A recent article in the Scottish press implied that planting trees may not tackle climate change. But the research paper on which the article is based is clear: the issue isn’t with tree planting overall, but with planting on carbon-rich soil. The press article failed to highlight that if the right trees are planted in the right place, they are effective against climate change.
What did the study show?
The new research by Friggens et al. is titled Tree planting in organic soils does not result in net carbon sequestration on decadal timescales. It shows that soils already rich in carbon – known as peaty soils in Scotland – released more carbon than the trees could absorb over the 40 year study period.
But it also provides strong evidence for establishing the right tree in the right place, for the right reasons. We’ve long advocated this at the Trust. The study shows that when deciding where to plant, it’s important to first consider the soil’s existing carbon levels. Landowners will already be aware of these levels based on their location, vegetation type and soil characteristics.
Trees lock up carbon as they grow, but carbon exchange also occurs in the soil. Carbon is added to the soil through plant litter and released by fungi and organisms known as decomposers. If trees are planted on soils already rich in organic carbon, it tips the balance so the soils release more carbon than the young trees can lock up over the coming decades.
Lead author of the research, Dr Nina Friggens, explains the implications. “Tree planting can increase carbon stocks in certain areas and ecological contexts,” she says. “But it is important to understand where in the landscape this approach is best deployed to achieve the best results for climate change mitigation.”
Planting in the right places
This is new evidence from the UK for a view long-held by conservationists and the Woodland Trust: there are places where it isn’t appropriate to expand woodland cover. Following lessons learnt in forestry, the UK Forestry Standard now prohibits planting trees on peat deeper than 50cm in the UK. In light of their research, the authors recommend that this should be extended to any soils with organic surface layers less than 50cm thick.
The UK has committed to reduce greenhouse gas emissions to net zero by 2050. That means we can’t afford to plant trees in soils that release more carbon than the tree can absorb in a generation. We need to plant them where they will be most effective.
Policy and professional guidance needs to improve
Tree policy in the UK is heavily focused on the carbon aspect of tree planting and woodland creation. But that focus neglects many of the other benefits trees offer. The value of our native woods for biodiversity is huge. The right trees in the right place also provide flood mitigation, clean air, timber and green spaces to enjoy for our health and wellbeing.
Experts agree that planting trees can only be a first step to creating a woodland ecosystem. To reap maximum rewards, we must focus not only on planting – or allowing trees to grow naturally – but also on the tree species and their future management.
As well as a climate emergency, we’re facing a nature emergency. Trees are one of the solutions to address these simultaneously. They are vital in tackling climate change and reversing biodiversity loss.
We need landscapes rich in native woods, trees and wildlife. Government policy must go further in committing to plant, protect and restore our woods and trees for the decades to come.
Forestry guidance on what is acceptable needs tightening further too. This latest research shows that when deciding where to establish trees, the carbon below ground must be considered for the trees to have a positive effect on carbon emissions. We need better advice on which trees to plant where, and how. The right trees in the right place are crucial for the future of people, climate and nature.
Karen Hornigold, Conservation evidence officer at the Woodland Trust
This article was first published as a Woodland Trust blog on 27 July 2020
Image by Sandra Graham