In one of his more daring exploits, Leng disguised himself as a chef working at logging camps to infiltrate the network of notorious logging baron, Try Pheap, an adviser to Cambodian Prime Minister Hun Sen.

He lifted the lid on Pheap's vast illegal logging operations in a 2013 report but the tycoon continued to expand his timber business across the country. This year, Leng filmed two major logging operations inside the protected area of Phnom Samkos Wildlife Sanctuary in the western Cardamon Mountains - both in concessions leased by Try Pheap.

What he discovered next is a scandal on an international scale.

Video footage from Try Pheap's timber depot on the outskirts of Phnom Penh shows huge quantities of luxury wood being loaded into shipping containers.

Al Jazeera tracked these containers and confirmed they travelled from Cambodia's Sihanoukville Port to northern Vietnam.

Try Pheap and his representatives have not responded to Al Jazeera's requests for comment.

Leng, the activist, says timber smugglers use illegal crossings dotted across the border as part of a rampant industry, with Vietnam effectively laundering then exporting illegally logged wood from Cambodian forests.

Almost half a million cubic metres of timber were smuggled from Cambodia to Vietnam between 2016 and 2018, according to a series of reports by international non-government organisation, the Environmental Investigation Agency.

In official correspondence seen by Al Jazeera, the Cambodian government accuses Vietnam of issuing permits for illegally logged timber, despite repeated warnings.

"There is still timber going across the border because there is a black market in that area," spokesman Neth Pheaktra tells Al Jazeera.

"These activities are illegal. That's why the Ministry of Environment, as well as other relevant ministries, and border officials are cracking down on forest crimes."

Vietnam's government says it "strictly prohibits illegal timber harvesting, transportation, processing and trade" and is taking measures to stop it.

From 2001 to 2018, Cambodia lost 2.17 million hectares of tree cover, equivalent to a 25% decrease, according to data analysis by Global Forest Watch.

The timber feeds an insatiable demand for rare wood in China, where prices for luxury timber furniture have soared. One bed made from Siamese Rosewood - which has been almost eradicated in Cambodia - reportedly was on sale for $1 million.

"Sometimes I cry. I feel disappointed because I'm not able to protect the forest," says Leng. "I see that the destruction is so big, but no one helps to protect it.

With huge profits to be made, Leng's investigations are undertaken at great risk.

Another Cambodian forest activist, Chut Wutty, was murdered in 2012 while investigating a logging company. Several more forest patrollers have been killed since, including three who were shot at the Vietnamese border last year.

Leng himself has received numerous death threats and had his equipment smashed.

"I know that this is dangerous work… No one dares to challenge the companies," says Leng. "Why do I challenge [them]? Because the companies have caused mass destruction to the forest."

In some cases, protected areas have been completely destroyed - such as Snuol Wildlife Sanctuary in eastern Cambodia.

Snuol Wildlife Sanctuary Data: NASA Landsat / USGS

The sanctuary was so severely damaged, the government removed its protected status in 2018 - conceding there was nothing left to protect.

Travelling in a four-wheel drive vehicle so old the rear brakes often erupt into plumes of smoke, Leng stops at the barren remains of the former sanctuary.

"Maybe 10 years ago there was jungle and a lot of forest and a lot of wildlife like elephants, tigers, rabbits..." he laments.

"The private companies came to destroy, to terminate the forest here."

Other sanctuaries, like Boeung Per in the north, are rapidly heading towards the same fate.

Boeung Per Sanctuary Data: NASA Landsat / USGS

But despite the forces stacked against him, Leng continues to race off deep into the jungle every time he gets a new tip-off of potential illegal activity.

"This has happened for 10 to 20 years - not just this year, and no one has been able to prevent it," he says.

But as long as there are precious forests to save in Cambodia, Leng will be on the frontline defending them.