Decades to establish, centuries to assemble: Soil microbial community development following woodland creation on agricultural land

Woodland creation on former agricultural land is a central component of UK and European land-use policy for climate change mitigation and biodiversity restoration. However, the long-term trajectories of soil fungal and bacterial community assembly following woodland creation, and the extent to which they converge toward ancient woodland communities, remain poorly resolved. In this study we examined whether belowground microbial communities progressively converge toward those of ancient woodland, if fungal and bacterial communities follow similar successional trajectories, and whether their development is associated with changes in soil properties, particularly soil organic carbon (SOC) and pH. We investigated soil fungal and bacterial communities across a > 250-year chronosequence in central Scotland (UK), spanning agricultural pastures, planted woodlands of increasing age, and ancient woodland comparators, using fungal ITS1 and bacterial (16S rRNA) metabarcoding at 35 sites. Ectomycorrhizal (EcM) fungi increased across the chronosequence, reflecting the progressive establishment of forest-associated symbioses during woodland development, from a mean of 8.2% in pastures to 36.4% in ancient woodlands. Arbuscular mycorrhizal fungi declined sharply following tree establishment, and SOC was the strongest correlate of both fungal and bacterial community composition. Most notably, fungal communities in planted forests remained compositionally distinct from ancient woodland even after 160 years of development, being more dissimilar to ancient woodland than ancient woodland sites were to one another. The persistence of compositionally distinct fungal communities after 160 years highlights the difficulty of recreating the soil conditions characteristic of ancient woodland, underscoring its irreplaceable conservation value and the need to consider soil condition as a core component of woodland restoration and monitoring alongside aboveground vegetation.