Effect of climate on cork-ring width and density of Quercus suber L. in Southern Portugal

Cork rings have been extensively used in dendroecological studies of the Mediterranean evergreen cork oak (Quercus suber L.). Through measurements of cork-ring width only, strong relationships have been found between cork-ring widths and climate parameters. To our knowledge, cork-ring density, which is an important cork quality attribute, has never been used in any dendroecological study to explore physiological responses of the cork oak to climate change. In this study, we measured cork-ring width and density over 50 years (1962-2013), corresponding to five consecutive cork harvests, and analyzed their inter-annual fluctuations in eight trees from two different sites, a wetter peneplain area (Benavente) and a drier mountainous area (Grandola). Our results revealed a statistically significant correlation between cork-ring width and density (p < 0.05) at both sites; however, it was negative (r =- 0.261) and relatively weaker at Benavente, and positive at Grandola (r = 0.410). Moreover, a direct relationship between cork density and spring-summer temperature was found only at Grandola. Here, cork growth sensitivity to inter-annual variability of precipitation and temperature in summer-autumn of the current growth year (p < 0.01) was the highest and, respectively, positive and negative. We conclude that in drier Mediterranean environments, summer drought stress might negatively affect both cork growth and density, by changing tree physiological processes involved in cork formation, resulting in narrower and denser cork rings. Considering scenarios of future climate change effects in Southern Europe, in particular the longer drought periods and rising temperatures, it seems that both cork yield and cork density will be affected, with implications on cork quality.