ALBERT

Description: Aims Facilitation has been reported in a wide range of plant communities, with evidence of positive interactions between beneficiary and nurse plants shifting during their ontogenetic development. This study explored the hypothesis that shrubs of Juniperus communis subsp. nana (thereafter Juniperus ) play a crucial role in the successional sequence of plant communities acting as nurse for different species, but only after reaching a certain size. In addition, we examined whether plant–plant interaction changes during ontogenesis of these shrubs in the presence of contrasting disturbance regimes in terms of substrate stability. Location Sibillini National Park, central Italy. The vegetation is semi-natural dry grasslands (92/43/EEC Habitats Directive: Habitat 6210 - Festuco-Brometalia ). Methods Field measurements were carried out to assess the effects of Juniperus on (1) the distribution of co-existing vascular species, (2) the above- and below-ground microclimate, and (3) changes in soil fertility and hydrology. Results The capacity of Juniperus shrubs to facilitate heterospecific plants considerably increases during its ontogenetic development, i.e. small shrubs mainly compete for resources with local vegetation, whereas large shrubs act as nurse plants for herbaceous and especially for woody species. The facilitation effect was slight, albeit significantly higher in the disturbed area than in the more stable one. Juniperus was able to promote the formation of an island of fertility under its canopy by accumulating a considerable amount of organic matter, N, P, Ca 2+ , Mg 2+ and K + in a few decades. Juniperus shrubs improve soil hydrological properties and mitigate the daily range of soil temperature, reducing the exposure of co-existing plants to high temperatures and water loss through soil evaporation, particularly during the growing periods in spring and summer. Conclusions This study demonstrates that biogenic amelioration of soil quality, coupled with the mitigation of below-canopy microclimatic conditions, control the establishment and growth of co-existing plants during Juniperus shrub development. In Mediterranean mountains, plants face extreme weather conditions. Juniperus communis , thanks to low stature and compact structure, is among the organisms best adapted to the critical combination of low winter temperatures and dry summers. This study demonstrates that biogenic amelioration of soil quality, coupled with the mitigation of microclimatic conditions, controls the performances of coexisting plants during Juniperus shrub development.