In the valley of Espot, central Iberian Pyrenees, two congeneric species coincide in the same habitats, showing a similar encroaching ability as prostrate shrubs expanding on former rangeland. Out of this valley, however, while Juniperus communis is widespread from regional to Holarctic levels, J. sabina is only sparsely found in distant Palearctic regions, where it remains restricted to scarce locations. We wanted to document and understand the dynamics of both species as drivers of landscape transformation, and to unveil which differences may explain their contrasting biogeographic performance. To do so, we analysed them in terms of plant traits, phenology, primary growth, population structure, and role in the landscape. Juniperus communis and J. sabina showed similar strong investment to stress resistance, according to the functional traits measured (SLA, LDMC, leaf lifespan), although both species expanded laterally at similarly noticeable ratios. The populations of J. communis included more balanced age classes and showed good recruiting ability, which would enhance the active role of this species along secondary succession. In contrast, J. sabina showed a poor recruiting ability, with populations lacking juveniles and including notably old shrubs. Longevity and clonal performance in this species would explain its persistence only in particular locations scattered over distant regions. Therefore, contrasting plant distribution and ecology – i.e. widespread and active colonizer vs. secluded and relict– would be based on a few interspecific developmental and reproductive differences.