While travelling through the southern Alentejo, you’ll pass mile upon mile of cork oak groves – so it may come as no surprise to learn that the district provides around fifty percent of the world’s entire supply of cork. It has been an important Portuguese export since the late nineteenth century and a major crop for over seven hundred years.

Cork (quercus suber) consists of a layer of spongy cells called phellogen that appear under the bark during the first year of growth. The cells grow radially outwards to form a durable, impermeable material with excellent thermal properties – it’s ideal to guard the tree against pests, fire and extremes of temperature, and also ideal as a material for humans to exploit. Importantly, the cork tree is also able to regenerate itself when a layer of cork is removed throughout the tree’s life (usually over a hundred years) – and each regenerated layer is thicker than its predecessor. Using a curved axe, cork farmers are therefore able to strip away rectangular layers of cork every nine years, the time it takes for the cork layers to be 4–6cm thick – ideal for wine stoppers. The world’s most productive cork tree, the 230-year-old Whistler Tree, in the northern Alentejo, has produced enough corks to stop up 100,000 bottles from a single harvest.

Cork trees cannot be harvested until they are at least 25 years old, and as a result, cork groves tend to be superb habitats for wildlife. Unfortunately this self-sustaining crop is under threat because of the growth in plastic and screw-top wine stoppers, forcing many farmers to rip up the cork groves for more viable crops, and destroying ancient habitats in the process.

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