Variation of cork porosity along the stem in harvested cork oak (Quercus suber L.) trees

Abstract

Cork porosity, due to lenticular channels, varied along the stem of Quercus suber L. Lenticular channels’ area, rather than their number, decreased upwards along the stem. Area decrease was observed regardless of tree size and of its intrinsic porosity. The cork of Quercus suber L. is radially crossed by lenticular channels, defining cork’s natural porosity. It has been suggested that porosity decreases from the stem base upwards, but studies on such variation have not yet been presented. Three main research questions were addressed: (i) how large is the variation of cork porosity upwards along the stem; (ii) how does porosity variation relate with porosity traits, namely the size and number of lenticular channels and (iii) how much does porosity vary with stem height and between trees. We set up a study at tree level to quantify the porosity of cork samples from fixed stem heights. Our statistical modelling approach was based on linear mixed-effects models, given the nested structure of the data. In the model fitting, porosity was described as a function of tree stem height, while the random effects explained the source of variability introduced by different tree size (as given by stem diameter at breast height, Dbh) and porosity (as given by intrinsic porosity, CPbh). The lenticular channels’ area rather than their number consistently decreased up the stem. The area proportion of the lenticular channels in the cork tissue (i.e. porosity coefficient) decreased by about 1.4% per metre upwards along the stem, regardless of tree size and of its porosity. Our findings highlight that the lenticular channels’ traits greatly vary among trees, much more than within-tree, which may be an important clue to predict variations in cork properties, for decision-making on cork oak management.