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On a cold morning this fall, I was clinging to a paracord line, descending through heavy brush on what felt like a near-vertical hill in Shasta-Trinity National Forest, in front of me a team of influential ecologists, behind me a team of affable, heavily armed law-enforcement agents. We were deep enough into the forest—bushwhacking into vegetation, miles and miles down a fire road—that anything human-made would have looked out of place. Then I saw a suitcase. A child’s coat. Pots and pans. A pair of jeans. Chapstick. Ramen packages. It looked a little like the site of a plane crash, except for the thousands and thousands of dead marijuana plants.

We’d arrived at a “trespass grow,” one of hundreds in the state of California. If you bought weed on the black market recently, there’s a good chance it came from a grow like this one. There’s a good chance, then, that your joint was contributing to a spiraling environmental catastrophe.

Trespass grows are not just an eyesore and a misuse of public lands. Growers on the sites use powerful illegal pesticides. That makes the grows a severe threat to the forest ecosystem, from the microbes in the dirt all the way up to apex predators like eagles. They are an object lesson in the invisible harms of the black market—and in the invisible harms of cannabis prohibition. As long as there is a strong incentive to produce and sell pot illegally, there will be a strong incentive for cartels to set up sites like the one I visited, and thus to damage delicate wildlands and poison countless animals.

The grows are almost invisible, hidden deep in national forests. (Growers seem to avoid the country’s national parks, because they are more heavily touristed and monitored.) Cartels set up temporary encampments, leaving a few intrepid workers to oversee tens of thousands of marijuana plants. When the plants mature, teams come in, harvest and dry the cannabis, and pack it out on foot. The sites are difficult to prevent and, due to their isolated and atomized nature, difficult to stamp out, the Forest Service agent Kevin Mayer told me while we trekked in the woods.

The grows are also sophisticated, if not much to look at. Growers identify pockets in the forest that have ample natural light—such as burn scars on southern-facing slopes—as well as water from natural springs, lakes, or streams.

In subtle ways, the trespass grows pervert natural ecosystems: They are greener, wetter, and emptier than the surrounding forest, attracting animals to them. “Our camera data shows that animals use these sites with higher frequency than they use the normal forest,” said Greta Wengert, a founder of the Integral Ecology Research Center (IERC), which has done extensive studies on the environmental damage from trespass grows, as she picked through the detritus the cartels left behind.

Trespass grows also hurt the natural environment in not-so-subtle ways. Growers divert an estimated 9 billion gallons of water a year, enough to supply 30,000 homes. In some places, that diversion accounts for a quarter or even half of a given watershed’s total surface flow. To protect their plants, the cartels use rodenticides and pesticides that are banned in the United States. These chemicals are eaten by animals, and make their way into the groundwater. More than 90 percent of California’s mountain lions have been exposed to pesticides, as well as 80 percent of Pacific fishers and 70 percent of northern spotted owls.

Gabriel Mourad, the IERC’s other founder, told me that the problem is systemic: dangerous anticoagulant rodenticides are now everywhere in the forest’s food chain. They are in the soil that gets eaten by bugs that get eaten by birds. They are in the small rodents that get eaten by fishers that get eaten by coyotes. They are in the water, hurting fish. And they are hurting people, too.

That likely includes the growers working with the cartels—including, potentially, the kids who seem to come visit their parents on-site, judging by the toys and tiny clothes in the detritus. At the site in Shasta-Trinity, there had been pesticides near where the growers were preparing and eating food, making trace contamination likely. Scientists and law-enforcement agents working on these sites are also being exposed to noxious chemicals. Mayer described fellow officers’ respiratory and eye problems, and said they all worried about the increased risk of cancer.

Law-enforcement officials have made continual arrests of growers on public lands. But Mayer said that the Forest Service lacked anything close to the resources to solve the problem on its own, particularly given how vast the country’s national forests are and how underfunded the service is. A sophisticated computer algorithm had helped locate the grow site we visited, but Mayer said officials were still finding only half of the sites out there.   

Those hundreds of sites produce 40 to 70 percent of the black-market cannabis in the United States. “They pull thousands of pounds off of these sites, which also severely undercuts the legal market,” Jackee Riccio, a conservationist, told me. “This is the opposite of cannabis culture. This style of cultivation is to cannabis as ivory poaching is to wildlife conservation.”

The ecologists and conservationists on my visit are part of a new group called the CROP Project, which is pushing for more public awareness of trespass grows and increased resources to preserve and restore the damaged forest. “The vast majority of people are not even aware the problem exists,” Rich McIntryre, the CROP Project’s director, told me.

But as I stood in the wreckage of the trespass grow, imagining children visiting their relatives working on the temporary farm and playing on rodenticide-contaminated soil, and imagining enjoying a joint laced with a drug so potent that American agribusinesses are barred from using it, I thought it seemed obvious that the biggest fix would be the best one. The cannabis grown unlawfully on California’s public lands is destined for the many states where pot is still illegal, and where demand remains high. Making marijuana legal at both the state and federal levels would help smother the black market—eliminating the profit motive that prompts the cartels to build these trespass grows at all.

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