Farmers converting long-established grasslands to forests under carbon plans may not be realizing widely envisioned climate change benefits, a new study suggests.
Scientists warn that planting trees on pastures can cause significant losses in soil carbon, potentially reducing overall climate benefits and raising important questions for landowners weighing carbon credit income against forestry subsidies.
The findings, led by researchers at the University of Stirling, suggest that while trees absorb carbon above ground, the soil beneath some forests can lose significant amounts of stored carbon over time. This factor is not always fully reflected in carbon accounting systems.
Professor Jens Arne Sabke said policy makers and land managers needed to look more closely at what was happening underground.
“Our findings highlight that we cannot rely too heavily on forests to mitigate the effects of climate change, as there is still much we do not understand,” he said.
“Despite the accumulation of tree biomass, we may be losing carbon capital – long-term stored carbon in soils and ecosystems – to the atmosphere.”
The warning is based on research examining lowland pine forests in Scotland. There, trees were once planted on grasslands for a long time.
Scientists took soil samples from 16 sites, some dating back nearly 70 years, to assess carbon levels and how stable that carbon remains over time.
They found that the soil beneath a mature pine forest contained about half as much carbon as the adjacent land that had remained as grassland.
The researchers also calculated that the carbon lost from the soil was equivalent to about a third of the carbon that trees absorbed from the atmosphere, significantly reducing the net benefit to the climate.
Additionally, carbon left in forest soils is found to be less stable and may be more susceptible to decomposition and release in the future.
The research is reflected in a new commentary co-authored with Dr Thomas Parker of the James Hutton Institute in the journal Global Change Biology, which highlights extensive evidence that soils deep in forests may be less reliable long-term carbon stores than previously assumed.
The findings are particularly important for agricultural businesses as tree planting forms a key part of the UK’s net zero policy and financial incentives are linked to schemes such as the Forest Carbon Code.
Dr François-Xavier Joly, from the French Research Institute for Agricultural Research (INRAE), who led the Scottish study within Professor Sabke’s research team, said carbon plans needed to take into account impacts on soil.
“There are important economic incentives for landowners to plant more trees, and these are linked to the putative benefits provided by vegetation conversion to forest,” he said.
“Our study adds an important dimension to these plans by revealing the effects of afforestation within the soil.”
He added that forest carbon codes and similar programs “need to take into account potential soil loss.”
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 essential to the well-being of humans and the planet for a variety of reasons, but we need to recognize that forests are not a silver bullet that will solve all problems,” he said.
“There are complexities and trade-offs that need to be understood to maximize the net benefits from forests.”
Forest Research climate change scientist Dr Mike Parks said further research was needed to understand how soil type, tree species, root systems and soil depth affect long-term carbon storage.
“Further research is needed to better understand carbon storage in soils,” he said.
The research involved collaboration with Colorado State University and the Forest Research Institute and was funded by joint grants from the National Environmental Research Council and the National Science Foundation.
The findings do not suggest that tree planting should be stopped, but it does indicate that farmers considering converting grasslands to forests may need to weigh soil carbon impacts alongside subsidy income, biodiversity capture and long-term land use planning.
