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

We describe the structure of two contrasting (elevation, topography, climate, vegetation, soil) alpine forest-pasture ecotones located in the Central Pyrenees (sites Ordesa, O, and Tesso, T). We define ecotone structure as the spatial distribution of trees of different size classes and growth-forms and the relationship between these aspects and the spatial distribution of understory vegetation and substrate. The studied ecotones are dominated by Pinus uncinata Ram. and have been little affected by anthropogenic disturbances (logging, grazing) during this century. One rectangular plot (30 x 140 m) was located within each site with its longest side parallel to the slope and encompassing treeline and timberline. The distribution of size and growth-form classes at site O followed a clear sequence of increasing size downslope from shrubby multistemmed krummholz individuals to bigger arborescent trees. At site O, regeneration was concentrated near the krummholz area and over rocky substrates. This suggests that krummholz may modify microenvironment conditions and increase seedling survival. At site T, regeneration was abundant above the treeline where the cover of the dominant understory shrub (Rhododendron ferrugineum) decreased. In both ecotones the diversity of plants was higher above the treeline than in the forest and decreased going downslope coinciding with the increase of P. uncinata cover. The reduction of plant diversity appeared above the current timberline. At site O, the decrease was steep and spatially heterogeneous what may be due in part to the edaphic heterogeneity. At site T the change was abrupt though smaller. The relationships between the plant community and tree regeneration should be taken into account in future ecological studies of treeline pattern.