Tracking the leading edge of Fagus sylvatica in North-Western Iberia: Holocene migration inertia, forest succession and recent global change

•The population of F. sylvatica in North-Western Iberia is expanding.•The Iberian Atlantic region can be considered one of the F. sylvatica leading edges.•Colonization and replacement of Q. petraea forests by F. sylvatica ones is confirmed. The Iberian Peninsula constitutes the south-western limit of the European distribution of Fagus sylvatica L. However, although the Iberian North-West extreme would appear to be climatically suitable for the species, F. sylvatica is not present there. This fact has led to the suggestion that the species is not yet in equilibrium with the climate and that there is a migration lag in the post-glacial expansion of the species. The objective of this work is to understand the main biotic and abiotic factors driving F. sylvatica distribution and population dynamics in the Iberian Atlantic biogeographical region over recent decades. Furthermore, in the light of other studies which suggest that the expansion of F. sylvatica in North-Western Iberia might be related to the retraction of Q. petraea (Matt.) Leibl. forests, we also study the demographic trends of both F. sylvatica and Q. petraea along with oak-beech interaction processes to infer forest succession dynamics. Using data from the last two cycles of the Spanish National Forest Inventory (1986–2012) for the study area (the Iberian Atlantic biogeographical region); dominance, population structure, recruitment and basal area increment were analyzed in different forest types for the two target species. General linear models and Bayesian structural equation modelling techniques were also applied to study the direct and indirect drivers of recruitment and forest succession. Contrary to what might be expected under the current conditions of climatic change, the population of F. sylvatica in North-Western Iberia is expanding, the basal area increment of F. sylvatica increasing westwards and new recruitment occurring in the lowlands. Accordingly, the Iberian Atlantic biogeographical region may be considered one of the leading edges of F. sylvatica. Our results also identify an inter-specific relationship between Q. petraea and F. sylvatica which negatively affects Q. petraea. The findings demonstrate that Q. petraea forests are being replaced by F. sylvatica forests in North-Western Iberia. These results not only confirm a previous biogeographical hypothesis but also provide new leads for forest management and conservation strategies under current and future climatic conditions.