Tracing 80 Years of Forest Succession and Species Trajectories Using Historical Aerial Photography and Sentinel-2 Imagery

Abstract

Global land abandonment drives secondary forest succession in mountains, yet long-term spatial dynamics remain poorly quantified. We traced 80 years (1945–2025) of forest expansion across Greece’s Rhodope Mountains using an integrated remote sensing framework. Historical baselines (>25% canopy closure) were derived via Object-Based Image Analysis of 1945 panchromatic orthophotos. Because grayscale media lacks spectral species resolution, taxa trajectories were inferred by intersecting 1945 canopy footprints with field-validated 2025 Sentinel-2 classifications. Landscape metrics and spatial statistics quantified colonization pathways for eight dominant taxa. Unconstrained natural forest expansion drove massive structural coalescence, doubling the Largest Patch Index and reducing patch densities universally. Spatial patterns corresponded with known dispersal mechanisms: heavy-seeded broadleaves (Fagus sylvatica, Quercus spp.) expanded conservatively near historical cores, whereas wind-dispersed pioneers (Pinus sylvestris, Betula pendula) colonized distant environments. Notably, Picea abies shifted downward (−204.7 m) toward valley floors. This was an empirical shift interpreted as a response to grazing cessation and potential reliance on riparian micro-refugia, rather than upward temperature-driven migration. While spontaneous coalescence enhances connectivity, unchecked homogenization threatens open-habitat biodiversity, requiring targeted silvicultural interventions to maintain landscape heterogeneity.