But despite the rise in arrests and in the seizure of marijuana plants, use of pot in California has increased slightly, said the report, part of a nationwide study released Thursday by a Virginia researcher.

In both California and the United States as a whole, "we keep arresting more and more people, but it's not having a deterrent effect," said Jon Gettman, an adjunct assistant professor of criminal justice at Shenandoah University in Winchester, Va.

Nationally, Gettman said, marijuana arrests have doubled since 1991,but marijuana use is unchanged.

Gettman is a former director of the National Organization for the Reform of Marijuana Laws. He said he favors the legalization of marijuana.

Gettman's report came a day after state officials announced that the state-federal Campaign Against Marijuana Planting had seized a record 4.4 million pot plants in California this year, up from 2.9 million in 2008.

Gettman's study is based on state and FBI arrest records and other government data from 2003 through 2007. It said California officers arrested 61,375 people on marijuana charges in 2003 and 74,024 in 2007, an average increase of more than 5 percent per year. Eighty percent of the arrests in 2007 were for marijuana possession, the report said.

Nationwide, the annual increase during the same period was just under 4 percent, the report said, although California's marijuana arrest rate, compared to its population, remained among the nation's lowest.

The report also found a large racial discrepancy in arrests.

African Americans were about 20 percent more likely than whites to use marijuana in 2007, but the arrest rate for blacks on marijuana charges was nearly 270 percent of whites' arrest rate, the report said.

Gettman said he found similar disparities nationwide and in most major cities, including San Francisco.

"I don't believe it's racially motivated," he said. Among the possible contributing factors, he said, are "more intensive patrolling" by police in minority neighborhoods, and the presence of marijuana when people are arrested for other crimes.

Overall, the report said, marijuana use increased in California by 0.73 percent a year in the four-year period, while nationwide use declined by 0.21 percent a year.

By geographic zone, the state's northernmost counties, which include the prime marijuana-growing areas of Humboldt and Mendocino counties, ranked 12th out of 350 regions in the nation in pot use by their residents. A region consisting of San Francisco, Marin and San Mateo counties ranked 15th.

E-mail Bob Egelko at begelko@sfchronicle.com.

This article appeared on page C - 4 of the San Francisco Chronicle