Structure and recent recruitment at alpine forest-pasture ecotones in the Spanish central Pyrenees

Abstract

Treeline ecotones are intensively studied to quantify the response of vegetation to environmental changes. We describe here the size, growth-form, and spatial distribution of trees in two alpine forest-pasture ecotones located in the Central Pyrenees (Ordesa and Tessó), dominated by Pinus uncinata Ram. and little affected by anthropogenic disturbances during the last century. Total variation of tree data was partitioned into spatial, environmental, combined, and unexplained components. The influence of understory plants on P. uncinata recruitment was assessed using cover data and detrended canonical correspondence analysis (DCCA). Climatic effects on recent recruitment were also investigated at Tessó. Most individuals at Ordesa were multistemmed and shrubby (krummholz). Krummholz and seedlings showed significant spatial correlation. The spatial component of variation of tree data was greater at Ordesa (26%) than at Tessó (4%). DCCA revealed a significant effect of elevation on the spatial segregation of the size and growth-form classes across both ecotones. At Tessó, recruitment was concentrated close to the treeline, where the cover of the dominant understory shrub (Rhododendron ferrugineum) decreased. The abundance of young individuals in the upper part of this ecotone might produce a future upward displacement of treeline. During the last 60 years, the important episodes of recruitment at Tessó were favoured by wet and warm springs and summers. Ordesa showed a clear spatial trend of distribution of P. uncinata individuals. Most individuals were krummholz, located at greater elevation. Recruitment was associated with krummholz type vegetation, which could buffer seedlings (nurse effect) from the harsh environment and thereby favour the establishment of seedlings.