Authorities say they have seen an increase in outdoor marijuana operations run by Mexican drug cartels. In the past several months, federal agents have found nearly $55 million worth of pot plants in national parks and on federal lands in California, Colorado and Idaho.

On Thursday, authorities closed a section of Sequoia National Park in California so they could destroy marijuana plants discovered near a cave filled with crystals that is a popular tourist stop. Most of the marijuana already had been harvested. Authorities estimated the plants were worth more than $36 million.

In June, federal authorities seized 2,250 marijuana plants from California's Point Reyes National Seashore and Golden Gate National Recreational Area. That same month, hikers in Idaho found a site with 12,545 pot plants.

In the most recent Colorado case, the marijuana was found in "the remote, rugged terrain" of Pike National Forest, which is about 60 miles southwest of Denver. The DEA said it is the largest outdoor marijuana-growing operation ever found in Colorado, with an estimated value of $5 million.

"The persons who were involved in this criminal activity had no regard for the damage caused to the forest and environment by the waste they left behind," said Jeffrey D. Sweetin, special agent in charge of the DEA's Denver office. "The public's safety is also at risk for those who recreate on our public lands due to these trafficking groups operating there."

Authorities say they learned of the marijuana site from a passer-by. DEA said Mexican migrant workers had been recruited to work at the site and harvest plants, which were between 4 feet and 6 feet high.

Authorities tracked down two men associated with the site and arrested them last week, but have released few details about them, including their names.

Mr. Sweetin said growing marijuana on public land in the United States has become attractive to drug cartels as increased border security has made it more difficult to smuggle large quantities of marijuana into the U.S.

And, he said, outdoor operations can be set up for relatively little money. Typically, the sites are tucked away relatively close to all-terrain-vehicle trails and campsites at the parks.

Stopping the proliferation of these sites has become a priority for the National Park Service, which dedicated $3.3 million this year to stop growers at parks in the West, including Yosemite, Sequoia and Redwood national parks.

"Before this, the [National Park Service] had set aside a modest fund for marijuana interdiction - about $150,000 a year over the past five years - and parks competed for this money," said Jeffrey Olson, a spokesman for the Park Service. "The bulk of it went to the Pacific West Region, where most of the marijuana grow sites have been found."

Authorities are concerned about the legal ramifications of such sites and the environmental consequences, which, they say, are severe.

"The impacts are numerous," said Gill Quintana, head of the U.S. Forest Service's Denver branch.

He said these include "damage to the lands due to clearing the areas to prepare the garden site, trash left behind, chemicals used to grow the crop [seeping] into the watershed, and the public-safety issues associated with the recreating public coming in contact with these organizations while they're operating on our national lands."

Earlier this month, investigators in California said they were looking for marijuana growers tied to a Mexican drug cartel that they suspect of igniting the La Brea fire that charred more than 88,000 acres of the Los Padres National Forest in the remote Santa Barbara County mountains northwest of Los Angeles.

The fire, which erupted Aug. 8, is thought to be the first major wildfire in the state caused by drug traffickers, a U.S. Forest Service spokesman said.

In a statement, the Forest Service said the blaze was sparked by a "cooking fire in a marijuana drug-trafficking operation ... believed to be run by a Mexican national drug organization. ... There is evidence that the unburned marijuana garden area has been occupied within the last several days."

No arrests have been made in that case.