Standing under the sprawling branches of the largest cork oak tree in the world, it's obvious where The Whistler Tree gets its nickname. Songbirds perch in the canopy of the Portuguese Monumental Oak, which was planted in 1783 and is now approaching its 240th birthday.
But soon gangs of men brandishing axes will be coming for it amid the sweltering heat of the Portuguese summer. They move from tree to tree, swinging and striking their one-handed, fan-shaped axes with a precision built over many years.
They are not here to fell these ageing oaks, but to harvest a prized resource from the trunks.
First they cut deep into the bark, then twist their axes and use the handles to prise long cork planks from the ancient oak trees that cover Portugal's biggest province, Alentejo.
It's a skilled and demanding job in the hottest months of the year, when temperatures reach more than 40C. If they cut too deep, the tree will be damaged, risking future harvests and ultimately the tree's survival. Too shallow and the planks aren't good enough to make the finest cork stoppers for the wine industry.
Each group of cork cutters work together quickly and carefully to remove just the right amount of bark, stacking the planks for collection before moving on to the next tree. Each newly-cropped tree looks as though it has had orange socks pulled up its trunk, and the year of the harvest daubed on it in white paint.
Pause for too long in the dappled shade beneath the rows hundred year-old cork oak trees (Quercus suber, or sobreiro in Portuguese), and the men with axes will quickly disappear into the distance. It will be nine years before they return to this part of the forest to harvest the sobreiro again.
In the meantime, the trees will quietly serve another, larger purpose. As the world clamours to reduce the amount of carbon dioxide (CO2) in an effort to slow climate change, the cork oak tree is standing tall.
Like all trees, cork oaks absorb CO2and through photosynthesis lock away carbon for many years in their roots and branches. Planting forests is a commonly used approach to offset carbon emissions by polluting industries, but when the trees are harvested they are usually cut down and much of their stored carbon is released back into the atmosphere.
But cork oaks are one of the few commercial forests not felled for harvesting. This means the cork tree's carbon storage capacity keeps increasing during the 200 or more years the trees can live.
Around 13 billion cork stoppers are produced globally every year for use in bottles of wine (Credit: Alastair Leithead)
Most of the carbon remains locked in the tree as it continues to grow. Although cork products contain some of the absorbed carbon, they can have a long life after being cut from the tree. Cork can be recycled and is slow to break down even when discarded.
"They are a carbon sink," says António Rios de Amorim, the fourth generation chief executive of the 150 year old Amorim cork empire – the world's largest producer. "For every single tonne of cork produced we are talking about 73 tonnes of CO2 that are captured."
His figures come from a report by consultants PricewaterhouseCoopers, commissioned by Amorim, which also claims 392g (13.8oz) of carbon is sequestered by every cork stopper. A separate study on cork insulation boards found it was the only material with a negative carbon footprint.
You might also like to read:
- The forest tended by an elusive giant
- The tree that conquered our cities
- Why the world's tallest trees are dying
In fact, the Portuguese cork association APCOR claims cork forests retain and store 14 million tonnes of CO2 every year, while scientific research in Portugal by the Instituto Superior de Agronomia (ISA) has confirmed that cork products are indeed carbon negative – storing more carbon than is used in their production. But calculating the total carbon footprint of anything is complicated – transport, processing and the fate of every product made from that resource must be considered.
So, can making better use of cork do more to help the climate?
The wild species that inhabit these areas no longer have alternatives, and so will be at risk if the Montado land-use is changed – Helena Serrano
Cork was first used by the Egyptians and Persians for fishing floats, then by the Ancient Greeks and the Romans who also made sandals and used it to seal amphorae jars. It wasn't until the late 1700s before glass bottles became the wine vessel of choice and their intimate relationship with the humble cork stopper began. Today there are 2.2m hectares (8,494sq miles) of cork forests growing around the world, producing around 13 billion cork stoppers every year, which are used in about two thirds of the roughly 20 billion bottles of wine sold annually.
In recent decades, the material has faced increasing competition from screw tops and synthetic stoppers, but the cork industry has been mounting a concerted comeback in the past few years. Cork oaks grow for an average of 25 years before their first bark can be removed and then it takes a further two harvests – 18 years in total – before it can produce cork of good enough quality to be used as a stopper.
The trees are endemic to the Mediterranean bowl on the edges of Europe and North Africa, but are particularly concentrated on the Iberian peninsula. Portugal produces around half the world's cork and grows a third of the world's cork trees – most of it amid the rolling hills of Alentejo – the country's largest, poorest and least populated province.
The savannah-like plains, known as montado in Portugal and dehesa in Spain, are a biodiverse landscape of cork, holm oaks and olive trees. It is a place where black Iberian pigs snuffle acorns, deer and wild boar roam the shrub, alongside cattle, sheep and goats that graze the interwoven pastures. It is also a fragile landscape – an ecosystem which sustains the endangered Iberian lynx, and threatened Imperial and Bonelli Eagles. But it's also a man-made landscape which has thrived for hundreds of years on the proceeds of cork.
"The different levels of the Montado – trees, shrubs, grasses – are unique in providing the diversity needed for food and habitat for animals and a niche for some plant species," says Helena Serrano, from conservation organisation Portuguese Ecological Society. "Montado is considered a High-Nature-Value-Farmland with high biodiversity and it is an ecosystem that has been shaped by humans for hundreds of years for cork, acorns for animal feed, and extensive pastures. This traditional type of management is not disruptive to biodiversity, which has adapted and endures because of it. The wild species that inhabit these areas no longer have alternatives, and so will be at risk if the Montado land-use is changed."
António Freitas has worked across the Alentejo cork forests for more than fifty years and can immediately spot the best planks by their weight, thickness and quality. "See the gaps in the cork and the fissures at the edge – that is not good," he says. Instead they are looking for undamaged, lower density cork, which means it will have greater elasticity and create better stoppers, Freitas explains. Researchers have recently found that they can detect the quality of cork a tree might produce using a method known as epigenetic analysis. This looks for subtle changes along the DNA molecule that later how the genetic information is processed by the tree's cells.
The bark is cut from cork oaks in planks using axes every nine years, but it can take 25 years before a tree is mature enough to be harvested (Credit: Alastair Leithead)
Freitas, however, can pick out the cork of highest quality almost intuitively, thanks to decades of experience. He knows which planks will be crushed and composited to make insulation, which will be perfect for the discs that make up the mushroom-shaped champagne corks, and which will be suitable for sealing the world's finest wines.
Freitas has seen many changes over the decades: skilled workers are becoming scarce as the countryside population has emptied into towns and new technology has arrived. But the wages of cork-cutters remain high. "It's the best paid job in agriculture and forests in Portugal," says João Sobral, a cork buyer. "A good cutter is a guy who doesn't hurt a tree, who extracts the most cork with the fewest cuts and takes a lot of cork in a day."
New technology in the form of a small smart chainsaw is starting to help with the shortfall in skilled workers. The saw has moisture sensors which only allow the blade to cut the drier cork bark and not to reach the living tree. Then the men with axes move in and can remove the planks more quickly and efficiently. "It's still work in progress," says Sobral, "This is not the end of it. This is still a prototype for us."
And there have been other breakthroughs in recent years too. Artificial intelligence, combined with new technologies emerging from materials science, also play an important role on the production lines and factory floors of a sprawling industrial complex just south of Porto.
"Here we make the very high end natural whole corks," says Carlos de Jesus, the head of marketing and communications for Amorim Cork. "We're looking at those strips of the very best cork planks, and we are looking at a series of cylindrical holes punched horizontally, never vertically, but horizontally along the cork plank."
These have the highest value, but the off-cuts and poorer quality of cork planks are also granulated and compressed to make cheaper stoppers and a whole range of other products.
Cork oaks grow for an average of 25 years before their first bark can be removed and 18 years in total before it can produce cork of good enough quality to be used as a stopper
One of the cork industry's biggest challenges has been to tackle concerns surrounding cork taint in wine. This occurs when fungi grow in minute airspaces within the cork and produce a harmless but unpleasant tasting chemical called trichloroanisole. If the trichloroanisole accumulates in the cork, it can contaminate the wine in a bottle and give it a distinctive musty "corked" taste.
The fungi generate this chemical from environmental contaminants known as chlorophenols – which in the past have been used in biocides used to control pests. Efforts to cut their use have helped, but chlorophenols have also more recently been found to occur due to a reaction between naturally occurring phenol and chlorine from cleaning and sanitation products that escape into the environment in waste water.
This has lead cork producers to develop improved methods of testing for trichloroanisole. Treating cork with supercritical fluid technology – the same method used for decaffeination – cleans corks of contamination and has boosted confidence in natural stoppers.
But the cork industry has also been searching for other ways to make more use of the material, which would then have knock on effects for the climate by allowing cork forests to increase in size and flourish.
Cork has many useful properties: it's a light, durable, impermeable, elastic, compressible and fire-resistant material which can be used by the construction industry for thermal or acoustic insulation and for absorbing vibrations. It's also used to make noticeboards and ornaments, shoes, flooring and insulation.
Yet it is finding ever more inventive uses, such as a base for artificial sports field turf as an alternative to synthetic fillings. It can also be compressed into shock-absorbing blocks and expansion joints for concrete bridges.
The montado landscape in Portugal and Spain has been maintained by careful management over the centuries and supports endangered species such as lynx (Credit: Alastair Leithead)
Cork is now being used in electric cars to protect batteries from heat and vibration, and its heat absorbing properties have protected space rockets from the Space Shuttle programme to Nasa's new Artemis spacecraft which has Portuguese cork in its nose cone. Waste cork dust is also burned as an alternative to fossil fuels to power some of the production lines.
Cork stoppers can also be collected, recycled, granulated and put to many of these different uses, with cork recycling programmes having been introduced across the world, including Recorked in the UK and ReCORK in the US.
Material scientists in Amorim's iCork lab are also experimenting by combining cork with rubber and bio-based or biodegradable polymers to develop other new uses including injection moulding.
"It's [got] the advantages of cork mixed with the advantages of plastic," says Álvaro Batista, who is prototyping new materials to replace the foil top on wine bottles, to allow rolls of sheet cork and to be used for injection moulding. We receive a lot of materials from other industries...to mix with cork to improve the properties of cork and of course reusing raw materials...to help to solve sustainability problems," he says.
The cork oak is Portugal's national tree, protected since 1209, and cutting one down or even pruning it without permission can land you with a hefty fine.
Twenty years ago the symbiotic relationship between ecosystem and industry was threatened by the rise of plastic corks, aluminium screw caps and the dreaded cork taint. The problem now is having enough cork – demand is growing and the value of Portuguese exports reached an all-time high of €1.13bn (£921.29m/$1.09bn) in 2021.
The biggest threats to the trees are intense wildfires and drought which are increasingly a risk amid the extremes of climate change. The warming climate also changes the threat posed by disease from moths, beetles and weevils, which burrow into the bark carrying fungi and bacteria.
So while cork oaks could help to combat climate change, they are also at risk from it. Without incentives or a viable carbon market other threats from cattle farming and urban expansion could also cause the valuable montado landscape where the oaks grow to dwindle.
The Whistler itself is a sign of just what could be lost. The men wielding the axes left it untouched this year, but its next trim will be the tree's 21st of its long life. They estimate this could surpass the tree's own record-breaking 1991 yield of more than 1,200kgs of cork – enough for 100,000 stoppers. Locked inside them will be the carbon from nearly a decade of growth.
* Alastair Leithead is a former BBC foreign correspondent now attempting to live off the grid in Portugal on a 17 acres (seven hectares) plot of land that includes cork oaks, olive trees, pine and eucalyptus. He also writes a blog about wine in Portugal.
Join one million Future fans by liking us on Facebook, or follow us on Twitter or Instagram.
If you liked this story, sign up for the weekly bbc.com features newsletter, called "The Essential List" – a handpicked selection of stories from BBC Future, Culture, Worklife, Travel and Reel delivered to your inbox every Friday.