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.
Stage
Work in ProgressDirectory of Expertise
Purpose
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.
Results
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.
Figure: Newly planted Sitka Spruce on ground prepared by mounding (Photos: Ruth Mitchell)
Key points to arise from the review were:
- Afforestation of organic rich soils is likely to lead to losses in soil carbon not offset by increases in above ground biomass carbon in the early decades following tree establishment.
- Faster soil organic carbon accumulation occurs where trees are planted on cropland than on grassland or more organic rich soils.
- Soil carbon accumulation in the F layer of the soil (the layer composed of fresh and partially decomposed leaf litter) may easily be lost from the soil due to disturbance. As such the F layers long-term contribution to soil organic carbon storage cannot be guaranteed and consideration should be given to where in the soil profile the increases in carbon occur.
- While one genera or type of trees (e.g. conifer v broadleaf’s) may generally store more carbon than another, even within genera there are differences between species in their carbon sequestration potential and this also varies with growing condition.
- Ground preparation for tree planting such as ploughing, mounding, and scarifying are known to cause soil carbon losses because they alter abiotic properties such as soil aeration, moisture and temperature. However, there is also increasing evidence that tree establishment with minimal soil disturbance e.g., slot planting or even natural regeneration can cause soil carbon losses on organic rich soils in the early years following establishment.
- The mechanisms for soil carbon losses following tree establishment with minimal or no soil disturbance needs further work but are likely to be due to ‘biological priming effects’, (the impact of the tree leaf litter and root exudates (fluids emitted through the roots of plants)) and because the fungi associated with the tree (mycorrhizal fungi) have enzymes capable of breaking down the carbon locked in the soil.
Benefits
In Scotland, tree planting on deep peat (soil having an organic (peat) layer of depth greater than 50 cm) has been banned, while grant funding still available to support planting on soils with less than 50 cm peat. However, the latest advice recommends restricting medium and high soil-disturbance cultivation to soils with an organic layer less than 10cm. This review shows the importance of implementing this guidance so that tree planting on soils with less than 50 cm peat does not to lead to carbon losses.
In addition, this review suggests that further work is required on the impact of tree establishment with minimal or no soil disturbance on soil organic carbon stocks on shallow organic rich soils.
Short-term maximum soil carbon gains are most likely to be achieved by planting on mineral soils, which have lower soil carbon. However, this may require trade-offs with other land uses such as crop production. The review highlights that while in the long-term tree planting may achieve many benefits in addition to carbon storage such as biodiversity or flood mitigation; if the key objective is increased carbon storage by 2045 to mitigate climate change, then tree planting requires a careful choice of site (particularly soil type) and tree species.