Is Cork Growth a Reliable Proxy for Stem Diameter Growth in Cork Oak (Quercus suber L.)? Implications for Forest Management under Climate Change in Mediterranean Regions

Abstract

Cork-ring widths have been extensively used in dendroecological studies assessing the relationship between cork growth patterns and climate (precipitation and temperature). Generally, cork growth is assumed as a proxy for stem diameter growth to address cork oak (Quercus suber L.) growth sensitivity to climate and cork yield modeling. Cork growth represents a large part of stem radial increment in this species due to the enhanced activity of phellogen when compared to the cambium activity; thus, similar inter-annual variations of cork-ring widths and tree diameter growth might be expected. However, so far, the influence of rainfall and temperature on stem diameter growth has scarcely been addressed; moreover, it is still not clear whether tree size relates, and in what way, to the variations in radial growth of cork and stem diameter and whether these reflect (proportional) quantitative variations in stem basal area growth. In this study, we computed the annual growth of cork and of stem diameter at breast-height in data series of 47 trees, from 2000 to 2012, corresponding to a full cork production cycle. Results showed a tight link between cork-ring width and stem diameter growth indices. However, while cork growth strongly correlated with climate conditions in autumn–winter prior to the growing season, stem diameter growth correlated with climate conditions of the current growing season, and, more importantly, it was tree size-related. The extrapolation from cork-ring increments to stem basal area growth is likely to progressively underestimate tree growth and biomass increment in larger cork oaks and to further bias it due to climate change effects in the Mediterranean region.