SEATTLE — Federal data show the number of live marijuana plants eradicated in outdoor and indoor grow operations has dropped in most states over the past three years, while the amount of bulk processed marijuana seized has doubled in that time.

And authorities can’t pinpoint exact reasons.

One thing is known: California, which provides the lion’s share of the millions of plants eradicated every year in the United States, saw a 46.5 percent drop in plants eradicated from 2010 to 2011, bringing down the nation’s overall numbers.

“You can’t attribute it to one factor,” said Casey Rettig, spokeswoman for Drug Enforcement Administration in San Francisco.

Shifts in tactics from growers, weather patterns and budget cuts to local and state enforcement agencies have played roles in the decrease in eradication, DEA and local officials said.

In 2010, authorities seized 10.32 million marijuana plants from outdoor and indoor growing operations, according to DEA data. By 2011, that number had dropped to 6.7 million plants — a 35 percent decrease. Those numbers are amplified by California, where 7.3 million plants were eradicated in 2010. The number dropped to 3.9 million plants in 2011, a 46 percent decline.

In that same time span, 37 states saw their eradication results drop. Data for 2012 is not yet available.

One of the most dramatic shifts came from Idaho, which saw its eradication results shrink by more than 98 percent from 2009 to 2011 — from 77,748 plants to just 786. Although, the Caribou County sheriff’s office reported raiding a farm with 40,000 plants this week.

In order for a marijuana plant to be counted, it has to have a root structure, even if the plant is just inches tall, and be mature enough to yield the buds that contain the drug, said Doug James, DEA spokesman for the Washington, Oregon, Idaho and Alaska region.

But while the number of plants eradicated has dropped, the number of pounds of bulked processed marijuana confiscated has increased from 53,843 pounds in 2009 to 113,167 pounds in 2011, the data collected by the DEA shows.

Some of this was seized along with eradications at growing locations. But DEA officials said the data also includes marijuana seized during traffic stops and other operations, and it’s hard to pinpoint where marijuana comes from.

“You can’t necessarily measure what’s out there,” Rettig said.

In California, Rettig said, growers are switching from large scale forest farms with many plants to smaller, less visible plots where they can grow fewer, but bigger and higher yielding plants.

There has been a change in where these plots are grown, too, with many operations moving to the agricultural region of the Central Valley, Rettig added.