Cork-ring width responds to climate depending on local site dryness

Cork, the bark of cork oak (Quercus suber), is a major non-wood forest product in the Mediterranean Basin, but its future production will depend on forecasted warmer and drier conditions. The comparison of topographically contrasting sites, subjected to different soil water availability, may be used as an analogous situation to projected aridification. We quantified cork and wood production and cork responses to climate variability and drought indices at dry (slope ridge) and wet (valley bottom) sites located in a relict cork oak population in north-eastern Spain. We also calculated intrinsic water-use efficiency cork (iWUE) by analysing C isotope ratios (delta 13C) in annual cork samples. In the wet site, tree- and cork-rings were wider than in the dry site, where cork delta 13C and iWUE were higher, particularly during moderate droughts. Cork and radial growth covaried in both sites and the slopes of their linear regressions were similar between dry and wet sites. In the dry site, cork width increased as June-July soil moisture did, whereas cork iWUE decreased as May soil moisture increased. Moist soil conditions in the prior October and March also enhanced cork growth at both sites, whilst wet prior winter conditions reduced cork iWUE at the dry site. Our findings show how cork production depends on local soil water availability. Cork delta 13C can be used and combined with wood information to trace the physiological status of cork oak trees in response to drought stress.