Earlywood vessel lumen areas were measured in 72 consecutive tree rings in wood cores from oak (Quercus robur L.) trees in a maritime woodland. This anatomical time series was statistically correlated with climate data for the same time span. There was a strong dependence of earlywood vessel lumen area on rainfall between February and April, which reflects the role of water availability in vessel ontogeny. By inversion, earlywood vessel lumen areas can be used as a proxy to reconstruct spring precipitation beyond the archived weather records. Such information may be of value in the context of climate change.

• Wood anatomical features measured in dated tree rings have often proven to be of ecological value. However, little is known about the suitability and power of such measurements studied in a year-to-year basis as is done in dendrochronology. • The present work is based on a comparative analysis of 60 dated time-series of earlywood features of chestnut (Castanea sativa) grown in the climatic context of the Southern part of the Swiss Alps. • It has been shown that the earlywood vessel area is a suitable ecological indicator. This variable, although not very sensitive, contains environmental information that is different from that stored in all other ring-width and earlywood features we considered. The vessel size is mainly related to the temperature during two physiologically crucial periods for vessel growth: the end of the previous vegetation period (during reserve storage) and the onset of cambial activity (during cell division and vessel differentiation). • Our work shows that the mean vessel size of the ring-porous chestnut contains ecophysiological information that can be used for research in dendrochronology.

• The aim of this study was to identify the climatic signal contained in the earlywood vessel size of the ring-porous chestnut (Castanea sativa) and the physiological processes involved in the underlying mechanisms. • In order to assign the encoded signal to a specific physiological process, bud phenology and vessel formation were monitored along an elevation transect and chronologies of the size of the first row of earlywood vessels were retrospectively correlated with 40 yr of early spring temperatures. • The first vessels appeared in late April to early May, after encoding both a negative temperature signal in February–March (during tree quiescence) and a positive temperature signal in early April (at the time of resumption of shoot growth). • We hypothesize that February and March temperatures affect cambial sensitivity to auxin, preconditioning tree responses later in the season. Furthermore, April temperature is related to tree activation whereby new hormone production fosters vessel expansion.

Summary: This study addresses relationships between leaf phenology, xylogenesis, and functional xylem anatomy in two ring‐porous oak species, the temperate Quercus robur and the sub‐Mediterranean Q. pyrenaica. Earlywood vessel (EV) formation and leaf phenology were monitored in 2012 and 2013. Ten individuals per species were sampled at each of three sites located in NW Iberian Peninsula. EV areas measured on microcore sections were used to calculate the hydraulic tree diameter (D_h), in order to model relationships to phenology. Thermal requirements were evaluated using growing degree days (GDD). A species‐specific timing of growth resumption was found. The onset of EV formation and budburst were associated to a particular GDD in each species. The onset and duration of EV enlargement affected D_h (and EV size) in Q. robur, but hardly in Q. pyrenaica. The relationship between the timings of EV formation and xylem structure appears to be stronger for the temperate oak, whose larger vessels may result from thermal‐induced earlier resumption. In contrast, the sub‐Mediterranean oak would maintain a more conservative hydraulic architecture under warming conditions.