Plant species distribution across two contrasting treeline ecotones in the Spanish Pyrenees

Abstract

We describe the structure of two contrasting (elevation, topography,climate, vegetation, soil) alpine forest–pasture ecotones located in theCentral Pyrenees (sites Ordesa, O, and Tesso, T). We define ecotonestructure as the spatial distribution of trees of different size classes andgrowth-forms and the relationship between these aspects and the spatialdistribution of understory vegetation and substrate. The studied ecotones aredominated by Pinus uncinata Ram. and have been littleaffected by anthropogenic disturbances (logging, grazing) during this century.One rectangular plot (30 × 140 m) was located within eachsite with its longest side parallel to the slope and encompassing treeline andtimberline. The distribution of size and growth-form classes at site O followeda clear sequence of increasing size downslope from shrubby multistemmedkrummholz individuals to bigger arborescent trees. At site O, regeneration wasconcentrated near the krummholz area and over rocky substrates. This suggeststhat krummholz may modify microenvironment conditions and increase seedlingsurvival. At site T, regeneration was abundant above the treeline where thecover of the dominant understory shrub (Rhododendronferrugineum) decreased. In both ecotones the diversity of plants washigher above the treeline than in the forest and decreased going downslopecoinciding with the increase of P. uncinata cover. Thereduction of plant diversity appeared above the current timberline. At site O,the decrease was steep and spatially heterogeneous what may be due in part tothe edaphic heterogeneity. At site T the change was abrupt though smaller. Therelationships between the plant community and tree regeneration should be takeninto account in future ecological studies of treeline pattern.