My trees are all sonotube, skinned with expanding spray foam, with PVC limbs that attach to 45 degree PVC angles, mounted to the sonotubes. Two issues that I've found;I'm curious if anyone who's built large scale props have a tried and true method for attaching multiple pieces in a secure way.
1. Any long limb will be heavy. Reinforce the attachment points with wood on the inside.
2. No matter how much you waterproof them, they'll get moisture that will cause them to deform. That's where the reinforcement helps the most.
This fella here is the entrance to our Haunted Forest section. He's 8' (plus about another 1.5' for his upraised arm) and he breaks down into 6 pieces. The body is 2 parts, the arms come off and the hands come off of the arms. I can stand the arms and hands inside of the base for storage with only a about a foot sticking out of the top. He's been changed a bit since these pics, but It was all cosmetic.
Hope that gives you some ideas!