Chronologies of earlywood vessels and latewood width disentangle climate drivers of oak growth in a mild oceanic region

We obtained tree-ring chronologies for pedunculate oak along an altitudinal gradient of four sites in NW Iberia, which covers most of the species range within the region. Trees grow under Atlantic climate, with a rainfall maximum in autumn-winter, and a minimum during summer, but lacking a remarkable drought. Chronologies included several earlywood anatomical features and ring widths, which were compared to each other. Latewood width and a subset of six earlywood variables were selected for further analyses, considering the period 1954-2003. The lowest site considerably differed from the rest of the gradient due to its milder conditions. Earlywood and latewood were unrelated at the three upper sites, and both compartments responded to different climatic factors. Hydraulic conductivity determined by vessel size was used to describe tree performance, which was modulated by temperature along the gradient. We hypothesize that the main processes involved are the timing of earlywood formation and carbohydrate dynamics. Water availability during late spring and summer affected latewood width, but only at low elevation. This paper illustrates the complexity of analyzing climate-growth relationships in oceanic areas under the absence of a prevailing limiting factor, while providing a feasible explanation of potential mechanisms involved.