By Gram Slattery

ANGOL, Chile (Reuters) - A few moments of relaxation for Chilean logger Nelson Hidalgo were shattered one day last month when 12 armed men piled out of a pick-up truck and demanded he step outside his work-site dining trailer.

At rifle-point, the masked men forced Hidalgo and a handful of colleagues to the ground, according to the workers' accounts. Over the next hour, the armed men burned a minibus, an excavator, and other equipment belonging to Nylyumar Forestry -- Hidalgo's employer and a subcontractor for Chilean timber giant Celulosa Arauco.

As they left, the men scattered leaflets claiming allegiance to Chile's indigenous Mapuche people, many of whom assert that logging companies are trespassing on their territory and draining the natural resources that are their birthright. No one was injured. But Nylyumar estimates losses that afternoon came to $600,000, and the company's workers were shaken.

"Since then, I've been tense. My muscles are in pain, as if they've been shrinking," the gruff, middle-aged Hidalgo said.

Such experiences have been multiplying lately, industry and government data show, and weighing on Chilean logging, the country's second-largest industry after copper mining. Mostly focused on southern Chile, it is the source of 10 percent of the country's exports.

Twenty-five attacks by saboteurs claiming to represent Chile's Mapuche were registered in the first five months of 2016 by forestry subcontractors' union Acoforag. That resulted in an estimated 9 billion pesos ($13.5 million) in damage, up from 3 billion pesos in all of 2015 and 638 million in 2014, according to the union.

(Graphic showing sites of recent attacks: http://tmsnrt.rs/1tALt1k)

Latin America's two largest forestry firms Empresas CMPC and Empresas Copec subsidiary Arauco have been among the worst affected. Both declined interview requests.

Behind the attacks -- according to politicians and law enforcement authorities -- are two small, mysterious groups who say they want an autonomous Mapuche state. They are becoming increasingly well organized and sophisticated in their tactics, law enforcement, loggers, and politicians say.

The groups complain that the logging industry's introduction of water-hungry radiata pine and eucalyptus trees since the 1980s has damaged the ecosystem.

In the southern Araucania region, the percentage of land now used for forest plantations has risen to almost 20 percent of all surface area. Academics say agriculture has become more difficult as plantations have dried up the land, and according to many government measures, the towns in the most heavily logged zones are the poorest in the nation.

"Before, we had many water resources and natural spaces that were the center of our cultural lifeblood, our food, our spirituality and our strength," said Jose Osvaldo Millanao, a Mapuche leader, while sitting in a traditional, wooden Ruka hut.

"And today, the big forestry companies have destroyed it."

CONFLICT ESCALATES

Around 600,000 Mapuche live in Chile, concentrated in Araucania and Bio Bio, lush and hilly provinces roughly 400 miles (640 km) south of the nation's capital Santiago.

Ever since the Chilean army invaded Mapuche territory in a brutal campaign in the late 1800s, relations with the state have been fractious. When timber companies began replacing native forest with industrial plantations in the 1980s, some Mapuche fought back, setting plantations, trucks and machinery ablaze.

But such occasional incidents were not a major operational threat to companies. That has changed in the last 18 months, as attacks have accelerated.

Chile's southern trucking union said damage from trucks that were hijacked and destroyed rose to $2 million in January and February of 2016, from $500,000 for the same period last year.

Intentional forest fires, meanwhile, jumped in logging regions to 3,081 in 2015 from 1,826 in 2013, according to the government. Between those years, according to regulatory filings, CMPC's reported net loss to "forest-related disasters and other damages" rose to $40.5 million from $6.6 million, as a long drought fanned fires that arose both intentionally and naturally.

A high-ranking CMPC manager, who asked not to be identified as he is not authorized to speak to the media, said 4 billion pesos worth of its subcontractors' equipment had been burnt in 19 attacks between January and May of 2016. That was up from 16 attacks in all of 2015.

Valuable time is also being lost as insurance claims are processed, loggers say. The average Acoforag member bills 200 million pesos monthly, but replacement equipment does not arrive for months after an attack.

MYSTERY GROUPS

The recent spike in violence has been attributed to two key groups with perhaps as few as 60 members.

Calling themselves the Coordinadora Arauco-Malleco ("Arauco-Malleco Group") and the recently formed Weichan Auka Mapu ("Fight of the rebel territory" in the Mapudungun tongue), they have claimed responsibility for dozens of recent attacks, but there have been few successful convictions and questions remain about their identity.

A senior law enforcement source said the groups take precautions against being tracked, such as collecting shell casings after firing weapons and wearing bags over their shoes to hide footprints.

Officials also question to what extent the activists speak for the wider community.

"We know these groups are small, and knowing that, the question becomes whether they really represent the Mapuche cause," said Humberto Toro, the governor of logging-intensive Arauco province.

While it is difficult to gauge Mapuche sentiment overall as each community has an autonomous governing structure, two leaders in the town of Ercilla -- at the center of the conflict zone -- said arson against equipment was not justified. They considered other forms of direct action acceptable, such as occupying company-owned land.

They also say they are frustrated that their people are under increased scrutiny due to the attacks.

“We don’t know that it’s Mapuche burning these trucks,” said Millanao.

In an effort to combat the spike in forestry attacks, the contractors' union has recently installed a sophisticated airborne balloon camera above the worst affected zone of Araucania, though it has resulted in no convictions so far.

According to the CMPC source, the company has put restrictions on riskier operations, such as night transport on certain roads.

Authorities have pledged increased security. Many police officers are heavily armed and signs along the main highway in Araucania warn that the route is video-monitored. But authorities struggle to protect the often remote logging sites.

"There's this sense of latent fear," said Hidalgo, the threatened Nylyumar logger, whose crew now works with a police detail. "I'm always looking around to see if anyone is following me, to see if anyone is coming."

(Reporting by Gram Slattery, Editing by Rosalba O'Brien, Christian Plumb and Stuart Grudging)