Rewilding vs Woodland Creation for Carbon

By Ed Bradbury, Forest Canopy Foundation, 2022

Can man afford to stand back and let nature fix the problem? A mini study comparing carbon sequestration in artificial and natural regeneration.

Introduction:

Establishing new trees and woodlands is an effective way to sequester atmospheric carbon. In the UK, even slower-growing native species such as oak (Quercus robur) can sequester ~2.68 tonnes of CO2e in a single tree over 100 years (Cannell, 1999). Because of this, man is now turning to various forms of tree establishment as nature-based solutions to mitigate climate change.

Foresters define two types of tree establishment: artificial regeneration and natural regeneration. Artificial regeneration involves deliberate planting of seed or saplings. Natural regeneration refers to the process by which plants and woodlands are allowed to establish from seeds dispersed naturally from local sources. This study takes a practical look at how carbon outcomes of these contrasting approaches compare and the inherent costs and implications of sequestering carbon through each method.

It is important to look at this in the context of the climate emergency we are currently tackling as well as existing policy and funding frameworks. The IPCC have concluded that to avoid catastrophic warming in excess of 1.5^oC above pre-industrial levels countries must aim to meet net zero by 2050 (Masson-Delmotte et al., 2018). Therefore, carbon outcomes of the next 25 years are focussed on in this study. We explore this in a lowland context where potential for tree establishment is high in the UK.

Cost of Sequestration:

Financial viability is key to ensure projects are implemented at scale. To compare the costs of artificial regeneration to natural regeneration, a budget for each approach has been produced for a model 10ha scheme, managed for 25 years. Consistent rates for project management, planting and protection, and aftercare were used in each case. Income estimates are based on the Forestry Commissions England Woodland Creation Offer (EWCO) scheme.

Full costings are shown in Appendix 2.

The net cost of the natural regeneration project is estimated to be £139,767 whereas the cost of artificial regeneration is significantly higher at an estimated £200,867. However, when comparing this on a cost per tonne of CO₂ basis (based on WCC modelled rates -see previous section), it costs and estimated £174 per tonne through the natural regeneration approach but just £103 through artificial woodland regeneration. Therefore, it is more economical to sequester carbon through artificial regeneration than through natural regeneration.

Other Considerations:

There are several other factors to consider when comparing these practises for carbon sequestration.

• Establishment of woodlands via natural regeneration can also be unreliable with patchy tree cover at much lower densities than artificial regeneration projects. For example, a successful UK natural regeneration project established closed-canopy oak, ash and field maple with densities of up to 390 trees per hectare. Artificial regeneration projects in lowland England typically plant between 1800 and 2500 trees per hectare. In the initial stages of natural regeneration projects, it is common for only shrubland to develop by year 15 (e.g. UKCEH, 2021). Because of this, areas eligible for natural regeneration are highly constrained. Funding (EWCO) for these projects requires land to be within 75m of a seed source and may require remedial planting, ground preparation (scarification) and respacing to ensure outcomes are achieved.
• Active management of pests such as the grey squirrel is key to achieve desired carbon outcomes from woodland in the England and Wales (RFS, 2021). At present, the reduction in carbon sequestration from grey squirrel damage is estimated to be £9 million per year in the UK (RFS, 2021). If squirrels are not controlled in unmanaged rewilding schemes where natural regeneration is occurring, there may be a significant reduction in carbon sequestration.
• Species established is dependent on the local seed source for natural regeneration projects. This means trees that establish successfully are likely to be well matched to the current site conditions and eliminates biosecurity risks (Forestry Commission, 2021). These factors can be met to an extent in artificial regeneration projects through intelligent species choice and following established biosecurity protocols (e.g. Plant Healthy). Artificial regeneration allows foresters to choose species that may be best suited to growth under future climates which may benefits carbon sequestration long term.
• Timber is also a key consideration when exploring which form of woodland creation. Timber outcomes are closely linked to carbon sequestration as production is highly dependent on growth rates. If woodlands are harvested for timber this may compromise some of the carbon sequestered in the later stages of tree growth. However, there is a significant carbon substitution effect when timber is used instead of concrete or steel in construction (Weatherall, 2019). It is estimated that the carbon footprint of a timber frame building is 75% lower than steel buildings (IDEAL, 2020).
• Income from sales of carbon credits and timber is key to persuade landowners to pursue these projects.
• Minimal intervention in rewilding/natural regeneration projects and inherent connectivity with existing woodlands means natural regeneration projects have high biodiversity value. Biodiversity benefit in artificial woodland creation is dependent on species choice and therefore highly variable.

Summary

In summary, whilst both natural regeneration and artificial woodland creation are effective methods of sequestering carbon, artificial woodland creation is much more effective over 25 years than natural regeneration. It is critical that we drive maximal carbon sequestration ahead of 2050 to best ensure net-zero is met.