Cork trees The tree forms a thick, rugged bark containing high levels of suberin. Over time the cork cambium layer of bark can develop considerable thickness and can be harvested every 9 to 12 years to produce cork. The harvesting of cork does not harm the tree, in fact, no trees are cut down during the harvesting process. Only the bark is extracted, and a new layer of cork regrows, making it a renewable resource. The tree is cultivated in Spain, Portugal, Algeria, Morocco, France, Italy and Tunisia. Cork oaks are considered to be soil builders and their fruits have been shown to have useful insecticidal properties. Cork oak forests cover approximately 25,000 square kilometres in those countries (equivalent to 2,277,700 hectares). Portugal accounts for around 50% of the world cork harvest. Cork oaks cannot legally be cut down in Portugal, except for forest management felling of old, unproductive trees, and, even in those cases, farmers need special permission from the Ministry of Agriculture.