Natural woodland expansion into old-field sites is valuable for nature conservation and international commitments to expanding forests. However, successful colonisation of trees and long-term woodland health depend on interactions with established vegetation, land use history, and suitable microclimatic conditions. Understanding the effect of these factors is crucial for anticipating woodland expansion outcomes, given the long periods and large scales involved. We used a novel combination of airborne photogrammetry and field measurements to evaluate the key determinants of long-term success of Juniperus thurifera in a 20 km² region of central Spain. A canopy height model, trained on field-measured biomass, was used to map juniper stand biomass, and time-series of land-cover were used to estimate the age of regenerating woodland patches. These datasets allowed us to evaluate the drivers of variable success in natural colonisation using a space-for-time substitution within the expansion frontier. Dramatic land cover shifts have occurred over the last 34 years, including a 75 % reduction in agricultural land and a tripling of open woodland. Juniper woodland has expanded out from steep slopes onto flatter land, and into sites with less extreme irradiance. Increasing dwarf shrubland density within stands significantly reduced juniper biomass by up to 25 % in the oldest woodlands. Higher insolation was associated with faster biomass accumulation over time but lower biomass in new woodlands. New stands with low insolation showed greater biomass stocks for their age compared to old stands, indicating potential benefits from land use legacies in the absence of drought stress. Our findings show substantial juniper woodland expansion over the last 34 years, indicating different constraints on growth at the expanding front. Using field methods and high-resolution photogrammetry, we highlight the importance of local competition and abiotic conditions for expanding growth over large spatial scales and multi-decadal time scales.