The sound of the first crack stops the patrol dead. Halfway up the 3,407 metres of the Nyiragongo volcano, the rangers fan out in the direction of the chopping noise, leaving the path of shattered lava for the dense forest, assault rifles at the ready.

For a few minutes, the silence is broken only by the sharp reports of blade on wood. Then a confusion of voices, followed quickly by the sound of bodies crashing through the undergrowth towards us. The one ranger remaining on the path has no rifle so instead draws a machete. There is no way to know who or what is coming.

The footfalls veer away from us and down, then the first of the rangers emerges with a prisoner on the path above. Lying on his back on the sharp black rock is a barefoot young man; his two friends have escaped but his arms are being bound with his own belt. He is one of what are known as "carbonisateurs", the footsoldiers of a huge and lucrative illegal trade in charcoal that is threatening the survival of Africa's oldest national park.

Download the new Independent Premium app Sharing the full story, not just the headlines

It's a criminal network that thrives on the extraordinary misery stretched out among the valleys of Eastern Congo below, where "choppers" like this youngster are joined by "cooks", who feed the old-growth trees of Virunga National Park into earthen ovens, and the human "mules" who transport the 80lb sacks of charcoal on their own backs. The trade generates an estimated £20 million a year, money that goes to the corrupt officials who turn a blind eye to it and the renegade government soldiers and rebel armies who sell their protection.

These cutters cannot have been working alone and the rangers want to know where the kiln is. The forlorn man, who refuses to give his name, is clad in a filthy sweat-stained shirt with "Rooney" on the back. He gestures up the slope to where the clouds meet the volcano's rim. The march begins again, a steep climb up a channel burnt through the forest by the rivers of lava that poured from Nyiragongo when it last erupted in 2002.

Once again there is a signal for silence and lead ranger Bosco Hakizimnana veers off the rock and into the trees, running in a crouch. He emerges into a "clear-cut", an acre in size, with panicked people running in all directions.

At the centre of the clearing is the smouldering black dome of the kiln, standing above head height. Beneath the mound of packed, burnt earth, the trunks and branches of the protected forest are being slowly carbonised.

Of the dozen or so people rounded up by the rangers, there is only one man – the "cook" who was feeding the oven. The rest are women and children, dressed in rags – the human mules who will carry back-breaking loads two hours' trek down the mountain to the road below. Wandering in circles in the smoke is a child who can be no more than eight years old with a baby bound to his back. Many of the women are soldiers' wives, sent by their husbands, who know that the park authorities cannot arrest them.

The rangers smash the sides of the kiln and the dirt gives way to reveal the skeleton of tree limbs below. As oxygen reaches into the oven, flames consume the wood giving it the appearance of a funeral pyre. There is no time to mourn lost money or habitat. Hakizimnana is concerned that those who have escaped have gone to fetch their protectors. "We may be ambushed on the way back down," he warns.

The mules in plastic sandals are loaded with the "evidence" – lumps of charcoal that lie scattered in all directions, turning the forest floor into an open hearth.

The descending path is punctuated by dozens more clearings, like angry black scars on the face of Nyiragongo. Here and there are signs of better times, wooden steps dug into the hillside, and in one of the openings the crumpled wreckage of a few chairs. They are all that remains of what used to be a resting station when the volcano would attract tourists who paid more than £100 a time to climb to the rim and camp in view of the lava lake inside. The route has been closed since 2007, when the carbonisateurs overseen by the FDLR – a rogue army of Rwandan Hutus made up of veterans from the genocidal Interhamwe militia who fled across the border – moved into the area.

When the rag-tag caravan finally emerges at the rangers' station at Kibati the women are made to sit in a circle. Hakizimnana delivers a stern lecture telling them that the park "belongs to all of Congo" and that they must find other ways to feed their families. He finishes by asking them what they will do "when the forest is gone?"

this is a question that haunts Emmanuel de Merode. The boyish-looking 38-year-old is the director of Virunga National Park, probably the hardest job in conservation anywhere in the world. "We are losing as much as 15 per cent of the forest every year so we don't have a lot of time," he says.

And yet at first glance he seems to work in a veritable Garden of Eden. The park headquarters at the hill station of Rumangabo look out over patchwork slopes of banana thickets that seem to shine with fertility. The distant horizon is punctuated by the emerald cones of more than half a dozen volcanoes. Blue monkeys call to each other from the treetops and larger black and white colobus apes loll in the branches around de Merode's office, while patrols of chimpanzees occasionally pass through the grounds.

The park has an almost surreal cast of rare and endangered animals – from the okapi and forest buffaloes to the hippos of Lake Albert and the savvana elephant herds of Rwindi. It has more endemic species than any other park in Africa and more bird, mammal and reptile species too. And in the foothills of Virunga's volcanoes, where the mist meets the tree line, the park's most spectacular residents can be found. Half of the more than 700 mountain gorillas left in the wild live in the Virunga range.

"There is no other park in the world like this," de Merode says with conviction. And certainly none that faces the same existential threats.

The 30,000 square miles of what was established as the Albert Park in 1925 also mark out a corridor of unrivalled human misery. The reserve is both hideout and theatre of operations for a host of rebel armies, including the notorious FDLR and the bizarre and brutal child soldiers of the Mai Mai. The Hutu militia are being hunted down by the largely unpaid and undisciplined Congolese army who have been blamed for an appalling litany of human-rights abuses.

The park is hemmed in, too, by an estimated 600,000 internal refugees, camped on its fringes, the human fallout from years of relentless instability and civil war. And to the south of Virunga is the slum city of Goma, whose population has swollen to three-quarters of a million people.

De Merode, an anthropologist by training, who has been working in the Democratic Republic of Congo for 16 years, was a passionate critic of the way Virunga Park was run, until last year when the government turned to the Belgian and in his words said: "If you're going to criticise, show us you can do better."

"As a conservationist, to be given overall responsibility for Virunga ..." his voice trails off then comes back rapidly: "It's the greatest national park on the continent and it's in serious trouble. If you are serious about conservation then you have no choice."

His predecessor at the Institut Congolais pour la Conservacion de la Nature (ICCN), Honore Mashagiro, is now in jail after being convicted of orchestrating the killing of seven gorillas under his protection in 2007. Pictures of the murdered silverback (or troop leader) Senkwekwe being carried on a litter by mourning villagers made headlines around the world. The subsequent investigation led back to charcoal money and it emerged that the park director had ordered the executions as a warning to rangers threatening to disrupt the trade by protecting the forest.

The chaos and poverty of North Kivu has left 98 per cent of the people there dependant on charcoal. It's their only source of fuel, what they use to cook and to boil water. Even if there was a way to do so, de Merode explains, you cannot simply close down the charcoal trade, it would cause an immediate humanitarian crisis. "We cannot deprive the population of Goma of energy even if it costs us the park," he says. "Without boiled water there would be a cholera epidemic that would dwarf what we saw last year. You would have an immediate, massive crisis."

The rangers raids into the charcoal-harvesting areas, where they have destroyed hundreds of kilns and made scores of arrests, are designed instead to "squeeze supply" and make a show of strength to the traffickers.

What's needed in the meantime is an alternative fuel – and in the workshops at Rumangabo, the Belgian and his team may have found it. While this time last year the rest of the world was waiting for the forces of renegade General Laurent Nkunda to overrun Goma, de Merode was testing technology designed in South Africa to produce "biomass briquettes". Made from a compost of plant waste (like rice husks) they look like rings pressed from animal dung. Now that Nkunda is under house arrest across the border in Rwanda and an uneasy truce prevails in much of North Kivu, the briquette plan is being rolled out at extraordinary speed.

On a whirlwind tour of the factory set up to manufacture the presses at Rumangabo, the park director is a man in a hurry: "You have to take advantage of periods of relative peace," he says. "In Congo things come back to life quickly and if you work with that things happen fast."

In the afternoon sun, dozens of the wooden presses are being varnished and branded with the ICCN stamp. Together with the rest of the kit to start making briquettes, they cost £190 a piece and are being given to the communities around the park.

Local Congolese get everything they need to build a small briquette factory – two days' training, the kit and six months' follow-up support. In return they commit to producing at least four sacks per week, otherwise the press is given to someone else. In return the park guarantees it will buy the briquettes produced at a fixed price, and then sell them on to the market.

The speed with which things can change here is nowhere clearer than in Kiwanja, 15 miles north of Rumangabo. During last year's fighting the small town was the scene of an appalling massacre. In the space of two days in November, 150 people, most of them young men, were summarily executed by fighters from General Nkunda's CNDP.

Shabani Kabemba, now aged 45, remembers the soldiers going from house to house. "It was death," he recalls. "I thought I would die at any time." In the end, he survived but seven of friends were not so lucky and his family was forced to flee to Goma.

Now they have come back and underneath a mango tree in Kabemba's backyard they are pressing briquettes. "Before, my wife went to the forest to get firewood every day," he continues. "But in the bush there are soldiers who can take her."

This new family business is churning out three sacks a day which ICCN buys from them for about £4.50 each. Spread out among the baked-earth gardens and tin shacks of Kiwanja there are now 250 presses at work, and a waiting list of people wanting more.

Three thousand kits have been given out so far and the bulk of production is being sold on to international organisations running the camps for displaced people. There are plans afoot to set up a drying warehouse in Goma – and remarkably, by this time next year, the dung-coloured discs will be the largest employer in North Kivu if the expansion continues. In its report earlier this year, the influential UN panel of experts cited the briquettes as one of four key initiatives to stabilise the entire region.

"The briquettes aren't a success story yet," says de Merode, in the measured tones of someone not wanting to tempt fate. "But something interesting is happening."

The rutted road from Goma to Rutshuru, trodden by the feet of more than a million refugees, draws a line between the two sides of Virunga Park.

The southern sector is dominated by the gently rising contours of Nyamulagira volcano. The green canopy that rolls down from its summit looks unbroken from a distance. Only the grey columns of smoke that climb from the unseen kilns below the tree line warn of what Innocent Mburanumwe calls the "charcoal war" being fought to save the park and the wildlife it contains.

The head of the Nyamulagira sector, he has been shot at by government soldiers and besieged at his own headquarters for arresting carbonisateurs and he has seen dozens of his rangers killed, the last of them shot dead by poachers in August."We are losing the forest," he says and when it is gone the charcoal harvesters will cross the road into the Mikeno sector. Already the tendrils of smoke are beginning to climb the tallest of the volcanoes, creeping towards the gorillas.

A waist-high wall built from volcanic rocks is all that marks the perimiter of Mikeno. Beyond it, the old paths are overgrown and a steep ascent of jagged rock and thick red mud leads into a jungle which closes in overhead and has to be hacked open with a machete.

A network of bamboo thickets that dot the slopes offer breathing space in the claustrophobic vegetation, like yellowing summer pavilions with umbrella-shaped roofs of canes. And scattered across the carpet of dried leaves are the empty yellow husks of bamboo shoots, the gorillas' favourite food.

Nearby branches as thick as human arms have been broken with scarcely credible force and fashioned delicately into "night nests" padded with soft leaves.

Then a clumsy crashing noise announces the presence of Virunga's most charismatic animals. A juvenile male has tested his weight against the strength of a sapling and won with a snap. He lands in an effortless roll, like a black ball, unfurls himself and stretches. A mother scampers hurriedly past trailing an infant behind her, who gawks at the newcomers with huge eyes bulging in a tiny face.

A hollow rattle of a beaten chest sounds from beyond a wall of vegetation. A flurry of unseen movement shakes the jungle floor. Then a face appears. His flared nostrils catch the light beneath an endless forehead and the dense black fur of his immense shoulders gives way to a silvery grey that runs the length of his saddle-shaped back.

Kabirizi is the leader one of the two largest gorilla groups in the world, the dominant male in a family of 34 great apes. He rolls forward with terrifying speed to inspect his visitors at close quarters. Reassured of his authority, the 450-pound silverback returns to the shade of his bamboo pavilion.

His nervousness is understandable. War has already touched this family and the previous silverback was killed by Rwandan soldiers fleeing across the mountain in the wake of the 1994 genocide. And the sad truth is that – despite his immense strength and the life-or-death efforts of the park rangers he has come to accept cautiously into his family – Kabirizi is still not safe.

Virunga National Park, DR Congo, www.gorilla.cd