Earlywood vessel area of Quercus pyrenaica Willd. is a powerful indicator of soil water excess at growth resumption

We selected two sites dominated by the sub-Mediterranean oak Quercus pyrenaica Willd. close to its distribution boundary in northwestern Iberia, within a mountain region with a high winter precipitation. The sites differed in their soil water regime, corresponding to the edge of a peat bog, and to a moderate slope. We obtained tree-ring chronologies of total ring width (RW), and mean earlywood vessel area (MVA); their responses to climatic factors were compared for the period 1945-2002. RW presented a higher chronology quality than MVA, but was rather independent of climate, probably because of the presence of recurrent growth reductions. In contrast, MVA was closely related to precipitation during April and May, whereby a high water availability was coupled to smaller vessels. We found remarkable differences between the climatic signal of both stands, as trees growing on the peat soil responded later and with considerably lower intensity. We hypothesize that spring waterlogging causes that the response at the wettest site occurs only when soil desiccation begins, which results in a delayed climatic signal, and also lower intra- and inter-annual variation due to more homogeneous conditions. Climate-growth relationships at the driest site were mainly associated with the first row, whereas it is vessels expanding later in the season that show this relation for the moist site. Our results confirm that MVA chronologies are reliable proxies of both regional and local climatic conditions, but only a careful optimization by selecting vessel subsets does provide a complete view of their potential.