The area of abandoned farmland is increasing in Europe. With the ongoing biodiversity and climate crisis, it is expected that a large fraction of intensively cultivated agricultural land will be converted into forest. The creation of forested areas has the potential to foster rich and resilient ecosystems, providing services and habitats for specialist species. However, biodiversity recovery in forests planted on agricultural soils is limited by land use legacies and fragmented landscapes.

In Denmark, a new forest program aims to transform 250.000 ha of intensively cultivated agricultural land to forests partly by planting and partly by natural colonization. Natural colonization and regeneration are considered cost-effective methods to create and restore forests. Although multiple benefits to biodiversity are expected from these methods, other ecosystem functions as carbon sequestration, might not follow the same trajectory when compared to forest plantations as natural colonization may be slow and even arrested due to biophysio-chemical legacies in the soil and the characteristics of the surrounding landscape e.g. fragmentation.

We used an arroy of research platforms to study the patterns e.g. chronosequences (space for time substitution), repeated measurements and experiments. We have monitored the development in forest structure, soil biogeochemistry and biodiversity during the first 50 years after abandonment in planted forests and forests developed by natural colonization. The development in ecosystem carbon, nitrogen and biodiversity are compared to old forests and setting aside old forests.

Within the platforms, we investigated the soil and surface active arthropods and understory vegetation using eDNA and conventional methods. We complemented this with information on the forest structure measured with LiDAR and ground truth measurements and soil biogeochemistry to investigate the drivers of biodiversity recovery in afforested agricultural land.

Results show that natural colonization (NC) enhances alpha and beta diversity in arthropods and herb species richness, driven by habitat heterogeneity, while beech plantations (PL) foster arthropod communities resembling older forests. Soil organic carbon (SOC) increases in the top 10 cm for both NC and PL compared to agricultural sites, but PL shows reduced SOC at deeper layers (10-25 cm). Nutrient dynamics follow expectations, with NC sites having higher phosphorus and total nitrogen than PL. Soil acidity in PL aligns more closely with mature forests, whereas NC exhibits grassland-like traits. Community composition varies significantly across land use types, with soil properties explaining only part of the variation, indicating roles for factors like moisture and competition. Overall, at the early stages, NC supports biodiversity and herb richness, while PL accelerates forest-related species development and soil conditions similar to mature beech forests. In the presentation, we will further examine the drivers of the different trajectories.

In the chronosequence, we found that different species groups responded to different aspects of forest development: e.g. the plant communities are shaped by light availability and disturbance. Furthermore, structural heterogeneity, canopy cover, and vicinity of old forest drive the abundance of forest specialists. However, communities did not develop to resemble the old forest reference. Indeed, while many forest associated plant species appeared with time, only those with long-range dispersal capability colonized the plantations. Although, soil properties did not fully resemble the reference, the nitrogen (N) and phosphorus (P) cycles tightened and available N and P approached the levels in the old reference forests. We, therefore, confirm that forest structural diversity and connectivity can promote potential biodiversity, but a lack of seed sources, dispersal limitation, and soil legacies affect the establishment and recovery of specialist communities.

Land-use change, Afforestation, Natural colonization, Biodiversity, Ground vegetation, Insects, Forest structure

Inger Kappel Schmidt

ORAL