Long‐Term Effects of Woodland Creation on Soil Carbon Stocks and Aggregate Distribution in Central Scotland

Abstract

The importance of increasing woodland cover in mitigating climate change is widely recognised, yet quantification of the long-term impact of maturing woodlands on soil structure and carbon stocks remains limited. This study evaluates soil structure and carbon content across a chronosequence of UK woodlands established over the last 160 years and old-growth woodland (250+ years old) compared to pasture sites. Soil organic carbon (SOC) stocks were significantly higher in woodland soils than in pastures, with mature and ancient woodlands containing up to 88%–111% more SOC ─ primarily due to the development of an organic layer that contributed up to 42% of total carbon stocks. In contrast, differences in carbon within mineral soil layers (0–15 cm, 15–30 cm) were minimal, indicating that afforestation-driven carbon gains are largely restricted to surface horizons. Woodland age had a large effect on carbon concentrations within water-stable aggregates (η2 = 0.58, p < 0.001), with macroaggregates (> 2000 μm) exhibiting the greatest increases (Cohen's d = 1.28). Mid-aged woodlands (31–80 years) displayed particularly even carbon distribution across aggregate size classes, while older stands showed accumulation in finer fractions (< 250 μm), suggesting progressive carbon stabilisation as forests mature. Aggregate stability, measured as mean weight diameter (MWD) from wet-sieving analysis, was significantly higher in woodland surface soils (0–15 cm) compared to pasture (p < 0.001), with older secondary woodlands (81–160 years) showing the greatest improvements. Critically, MWD was not a statistically significant predictor of SOC stocks or carbon concentration in mineral soil layers (0–15 cm and 15–30 cm; p > 0.05, η2 < 0.01), confirming it as an unreliable indicator of carbon status in these systems. This study highlights the essential role of organic layer development in woodland carbon sequestration and provides detailed evidence that woodland age influences not only soil carbon quantity but also its distribution and physical protection within the soil matrix. These findings have practical implications for land management and carbon accounting practices in rewilded and afforested landscapes.