How resilient is Quercus suber L. to cork harvesting? A review and identification of knowledge gaps

Cork oak (Quercus suber L.) woodlands are ecologically sensitive and human-supported areas which strongly rely on the high market value of cork for their sustainability. Cork harvesting is a forestry practice specific to this Mediterranean evergreen oak, whereby the bark (cork) is periodically removed from stems and branches over the whole tree lifetime. This practice is a strain factor for the tree, particularly because it is performed during the most stressful time of the year, and also because stripped oaks become more vulnerable to pathogens and environmental hazards such as wildfires. This review gathers and analyzes information about tree responses to cork harvesting. Cork removal leads to phellogen destruction and subsequent regeneration. Upon each harvesting, the tree faces important losses of water, released from the suddenly exposed living cells, and of cork. Immediate responses relate to stem healing mechanisms, leaf stomatal closure and prompt production of new cork layers by the restored phellogen, the latter actually representing the most evident response of Q. suber to cork harvesting. Further strategies to compensate for water and cork losses are insufficiently understood, but possibly include prolonged reduction of leaf transpiration (and consequent reduction of photoassimilation), increased water uptake from the soil, use of reserves and changes in whole-tree energy allocation to support cork regeneration. Cork stripping by unskilled workers may leave wounds on the tree, which eventually hinder cork regeneration and lead to premature tree death. The relationship between current cork harvesting practices (e.g. harvesting intensities or periodicity) and tree resilience is not quite established. Higher stripping pressures seem to trigger more pronounced or drastic tree responses and/or to enhance their decline. A major conclusion is that the available information is still too scarce to allow for a consistent evaluation of the effects of cork harvesting on cork oak and on its associated woodlands. So far, few studies have addressed how tree resilience relates to harvesting intensities, a crucial issue to enable improved tree and stand management strategies and to ensure adequate conservation of these sensitive forest systems.