Natural tree colonisation of organo-mineral soils does not provide a net carbon capture benefit at decadal timescales

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 targets. 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, as well as tree inputs, soil physicochemical properties, and soil community composition 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. Increased soil carbon stocks along the transect was associated with increased soil moisture and decreased carbon-to-nitrogen and carbon-to-phosphorus ratios, and changes to the soil bacterial and fungal community compositions. 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 after 25 years.

However, trees might be established onto carbon-rich soils for a range of other ecosystem services, in addition to or instead of carbon storage benefits, such as biodiversity benefits, flood alleviation, water purification, and recreation. The trade-offs between soil carbon losses and these other benefits should be considered, and given that belowground carbon losses are offset by aboveground tree carbon gains, sparse natural tree colonisation could provide a mechanism for tree establishment whereby there is no net ecosystem carbon loss or gain on a decadal timescale, but other ecosystem services are increased.