Wood is not a good conductor of electricity, but a tree also has a lot of moisture which is a fairly good conductor. During the lightning time there is a difference in voltage between a cloud and the ground.This voltage tries to flow from the high voltage area to low area so it looks for tallest conductive object like trees,towers,highest buildings.
Possibility is if thunder strikes on trees the person who stands below the tree also get a strike or branches from tree may fell on him. Also its better during thunderstorms if you get stuck some where reduce the earth conduct as maximum as possible.This increase a possibility not to select you as object to pass the electricity.