TREE planting is one of the accepted methods of tackling climate change, but now Scottish scientists have found evidence that it might not actually help to reduce carbon in the atmosphere.

Experts at the University of Stirling and James Hutton Institute looked at four Scottish locations where birch trees were planted onto heather moorland – and found there was no net increase in ecosystem carbon storage over the decades.

The team found any increase to carbon storage in tree biomass was offset by a loss of carbon stored in the soil.

Dr Nina Friggens of the Faculty of Natural Sciences, who led the team, said while the Scottish and UK governments have vowed to plant huge numbers of trees to address climate change, it might not be the best way forward.

The Scottish Government currently aims to plant 15,000 hectares of trees annually by 2025.

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Friggens (above) explained: “Both national and international governments have committed to plant huge numbers of trees to mitigate climate change, based on the simple logic that trees – when they photosynthesise and grow – remove carbon from the atmosphere and lock it into their biomass. However, trees also interact with carbon in soil, where much more carbon is found than in plants.

“Our study considered whether planting native trees on heather moorlands, with large soil carbon stores, would result in net carbon sequestration – and, significantly, we found that over a period of 39 years, it did not.”

The tree-planting experiments were set up by the late Dr John Milnes of the Institute of Terrestrial Ecology in 1980 and Hutton Institute in 2005. The sites helped the team assess the impact of tree planting on vegetation and soil carbon stocks by comparing them to adjacent control plots featuring original heath vegetation.

They found that 39 years after planting, the carbons sequestered into tree biomass offset the carbon lost from the soil, and there was no overall increase in ecosystem carbon stocks.

Friggens went on: “When considering the carbon stocks both above and below ground together, planting trees onto heather moorlands did not lead to an increase in net ecosystem carbon stocks 12 or 39 years after planting. This is because planting trees also accelerated the rate at which soil organisms work to decompose organic matter in the soil – in turn, releasing carbon dioxide back into the atmosphere.

“This work provides evidence that planting trees in some areas of Scotland will not lead to carbon sequestration for at least 40 years – and, if we are to successfully manage our landscapes for carbon sequestration, planting trees is not always the best strategy.

“Tree planting can lead to carbon sequestration; however, our study highlights the need to understand where, in the landscape, this approach is best deployed in order to achieve maximum climate mitigation gains.”

The study’s findings will be relevant around the world as nations try to address the issues caused by climate change. Friggens said it’s important the methods used to tackle the crisis “are robust and achieve the intended outcomes”.

She went on: “Changes to carbon storage – both above and below ground – must be better quantified and understood before we can be assured that large-scale tree planting will have the intended policy and climate outcomes.”