Stand structure and regeneration of a mixed forest (Abies alba-Fagus sylvatica) in the Central Pyrenees, Ordesa National Park, Spain

The locations and biometrical characteristics of 2391 living and dead trees > 1.3 m tall of Abies alba and Fagus sylvatica, and the 378 understory shrubs o/Buxus sempervirens, were mapped in a 1.4 ha plot on the northern slope of Ordesa Valley to evaluate several hypotheses about stand structural development, tree species regeneration and coexistence. The plot is located in relatively undisturbed old-growth forest, but contains areas at low elevation which were formerly pasture. Abies is typically represented by many young trees and gradually declining numbers of trees in successively older size classes, whereas Fagus has greater numbers of trees in larger size and older age classes. This would imply a shift in dominance from beech to fir if the two species have similar mortality rates. We tested two hypotheses about the coexistence of ecologically similar species: (1) based on differentiation of regeneration niches, and (2) by means of different life history strategies (preference for survivorship or fecundity). Redundancy analysis (RDA) was used to determine if the two species prefer different habitats. The analysis of spatial patterns and interspecific associations by Ripley's K-function was used to estimate the role of competition among trees in forest dynamics. The data provide empirical support for both tested hypotheses, although it has been shown that their importance varies depending on the degree of environmental heterogeneity along the slope across the plot. Different life history strategies appear critical to the success of coexistence in moderate environment at lower elevations, where co-dominant species have overlapping regeneration niches. [fr] Dans une parcelle de 1, 4 Ha au versant nord de la vallée d'Ordesa nous avons cartographie à petite échelle et pris des données biométriques sur 2391 hêtres (Fagus sylvatica) et sapins (Abies alba) vivants ou morts mais tous s'élevant à plus de 1,3 m, ainsi que sur 378 arbustes de Buxus sempervirens dans le sous-bois. Puis nous avons tenté quelques hypothèses concernant la structure et la dynamique de la parcelle forestière, la régénération des arbres et leur coexistence. Bien que la parcelle fasse partie d'une vieille forêt peu altérée, dans sa partie inférieure quelques surfaces ont été paturées. Le sapin montre beaucoup de jeunes individus et progressivement moins d'arbres grande taille, à l'opposé, le hêtre présente un plus grand nombre d'arbres appartenant aux classes de haute taille et de grande ancienneté. En supposant que les deux espèces aient des taux de mortalité similaires, cela indiquer