U.S. authorities have found illicit marijuana crops growing in 61 national forests in 16 states this year, from California to Virginia, the Wall Street Journal reported Sept. 3.

That's up sharply from last year, when marijuana was found growing in 49 forests in 10 states, according to the U.S. Forest Service.

The grow operations present a variety of hazards. Some are booby trapped or monitored by armed guards. Trees are cut down, streams diverted, and land terraced to plant crops, which are sprayed with fertilizer and herbicides. A camp used by suspected marijuana growers was fingered as the source of a forest fire that scorched 88,000 acres in the Los Padres National Forest in California.

Federal agents have raided 487 marijuana plantations in public forests this year. About half were run by foreigners, typically Mexican drug cartels. Such operations have become more popular among traffickers as enforcement has increased along the U.S.-Mexican border.