What could cause bright green flames in a campfire?

Tonight we built a campfire for making s’mores, and all of the wood for the fire came from branches picked up in the yard. I helped the kids collect it, so I know there was nothing other than tree branches. By the time we saw the green flames, the newspaper we’d used to start the fire had completely burned down. I read that copper can cause green flames, but don’t know how copper could be on tree branches. What else could have caused the green flames?