The tree forms a thick, corky bark. Over time this bark can develop considerable thickness and this is harvested every 10–12 years as cork. The harvesting of cork does not harm the tree and a new layer of cork regrows, making it a renewable resource. The tree is widely cultivated in Spain, Portugal, Algeria, Morocco, France, Italy and Tunisia. Cork oak forests cover approximately 2.5 million hectares in those countries. Portugal accounts for 50% of the world cork harvest. Cork oaks cannot legally be cut down in Portugal, except for forest management felling of old, unproductive trees.