By Luis Fajardo
BBC Mundo, California
The United States is not just one of the world's biggest consumers of marijuana.
Capt Mayer spends his days seeking out illegal cannabis crops
According to UN figures, it is also a major producer.
The country's most populous state, California, ranks first for production and consumption, and local authorities are fighting a constant battle to eradicate marijuana crops.
That battle does not just take place on
farms or in private houses - it has moved onto public land and into national parks.
Captain Kevin Mayer is one of six Forest Service guards working in the Sierra National Forest, a nature reserve in California.
He is not the typical-looking gamekeeper - he carries an automatic rifle and spends the day patrolling the reserve looking for marijuana crops.
Capt Mayer is convinced that the amount of marijuana being planted in the Sierra National Forest is increasing.
"I think we will see more crops in the future. They will expand along the eastern coast of the US and towards the border with Canada," he told the BBC.
"There is a lot of money in this business."
We are shown an area where cannabis plants were recently destroyed, but the camp occupied by the clandestine growers was still to be cleared up.
Among the piles of rubbish and dried-out cannabis plants were shotgun cartridges and empty herbicide bottles left behind by those who were looking after the illegal crop.
"Somebody was making fresh corn tortillas here," said federal agent Robert Hernandez.
Cannabis crops are laboriously chopped down and removed
California has seen significant changes in marijuana production over the past 10 years.
"It's no longer a handful of people growing a bit of marijuana in the hills for themselves and their friends," said Jackie Long from California's Bureau of Narcotic Enforcement.
"Today, were seeing organised groups, such as members of California's street gangs, and drug-trafficking gangs from Mexico involved."
It is a growing problem for national parks - in 2005, 1.1 million marijuana plants were eradicated from state or federal public lands.
Speaking last year, director of National Drug Control Policy John Walters said tighter border controls had made smuggling drugs into the US from Mexico more precarious.
As a result, Mexican drug cartels were "turning our national parks into centres of international drug production and trafficking", he said.
The state department's anti-drugs report put the US production of marijuana in 2005 at 10,000 tonnes.
Figures from the California Justice Department show that $6.7bn (£3.36bn) of cannabis crops were eradicated in the state in 2006.
The number of destroyed plants was more than 1.6 million - a five-fold increase since 2001.
The US has pursued the policy of destroying drug crops in Colombia and other Andean countries by spraying them with herbicides.
But in the Sierra National Forest, environmental concerns mean cannabis plants are eradicated by being picked by hand, a labour-intensive task.
"Unfortunately, cannabis crops are next to trees and other plants. If we were to spray them with herbicide, we would kill the forest. So that is simply not done here," Mr Long said.