The Scottish Government is investing significant resources into expanding Scotland’s woodland cover to increase carbon sequestration and mitigate climate change. Wild animals are rarely considered in carbon storage policy. However, there is growing evidence that Scotland’s wild deer population could hinder targets for woodland creation. High pressure from deer can also harm the health of pre-existing woodland and therefore reduce the ability of Scotland’s woodlands to store carbon and off-set greenhouse gas (GHG) emissions.
The latest figures show Scotland’s wild deer population to be approaching 1 million individual animals. The need to control wild deer has led deer management to become Scotland’s largest terrestrial wildlife management challenge.
This short review explores the current state of knowledge on wild deer populations and how they effect carbon sequestration in Scottish woodlands. It gives an overview of the key factors and identifies areas where there is an absence of evidence.
- The impacts of deer fall into two categories:
- Direct – such as removing vegetation, preventing natural regeneration, and increasing mortality of mature trees.
- Indirect – promoting the dominance of less palatable species and reducing the quality of the plant litter that in turn can affect the nutrient balance.
- Analysis of the evidence indicates that the primary mechanisms by which deer interfere with carbon cycling in woodlands are largely identified. However, we found limited data that quantify the size of these effects. Therefore, more research is needed to establish the scale of the threat to carbon sequestration in woodlands.
- A larger body of evidence exists to demonstrate the effects deer have on above-ground carbon storage. Conversely, there were limited data on the less direct, but potentially significant, effect deer have on below-ground carbon stores.
- Although more data are needed to determine the significance of deer browsing in relation to meeting carbon sequestration goals in woodlands, reducing deer impact to a level where woodlands can naturally regenerate would likely increase woodland productivity and carbon storage.
- For Scotland’s natural landscape and woodlands to recover, deer densities need to be reduced and maintained around a <5 deer/km2 In some cases, deer fences may need to be erected temporarily to protect certain areas. Although, they are a costly solution.
This evidence review suggests the mechanisms by which deer impact carbon cycling have been investigated and mostly identified. However, very limited evidence was identified that examined the size and significance of these effects. Without these quantitative data, it is difficult to create an informed mitigation strategy. Hence, further research is needed in Scotland to fully understand this complex relationship.
Given that the actions taken to mitigate the climate emergency are time sensitive, action may have to proceed using the limited data currently available. Scholars and practitioners largely agree that reducing deer numbers in woodlands to a threshold believed to allow natural regeneration of the full assemblage of woodland plant species would be beneficial for plant productivity and therefore carbon storage. Thus, reducing deer numbers to a sustainable threshold could counter many of the adverse effects presented in this report. In addition, reducing deer numbers will be essential in protecting the woodlands that will help Scotland reach its goals of net zero by 2045.