Tree planting on grassland may cut soil carbon, scientists warn farmers

Farmers converting long-established grassland into woodland under carbon schemes may not be delivering the climate gains widely assumed, new research suggests.

Scientists have warned that planting trees on pasture could trigger significant losses of soil carbon — potentially reducing the overall climate benefit and raising important questions for landowners weighing up woodland creation grants and carbon credit income.

The findings, led by researchers at the University of Stirling, suggest that while trees absorb carbon above ground, soils beneath some forests may lose substantial stored carbon over time — a factor not always fully reflected in carbon accounting schemes.

Professor Jens-Arne Subke said policymakers and land managers must look more closely at what happens below ground.

“Our findings emphasised that we cannot over-rely on forests to mitigate the impacts of climate change because there is still so much that we don’t understand,” he said.

“Despite accumulating tree biomass, we may be losing carbon capital – the carbon stored long-term in soils and ecosystems – to the atmosphere.”

The warning builds on research examining pine plantations in the Scottish Lowlands, where trees had been planted on former long-term grassland.

Scientists took soil samples from 16 sites, some dating back nearly 70 years, to assess both carbon levels and how stable that carbon remained over time.

The results showed that soils beneath mature pine forests contained around half as much carbon as neighbouring land that had remained as grassland.

Researchers also calculated that the carbon lost from the soil was equivalent to roughly a third of the carbon the trees had absorbed from the atmosphere — significantly reducing the net climate benefit.

In addition, the carbon that remained in forest soils was found to be less stable, meaning it could break down and be released more easily in future.

The work feeds into a newly published commentary in Global Change Biology, co-authored with Dr Thomas Parker of the James Hutton Institute, which highlights wider evidence that deep forest soils may be less reliable long-term carbon stores than previously assumed.

For farm businesses, the findings are particularly relevant as tree planting forms a key part of UK net zero policy, with financial incentives linked to schemes such as the Woodland Carbon Code.

Dr François-Xavier Joly of the French Institute for Agriculture Research (INRAE), who led the Scottish study within Professor Subke’s research team, said carbon schemes must take soil impacts into account.

“There are important financial incentives for landowners to plant more trees; however, these are linked to presumed benefits brought by a change in vegetation towards forests,” he said.

“Our research has added an important aspect to these schemes by clarifying the consequences of tree planting within the soil.”

He added that the Woodland Carbon Code and similar programmes “must take account of potential soil losses”.

Dr Parker stressed that forests remain essential for biodiversity, water management and rural economies, but cautioned against viewing them as a simple solution.

“Forests are an essential for human and planetary well-being for a range of reasons, but we need to acknowledge that they are not a silver bullet for all our problems,” he said.

“There are complexities and trade-offs that need to be understood to maximise the net benefits that we gain from forests.”

Dr Mike Perks, climate change scientist at Forest Research, said further work was needed to understand how soil type, tree species, root systems and soil depth influence long-term carbon storage.

“More research is needed to better understand carbon storage in soil,” he said.

The research involved collaboration with Colorado State University and Forest Research and was funded through a joint grant from the National Environment Research Council and the US National Science Foundation.

While the findings do not suggest tree planting should stop, they indicate that farm businesses considering converting grassland into woodland may need to weigh soil carbon impacts alongside grant income, biodiversity gains and long-term land use plans.