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Quercus pyrenaica is one of the most widespread species in pure or mixed stands across the Iberian Peninsula. It mostly occurs in mountain areas within the Mediterranean region, but also dominates forests along the boundary to the Atlantic northern Iberia. Given this role as a transitional species, the understanding of its behavior is of great relevance in a context of climate change.

We analyzed five Q. pyrenaica stands in northwestern Iberia, located along a transect of increasing elevation in the Atlantic/Mediterranean biogeographical boundary. Tree-ring chronologies were obtained by measuring the earlywood vessel size, and the radial increment on a representative number of trees. Three variables were used, namely (i) the hydraulically-weighted diameter for the earlywood vessels in the first row (D-H-rl), (ii) for vessels outside this row (D-H-nrl), and (iii) the latewood width (LW). Variable chronologies were compared to monthly meteorological records, and to the North Atlantic Oscillation index (NAO).

LW variation was controlled by water availability during late spring-early summer all throughout the study area, but differences among sites were mostly driven by the presence of abrupt growth changes linked to forest disturbance regime. In contrast, earlywood responses were modulated by the topographic position. The three low-elevation sites, located at windward of a central mountain range, were related to environmental conditions during quiescence, whereas the two others responded at the moment of wood formation; DH-rl was more controlled by climate than DH-nrl. The close association between NAO and vessel size was in accordance with the elevation gradient.

Our results showed relevant signals related to micro-, meso-, and macroclimatic conditions, and pointed out to the existence of cause-effect relationships. Therefore, the combined time-series analysis of earlywood vessels and latewood increment is a powerful tool to understand the ecological behavior of marcescent oaks in marginal populations, which often dominate transitional areas.