Facilitation in mediterranean mountainsengineering role of juniperus sabina l. At community, population and individual levels

Resumen

Mediterranean mountains are harsh environments doubly constrained by low temperatures in winter and drought in summer. In these environments, biotic interactions play a key role shaping community structure and diversity. Along the Mediterranean Basin, prostrate junipers are conspicuous landscape elements that create fertility islands and facilitate the presence of other plants, both woody and herbaceous. In this PhD thesis, the effects of the prostrate Juniperus sabina on community organization, population structure and individual functioning are studied in Iberian Mediterranean high mountains. At the Iberian System, central-Eastern Spain, abandoned crops in mountain areas are currently being recolonized by the forest of surroundings. The progression needed to reach the final structure of pine forests with several junipers in the understory starts with the encroachment by bird-dispersed junipers, followed by pine establishment and dispersal. The facilitative effect of J. sabina is more important at the earlier stages of succession. In areas above treeline, the effect of J. sabina on population dynamics shifts depending on the abiotic stress degree. Establishment and growth of the cushion plant Hormathophylla spinosa are enhanced by junipers under higher stress, but more favorable conditions lead to higher densities of beneficiary plants and increase intraspecific competition. Establishment and fitness of Helleborus foetidus also increase under juniper canopies when stress is higher at the Sierra de las Nieves, southern Spain, and this implies changes in the way in which plants acquire and invest carbon resources. Facilitated plants with easier access to resources show more resource-acquisitive strategies than those living in open areas. Juniper shrubs not only influence community structure, but also plant functioning in a complex way that varies depending on the environmental context. Understanding the way in which these juniper species behave regarding to the main drivers of global change is crucial for preserving these singular ecosystems.