Increasingly trees are being promoted as a means to increase carbon storage and hence off-set climate change. However, it is critical to not only understand the gains in above-ground carbon (the carbon in the trees) but to also understand the impact of the trees on the carbon stored in the soil, and how these impacts differ between different soil types.
We reviewed recent literature on changes in carbon storage following woodland establishment. Studies across modelling work, experimental plots, and large-scale surveys indicated that in the initial decades following tree establishment soil organic carbon (SOC) increases following tree planting on soils with little SOC but declines on soils with a high starting SOC.
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Tree planting is among the most prominent of nature-based solutions to climate change because of its large potential to sequester carbon. Tree planting is a key policy to enable meeting the net-zero target of 2045 set by the Scottish Government. However, the effects of tree planting on soil carbon are unclear.
The assumption behind the policy of increased tree planting to mitigate climate change is that as tree biomass stores carbon, increased tree cover will lead to an increased amount of carbon being stored resulting in less carbon dioxide, a key greenhouse gas, being available in the atmosphere. Ultimately the assumption is that this will lead a reduction in climate change. However, this assumption ignores the amount of carbon that might be stored in the original treeless habitat and any changes that may occur to this carbon as trees establish. In particular, it is critical to take into account changes in the soil carbon during tree establishment. More carbon is stored in soils globally than in the vegetation and the atmosphere combined and a large proportion of this global store is stored in soils in northern and high latitude regions, including Scotland.
Figure: Mounding to prepare the ground for tree planting (Photos: Ruth Mitchell)
Given the policy relevance of this topic globally, and our urgent need for climate action, there has been a rapid increase in scientific research (and the corresponding literature) studying the impacts of tree planting on carbon storage. In particular, Mayer and colleagues (2020) have provided a detail review on the influence of forest management activities on soil organic carbon stocks: ”Tamm Review: Influence of forest management activities on soil organic carbon stocks: A knowledge synthesis”.
Our review does not attempt to repeat the work of Mayer and colleagues but rather focuses on new literature that has become available since their work.
Our work specifically focuses on:
- Impact of afforestation or reforestation on soils and whether this increases or decreases both soil organic carbon and overall net ecosystem carbon sequestration and carbon storage.
- Timescales of changes in soil organic carbon, particularly with respect to changes in soil organic carbon in the first few decades following tree planting, as this timescale is most relevant to Scotland’s target of achieving net-zero by 2045.
- Differences between tree species or types of trees (conifer versus broad leaves) in their impact on soil carbon.
- Differences in soil organic carbon storage between different soil types.
Our review of the literature showed that across modelling work, experimental plots, and large-scale surveys generally soil organic carbon (SOC) increases following tree planting on soils with low SOC but declines on soils with a high starting SOC. While in the long-term tree establishment on more organic rich soils can lead to net ecosystem gains in carbon in the short-term (the first 20 years following tree planting) there is little evidence that planting on organic rich soils increase ecosystem carbon storage. Thus, tree planting to achieve significant carbon sequestration within the timeframes relevant to achieving net zero, requires careful choice of site (particularly soil type) and tree species.