Similar functional structure and encroaching dynamics in two Juniperus species with contrasting distribution patterns

Abstract In the valley of Espot in the central Iberian Pyrenees, two species of juniper, Juniperus communis and J. sabina , co-occur in the same habitats, showing a similar encroaching ability as prostrate shrubs expanding on former rangeland. Outside of this valley, however, J. communis is widespread from the regional to the Holarctic level whereas J. sabina is only sparsely found in mutually distant Palaearctic regions, where it remains restricted to scarce locations. We set out to analyse 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 their role in the landscape, all along an elevation gradient. Juniperus communis and J. sabina showed similar strong investment into stress resistance, according to the functional traits measured (SLA, LDMC, leaf lifespan), and both species expanded laterally at similarly noticeable ratios. Along the elevation gradient, we found shifts in functional traits and performance that respond to environmental differences. The populations of J. communis included more balanced age classes and showed good recruiting ability. By contrast, J. sabina exhibited a poor recruiting ability, with populations lacking juveniles and including notably old shrubs. The longevity and clonal performance of this species would explain its persistence only in particular locations scattered over distant regions. Therefore, the contrasting plant distribution and ecology of the two species (i.e. a widespread and active colonizer vs a relict growing in secluded locations) appears to be based on a few interspecific developmental and reproductive differences.