When the whole tree breaks, it becomes a set of rigid bodies and how it falls will be calculated differently for each client.. so the trees wont fall the same way each time, and also how they fall for one client, doesnt nessecarily mean they fall the same for another client. Breakage is synced, when a tree breaks, it breaks for everybody, just falls slightly differently(this is to save network bandwidth.. having to transmit position, rotation values for each piece of broken tree/particle etc every frame to all clients would kill performance..)