The protesters perched themselves in the trees for hours but were eventually talked down.
Four activists were booked on charges of trespassing.
One of the tree sitters spoke to Eyewitness News by phone from a tree during the bulldozing.
"They're just knocking over trees. One just dropped 10 feet below us. They definitely need to back off because they're seriously going to endanger us with these cranes," said John Quigley, who camped out for 71 days in an oak tree in Santa Clarita in 2003.
County officials said they needed to clear the grove of trees to ensure the integrity of the nearby Santa Anita Dam, which provides most of the drinking water to /*Arcadia*/ and /*Sierra Madre*/.