Think about the last bottle of wine you drank. Was it sealed with a natural cork? A synthetic plastic closure? A screw top cap? Where does cork come from, and what’s with all the buzz about cork trees being endangered? To investigate, we ventured off to Alentejo, a rural region in south-central Portugal, and one of the world’s largest sources of cork oaks. All told, Portugal produces about 75% of the world’s cork, and about 75% of this goes into wine bottle stoppers. About 33% of all cork trees grow in Portugal, and 95% of these are in the Alentejo region.

At Herdade da Maroteira, Philip Mollet guided us on a tour of his 540 hectare farm. Much of the land is forested, with approximately 2/3 covered by cork forest, 1/3 covered by stone oaks, and some cleared land for vineyards and livestock. Mollet is a 5th generation farmer, whose family originally hailed from Britain. As the story goes, in the beginning there were two brothers who were on their way to Australia. They stopped in Porto to make repairs on their boat and look for cork stopper resources. The brothers ended up traveling to Alentejo, where they found this particular cork farm. One brother continued on to Australia, while the other stayed in Portugal. Later on, the brothers arranged for a cultural swap, with one brother sending eucalyptus to Portugal and the other brother sending cork acorns to Australia. However, the acorns that were sent were sterilized—nothing like a little sibling rivalry to help foment family feuds!

Unusually, a cork oak has two layers of bark—the cork, which is the outer layer, and the inner bark. After the cork has grown to sufficient thickness, you can strip it by hand using an ax with a curved blade. To date, no mechanical harvesting method has been developed, and it requires skill and experience to harvest cork without damaging the tree. Workers must be able to gauge the cork sheet’s thickness, and not cut too deeply into the tree, or they will cause irreversible damage. Done properly though, cork is a renewable resource, and a healthy tree will produce cork almost indefinitely, or until its life expectancy of 600 years is up.

Cork can first be harvested at about 25 years of age, but this virgin cork is considered low-quality and is worth about the price of the stripping. Thereafter, cork is usually harvested every 9-10 years. To keep track of the last harvest, each tree is marked with a number to indicate the last year it was stripped. For instance, a “6” means the tree was last stripped in 2006.

When do you harvest cork? As it turns out, the cork layer usually sticks to the tree like glue, but there is a narrow 3-week window each year when you can strip cork. As the weather moves out of cold temperatures into warm ones, the trees “sweat” and it is possible to separate the cork from the inner bark. The timing of this window varies from region to region, and depends on humidity as well.

At Herdade da Maroteira, cork harvesting takes place for about ten days in June, and is done primarily by a team of 12 men with axes. Any more than that, and it becomes difficult to supervise workers and make sure they are doing the job properly, said Mollet. They are supported by a back-up team of employees who drive the tractors, paint numbers on the trees, and stack the cork sheets.

The quality of cork depends on the thickness and density of the cork sheets—the greater the better. Cork is traded in units called arroba, which is equivalent to 11.5 kg in Spain and 15 kg in Portugal. Top-notch cork, with high density and thickness, is sold for €40-50/arroba, while lower quality cork might be sold for as little as €8/arroba. Mollet’s cork is middle of the range, with high density but average thickness, and sells for about €18/arroba.

What about all the rumors of a worldwide cork shortage? According to Mollet, cork production is fully sustainable and there is no truth to these claims. “The problem is the wineries,” he said. “Everyone’s looking at cost.” He explained that natural corks cost 28¢ each, while lower-grade 1+1 conglomerate corks and powdered corks cost 8¢, and a 100% conglomerate cork costs 4-5¢. Meanwhile, plastic corks cost 3-4¢ each. Mollet lamented the rise of synthetic closures and said, “Look, this cork is ecological, biological and natural. If we lived in the US or the UK, we’d be marketing this product heavily. But the Portuguese government has their hands in their pockets. So, winemakers are moving to plastics to cut costs, not watching quality and not knowing what goes into the wine.”

In recent years, the demand for cork and value of the raw product has fallen sharply. At the market’s peak about ten years ago, Mollet was grossing about €120,000 for each cork harvest, but that figure has fallen to €42,000 today. Simultaneously, stripping costs total €30,000. And that does not even account for the year-round costs of maintaining the cork forest and fighting the coraebus undatus beetle that infects cork trees. It is a poor time indeed to be in the cork-growing business.

To verify Mollet’s claims, I did some research on the sustainability of Portugal’s cork forests on my own. The cork oak is indeed listed on the World Wildlife Fund website as a priority species, but not for overharvesting. As stated by the WWF, “Harvesting of cork for use in wine stoppers is entirely sustainable,” but “increased market share for alternative wine stoppers could reduce the value of cork oak areas, leading to their conversion or abandonment.” In other words, not using cork stoppers will hurt the continued preservation of the trees.

Mollet realized long ago that it was risky to be dependent on the vagaries of the cork market, and decided to diversify his farm into other lines of business. Today, the farm also raises pigs, produces wine, olives, olive oil, honey and has two agrotourism guesthouses for visitors. The pigs are actually owned by Spanish livestock farmers, and are sent to Mollet’s farm in the fall to gain weight. These prized Iberian pigs arrive in October weighing approximately 80 kg, and leave in March at 180 kg. Mollet is paid based on the amount gained by each pig, which comes to about €120 per pig. “I make sure that they are happy and comfortable,” he said. “We keep them as calm as possible, in a stress-free environment.” The pigs are rotated from section to section of the forest, as they feast on acorns dropped by the oaks. They will later be processed into prized jamon iberico and other meat products, for a total value of around €3,000 per animal.

Why harvest the cork if it is a money-losing prospect? Mollet paused to reflect on the volatile prices and pests threatening Portugal’s cork oaks. “People used to say, ‘Money doesn’t grow on trees,’ and we would respond, ‘Actually, it does.’ But now I’m making just enough money to keep the tractor running. Fortunately, we diversified into wine, but the whole cork industry is in trouble, and if something doesn’t change, the forest will die.” Without protection, it is likely that the forest will be converted to other uses and the trees logged away.

Did your wine purchase for tonight’s dinner just get a little more complicated?