This article appears as part of the Winds of Change newsletter
The question of trees and Net Zero is often a thorny one. Which trees, where? Is woodland regeneration better than planting? And, more importantly, is the whole thing part of a carbon credit greenwash that is encouraging us to think we are doing something while we go on burning fossil fuels> Meanwhile chewing up Scotland’s land market?
A paper published this week is another reminder of the fact that the trees and carbon story is not always a simple Net Zero boon. Scientists from The James Hutton Institute warned that initiatives that promote “self-establishing trees” - essentially woodland spread via self-seeding - - “may not always lead to carbon capture at the decadal timescales relevant to achieving net zero and mitigating climate change”.
Trees take longer than that to make a difference. Their time frame doesn't bend to the kind of short spans we humans think of as the long term of politics, international agreements and climate targets.
Studies like these serve as a reminder of the slowness of carbon removal by growing forestry - what's burned in a few moments, is only absorbed back decades down the line - and also that in focussing chiefly on trees, we have failed to properly emphasise something else: soil carbon, and more crucially, the possibility of its release from peaty Scottish soil.
There are many reasons to value trees. But this story that we have heard most, of trees and carbon offset, and of how planting millions might rescue us, puts trees too much front and centre of a carbon issue that is much more complex than it is often portrayed.
This latest piece of research by The James Hutton Institute, in collaboration with the University of Edinburgh and Forest Research, looked at what happened on heather moorland, when trees established from parent trees via seedfall, without human intervention.
After 25 years, the carbon captured in the small, sparsely growing trees did not exceed the carbon lost from the soil, meaning no carbon was captured following the colonisation of these previously unforested ecosystems by trees.
Why choose 25 years? Because that quarter century is significant. It takes us to the 2050 deadline of the Paris Climate Agreement, and beyond Scotland’s pledge to go Net Zero by 2045, partly by increasing tree cover.
It follows another study by The James Hutton Institute in collaboration with the University of Stirling, which found that planting birch and Scots pine trees in heather moorland ecosystems with carbon-rich soils was linked to soil carbon losses that were similar to the amount of carbon captured in the trees. In other words, “overall, no net carbon was captured in the first few decades following tree planting”.
So here we have two studies. One telling us that tree planting, when done in carbon-rich soil may not result in the benefits sometimes assumed within the time frame of the 2050 Net Zero project. The other tells us that even rewilding, or regeneration, also may not.
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There is, as Professor Mark Reed, one of the architects of the Peatland Code carbon credit system, told me, almost as much carbon stored in the UK’s peat soils as there is in all our forests and the forests of France and Germany combined.
The carbon builds up because the soils are waterlogged, so the biggest threat to this carbon store is the peat drying out. When trees colonise peat soils, their roots dry out the peat and the soil loses carbon, so much so that it cancels out the benefits of carbon sequestered by the trees. It is therefore not permitted, by law, to plant trees in deep peat.
The Scottish Government is, of course, trying to tackle some of this and has invested £250 million in peatland restoration over ten years. 10,000 hectares of peatland were restored between 2023 and 2024.
That said, trees can have a Net Zero role in the long run, particularly if we look beyond 2050. They also perform certain other important roles in our landscapes.
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As Naomi Housego, postgraduate researcher at the Hutton, who led the latest study, stressed: “Trees can also be established for other benefits such as increased biodiversity, flood alleviation, or recreation. Given that overall we found neither carbon gains nor losses, natural colonisation could still provide an approach for increasing tree cover that provides these ecosystem services.”
We can continue to celebrate trees, and what they do for us, but these studies are a reminder that, for all the big noise there is around tree planting and regeneration, a bigger element of our Net Zero story is really soil and, in particular, peat.
But they also tell us that our priority should be to reduce emissions from the burning of fossil fuels as rapidly as possible - rather than pin our hopes on some possible future trees to draw it back down.
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