Cork: the bark that grows back

Almost everything we take from a tree, we take once. Timber, resin and fruit are removed and the tree is either felled or left to recover for a season. The cork oak is different. Its thick outer bark can be cut away entirely, down to a living inner layer, and within a decade the tree has replaced it. A healthy cork oak may be stripped fifteen times or more over a life that can pass two hundred years.

The species, Quercus suber, grows only around the western Mediterranean, where hot dry summers alternate with mild wet winters. Portugal holds roughly a third of the world's cork oaks and produces about half of all the cork traded each year; Spain, Italy, Algeria, Morocco and Tunisia account for most of the rest. In Portugal the open woodland in which the trees grow is called montado. It is not a plantation. The oaks stand far apart, sunlight reaches the ground between them, and cattle, sheep and black pigs graze among the trunks.

No bark is taken from a young tree. The first cut is made at about 25 years of age, and what comes away is coarse and cracked. This virgin cork is ground up for flooring, insulation board and garden products. A second stripping nine years later produces material that is better but still uneven. Only at the third harvest, when the oak is past forty, does the bark come away in smooth slabs thick enough to be punched into wine stoppers, the use that pays best. Nine years must pass between every stripping, and the interval is written into Portuguese law.

Stripping is done between May and August, when the tree is growing fastest and the bark parts most easily from the trunk. There is no machine for it. A worker swings a long-handled axe, listens for the change in sound as the blade reaches the inner bark, and levers the slab free. Cut too shallow and the plank is wasted; cut too deep and the growing layer is injured, so the bark may return thin, or not at all. The skill takes years to learn, and strippers are among the best paid agricultural workers in Portugal.

What makes cork worth the trouble is its structure. Rather more than half of its volume is air, held in tiny sealed cells, about forty million of them in a single cubic centimetre. Walls of suberin, a waxy substance, keep each cell watertight. The result is a material that floats, springs back when squeezed, resists rot, and conducts heat and sound poorly. Those properties have carried cork a long way from the vineyard. It lines the walls of recording studios, sits beneath railway track as vibration padding, and has been used in the heat shields of spacecraft.

The trade has had a difficult generation. Screw caps and moulded plastic stoppers took a large share of the wine market, and some growers pulled out. The reply has been environmental rather than commercial. Montado supports lynx, eagles and a long list of plants found nowhere else, and the woodland holds carbon in its soil as well as in its timber. Industry bodies also argue that a stripped oak absorbs considerably more carbon dioxide while regrowing its bark than one left alone, a claim that independent researchers regard as plausible but difficult to verify.