Neighbourhood interactions and environmental factors influencing old-pasture succession in the Central Pyrenees

The shrub Buxus sempervirens and the trees Abies alba and Fagus sylvatica have recently recolonized old-pastures in the Central Pyrenees. We mapped all live and dead individuals (> 1.3 m tall) in a large forest plot in Ordesa Valley to examine the importance of density-dependent processes during recolonization. Biotic interactions were inferred from changes in horizontal structure and the influences of neighbours on tree survival. Buxus differentially influenced establishment and survival of tree species, thereby controlling future canopy composition and spatial structure. The rapidly invading Abies formed denser patches on elevated sites less occupied by Buxus. whereas Fagus preferentially established within shrubs. Abies reached densities which led to intense intraspecific competition and high mortality rates among saplings. Self-thinning in Abies led to smaller numbers of regularly spaced survivors, and greater relative dominance of Fagus. Disregarding intraspecific effects and abiotic environment, Abies survival was significantly lower under Buxus shrubs. which suggests that the spatial location and abundance of Abies was constrained by the location of Buxus. Fagus survival was not related to Buxus density, but remained significantly lower in denser Abies patches. The higher mortality of Fagus in denser Abies patches, and the resulting spatial segregation of the species, reflects asymmetric interspecific competition. Inhibition from Buxus shrubs and intraspecific competition prevent invading Abies from dominating and may therefore help in maintaining a mixed Abies-Fagus stand.