A Methodological Framework for Determining Whole-Ecosystem Carbon Storage in Carbon Sequestration Tree Planting Schemes: A Proof of Concept Survey

Corporate carbon offsetting schemes heavily rely on tree planting for carbon sequestration, and in Scotland the government presents tree planting as a key carbon sink in the face of climate change. However, carbon offsetting schemes rarely consider associated fluxes in soil organic carbon (SOC) driven by afforestation. Soils in woodlands store approximately 75% of the total ecosystem carbon, yet planting trees on existing carbon-rich soils or grassland may result in changes in soil carbon that take decades for tree biomass to recover. This study provides a methodological framework using the chronosequence approach (space-for-time substitution) to measure whole-ecosystem carbon storage for existing commercial carbon sequestration schemes. A proof-of-concept pilot study was carried out at the University of Edinburgh’s Rullion Green site, gathering 35 soil samples across unplanted baselines, a recent 2025 planting scheme, and a mature mixed woodland. The findings of this proof-of-concept study show that soil carbon in the newly planted area (5.06% SOC) was lower than in the mature woodland (7.64% SOC) and remained statistically similar to the unplanted baseline (7.11-8.65% SOC depending on slope). The carbon in the recently planted trees (4.10 t C/ha) did not offset the carbon losses from soil, with a drop in ecosystem carbon of 156.41 t C/ha compared to the baseline, and 418.23 t C/ha compared to the mature woodland. The methodology and trial study suggest that initial tree planting may not guarantee an immediate net increase in ecosystem carbon until trees become mature. The methodology therefore provides a tool to help mitigate greenwashing in the offsetting industry. The methodology will now be applied in further research to other sites across northern England to determine what the impact of tree planting for carbon sequestration is on soil in the UK.