Tree cover is often increased with the aim of increasing ecosystem carbon sequestration and mitigating climate change. However, when planting trees in ecosystems with carbon-rich soils, soil disturbance during ground preparation can cause soil carbon losses not counteracted by tree carbon gains at the decadal timescales relevant to climate change mitigation. Tree establishment via natural colonisation, which does not involve soil disturbance, might prevent these soil carbon losses, but this is unknown. We measured soil, ground vegetation, and tree carbon stocks and tree inputs along an 8 metre transect from single, native, 25-year-old naturally colonised trees (Pinus sylvestris or Betula spp.) onto Calluna vulgaris-dominated moorland, at sites with carbon-rich organo-mineral soils in the Cairngorms, UK.  Along the transect away from the tree, organic soil carbon stocks increased from 4.0 kg C m^-2 at 0.5 metres, to 6.0 kg C m^-2 at 8 metres. Meanwhile, carbon stocks in the top 10 cm of the mineral soil horizon, 3.6 kg C m^-2, did not vary. Ground vegetation carbon stocks increased only slightly, from 1.0 kg C m^-2 at 0.5 metres, to 1.3 kg C m^-2 at 8 metres. Mean carbon stock per tree was 32.4 kg, so overall, sparse natural colonisation resulted in no net ecosystem carbon gain.

Policy implications: Sparse natural colonisation of carbon-rich soils by low biomass trees might not result in net ecosystem carbon gains at decadal timescales, and instead lead to unforeseen soil carbon losses. Soil carbon changes should be taken into account when quantifying the climate change mitigation potential of natural colonisation.