When a tree branch touches a high voltage power line, the first result is typically a small arc at the touching point at first, and then, for some time, vapor emerging from the point of contact, and the point where the plant connects to the ground. While the branch gets hotter, there is more vapor along the branch and stem.
After a while, parts of the connecting starts to burn. The flames may later change to arcing.
I think that the resistance of the wood and bark will increase a lot during the time the vapor emerges, and then even more when it burns, which should dry the wood more.
Looking at a recording of the process, like this slow and detailed example, it seems like the resistance does not significantly drop during the process. Why is that?