Soil carbon and biodiversity response to Scottish Highlands rewilding

Rewilding is increasingly considered as a powerful tool to limit both the ongoing declines
in biodiversity and mitigate climate change, through its potential to restore biodiverse
habitats and increase ecosystem-scale carbon (C) storage. In Scotland, where ca. seven
million sheep and one million deer graze the land predator-free and prevent the natural
regeneration of trees, the removal of these large herbivores through fencing has allowed
natural forest regeneration in several small-scale experiments (see below). The visual
success of these initiatives has been embraced by the rewilding conservation movement
and has led to increasingly vocal calls to 'rewild' the Scottish Highlands. Rewilding
initiatives often infer that there will be wider holistic benefits (e.g., Warner et al. 2021),
however, confidence in the true benefits of rewilding, particularly biodiversity recovery
and C sequestration, is limited due to a lack of above and belowground data.
While these rewilding initiatives likely increase above ground carbon storage by allowing
tree growth, the overall change in ecosystem C storage also depends on the response of
the largest C stock in terrestrial ecosystems: the soil C stock. Recent findings from
experimental tree planting in the Scottish uplands suggests that while tree planting led to
an accumulation of C aboveground, it simultaneously led to decrease in C sequestration
belowground (Friggens et al. 2020), thereby cancelling out the aboveground benefits.
Preliminary data from a rewilding initiative at Ben Lawers Nature reserve suggest that this
decrease in soil C may also occur following the natural regeneration of trees.
Furthermore, soils are globally the largest reservoir of biodiversity, hosting more than
25% of the Planet's total biodiversity (Bardgett and van der Putten (2014). Holistic
benefits of rewilding on biodiversity thus largely depend on the response of the soil.
There is thus an urgent need to carefully considering the response of the soil to rewilding.
Can rewilding the Scottish Highlands increase ecosystem-scale carbon storage and
biodiversity, once the soil is considered?