conserved.

The results presented by Professor Huntley’s team show that such sites exist in the Highland landscapes that they examined. However, these sites are today mostly treeless moorlands because of their management for red grouse, red deer or sheep.

Although the team’s work focused upon Caledonian pinewoods, their results are of much more general, global, significance.

All forest ecosystems, and many other ecosystems globally, are dominated by long-lived perennial plants.

As a result, given the slow responses of such plants to climatic changes, many ecosystems globally are likely to have accumulated similar climatic debts to those of the remaining Caledonian pinewood fragments.

Conserving these ecosystems, and their component species, requires strategies that seek to identify sites that today, and in the future, will be climatically suitable for these ecosystems, the researchers said.

The research, published in the journal Conservation Letters, was funded by the Natural Environment Research Council (NERC).

Date: 2 March 2017

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