Rewilding offers a carbon capture alternative to tree plantations with a wealth of biodiversity benefits

Carbon stored in woody vegetation plays a crucial role in mitigating climate change, with tree planting commonly used for carbon offsets as a result. Young, growing woodlands can trap carbon dioxide while also providing other benefits to those living nearby – such as alleviating flood hazards and improving air quality.

However, there are drawbacks to these monoculture tree ‘plantations’; they incur their own carbon costs to set up and offer little for biodiversity. An increasingly popular alternative is natural colonisation through rewilding. While the benefits of rewilding for biodiversity are becoming better understood, the potential benefits for carbon sequestration in scrubland remain unclear – particularly the influence of herbivores on woodland creation.

New research has investigated this at the Knepp rewilding project, where large grazers have been reintegrated onto the land, formerly used for agriculture, following a period of natural regeneration. The study challenges traditional methods of estimating carbon storage, which have previously overlooked the impact of grazing herbivores on tree growth. Lead author Nancy Burrell stated:

"We've been so fixated on tree plantations as carbon offsets that we’ve neglected to examine or understand the contribution that could come from smaller, non-forestry species. We have no idea how much carbon they might store, not only above ground but also below ground in their roots – and how this storage may be stimulated and enhanced by the browsing of the free-roaming herbivores that drive rewilding projects."

Herbivores like deer can alter trees’ and shrubs’ growth patterns, with scrubland trees storing more carbon in their roots than previously thought.

Image: Charlie Burrell

The researchers employed the i-Tree Eco model, typically used in urban and forestry settings, to estimate carbon storage in a rewilded section of the Knepp estate. They compared its output with actual measurements from 39 trees across five species, which they unearthed, including their roots. The focus was on disentangling the relationship between tree dimensions and biomass, especially considering the influence of herbivore browsing.

The research revealed that the trees and shrubs have a much higher root-to-shoot ratio than currently predicted by carbon storage models – indicating that scrubland trees store far more carbon in their roots than previously thought. This finding suggests that herbivore browsing in rewilded landscapes does not stunt growth and reduce carbon storage, contrary to previous suggestions. Instead, the study supports the idea that trees under browsing pressure adapt by investing more in root growth, a strategy previously overlooked, which could enhance their carbon storage capacity.

This research has the potential to alter our perception and valuation of rewilded landscapes as a mechanism for carbon offsetting in the context of climate change. By demonstrating that these areas might be more efficient at carbon storage than previously believed, it provides support for the dual role of rewilded landscapes in biodiversity conservation and climate change mitigation efforts. It also underscores the importance of considering scrub, not just large trees, in carbon sequestration strategies.

The next step in this research is to develop a species-specific model to estimate the above and below ground biomass (and consequently carbon) in scrubland created by rewilding. The researchers aim to use this model to determine the carbon storage potential of current and future rewilding projects.

To read more about this research, published in Ecological Solutions and Evidence, visit: https://doi.org/10.1002/2688-8319.12301