Turning Mediterranean Farmlands Into Priority Habitats: Natural Expansion of Juniper Woodlands After Agricultural Abandonment

Abstract

Aims Natural woodland expansion into former agricultural land contributes to conservation and global reforestation goals. However, tree colonisation and woodland persistence depend on interactions among vegetation, land-use history and suitable microclimatic conditions. Understanding these drivers is essential for anticipating woodland expansion outcomes over large spatial and temporal scales.

Methods We combined airborne photogrammetry with field measurements to assess the long-term success of Juniperus thurifera woodlands—a Natura 2000 priority habitat—across a 20-km2 region of central Spain. A canopy height model, calibrated with field biomass data, was used to map juniper biomass, while time series land-cover maps estimated woodland age. This novel approach enabled evaluation of colonisation success using a space-for-time substitution along the expansion frontier.

Results Over the past 34 years, land cover has changed markedly, with agricultural land declining by 75% and open woodland tripling. Juniper stands have expanded from steep slopes onto flatter terrain and into areas with lower irradiance. Increasing dwarf-shrub density within stands reduced juniper biomass by up to 25% in the oldest woodlands. Higher solar exposure promoted faster biomass accumulation through time but limited biomass in younger stands. Contrarily, new stands under lower insolation showed greater biomass for their age, suggesting positive land-use legacies where drought stress was reduced.

Conclusion Overall, J. thurifera woodlands have expanded substantially over recent decades, yet growth constraints differ across the expansion front. With our innovative framework integrating high-resolution photogrammetry and field-based biomass models, our study underscores the interplay between local competition and abiotic factors in shaping woodland dynamics across space and time. This framework offers a valuable, scalable and transferable tool for monitoring and managing long-term ecosystem dynamics across larger spatial and temporal scales and informing habitat restoration and conservation planning under changing environmental conditions.