Structure and spatial variability of Pinus uncinata Ram. growth forms at two contrasting treeline ecotones in the Spanish Pyrenees

Abstract

We describe the spatial distribution of Pinus uncinata individuals classified according to their growth-form and size at two undisturbed altitudinal treeline ecotones in the Spanish Pyrenees (Ordesa, O; Tessó, T). At each site, a rectangular plot (30 ´140 m) was located with its longest side along the altitudinal gradient, and encompassing treeline and timberline. At both sites, living individuals were bigger and had a higher number of needle cohorts downslope. The distribution of size and growth-form classes at site O followed a neat sequence of increasing size downslope, from abundant shrubby multi-stemmed krummholz individuals with few needle cohorts (1-3) up to bigger uni-stemmed arborescent trees with several needle cohorts (4-12). In contrast, structural changes through the ecotone at site T were gradual, and they did not follow so neatly this sequence of increasing size downslope.