Soil carbon and biodiversity response to Scottish Highlands rewilding

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Rewilding is increasingly considered as a powerful tool to limit both the ongoing declinesin biodiversity and mitigate climate change, through its potential to restore biodiversehabitats and increase ecosystem-scale carbon (C) storage. In Scotland, where ca. sevenmillion sheep and one million deer graze the land predator-free and prevent the naturalregeneration of trees, the removal of these large herbivores through fencing has allowednatural forest regeneration in several small-scale experiments (see below). The visualsuccess of these initiatives has been embraced by the rewilding conservation movementand has led to increasingly vocal calls to 'rewild' the Scottish Highlands. Rewildinginitiatives 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 recoveryand C sequestration, is limited due to a lack of above and belowground data.While these rewilding initiatives likely increase above ground carbon storage by allowingtree growth, the overall change in ecosystem C storage also depends on the response ofthe largest C stock in terrestrial ecosystems: the soil C stock. Recent findings fromexperimental tree planting in the Scottish uplands suggests that while tree planting led toan accumulation of C aboveground, it simultaneously led to decrease in C sequestrationbelowground (Friggens et al. 2020), thereby cancelling out the aboveground benefits.Preliminary data from a rewilding initiative at Ben Lawers Nature reserve suggest that thisdecrease in soil C may also occur following the natural regeneration of trees.Furthermore, soils are globally the largest reservoir of biodiversity, hosting more than25% of the Planet's total biodiversity (Bardgett and van der Putten (2014). Holisticbenefits 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 andbiodiversity, once the soil is considered?