A case in Scotland with which I was peripherally involved determined that for the purpose of Rule 5 a Forestry Commission conifer plantation was "a structure". Apparently the court accepted the view that the forest was man-made (well, planted and managed), therefore it was by that definition a man-made structure.
I have a suspicion that this is entirely up to the court to determine, if it ever got that far, and doubt that it could be defined generally as a rule or guideline. The circumstances of the incident will almost certainly bear heavily on the interpretation.
Perhaps one of the legal bods on here might tell us more about this?