By Alistair Grant

PLANTING huge numbers of trees to mitigate climate change is “not always the best strategy”, a new study has found.

Experts at Stirling University and the James Hutton Institute analysed four locations in Scotland where birch trees were planted on heather moorland.

They found that, over decades, there was no net increase in ecosystem carbon storage, in which carbon dioxide is naturally removed from the atmosphere.

Instead the team – led by Dr Nina Friggens of the Faculty of Natural Sciences at Stirling – discovered that any gains were offset by a loss of carbon stored in the soil.

Dr Friggens said: “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.”

First Minister Nicola Sturgeon declared a “climate emergency” at the SNP conference last year, and has committed to reducing greenhouse gas emissions to net-zero by 2045.

The Scottish Government aims to plant 30 million trees a year by the end of this Parliament.

The moorland tree-planting experiments took place in the Grampians, Cairngorms and Glen Affric.

The sites allowed experts to assess the impact of tree planting on vegetation and soil carbon stocks, by comparing these experimental plots to adjacent control areas consisting of original heath vegetation.

Scientists measured soil respiration – the amount of carbon dioxide released from the soil to the atmosphere – at regular intervals during 2017 and 2018.

Meanwhile, soil cores were taken to record soil carbon stocks and tree carbon stocks were calculated using non-destructive metrics, including tree height and girth.

The study recorded a 58 per cent reduction in soil organic carbon stocks 12 years after the birch trees had been planted on the heather moorland.

Significantly, this decline was not compensated for by the gains in carbon contained in the growing trees.

It found that, 39 years after planting, the carbon sequestered into the trees offset the carbon lost from the soil – but there was no overall increase in ecosystem carbon stocks.

Dr Friggens said: “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.”

She added: “The climate emergency affects us all – and it is important that strategies implemented to mitigate climate change – such as large-scale tree planting – are robust and achieve the intended outcomes.

“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.”

Dr Ruth Mitchell, a researcher within the James Hutton Institute’s ecological sciences department and co-author of the study, said: “Our work shows that tree planting locations need to be carefully sited, taking into account soil conditions, otherwise the tree planting will not result in the desired increase in carbon storage and climate change mitigation.”