Recently having had an argument about a fence with my neighbour, I can
tell
you what happens here in Canada. It probably isn't all that different in
the
States as it's a common law issue, and Canadian & American common law both
have their roots (no pun intended) in British common law. According to the
local municipal standards office, one may not install anything on a
property
line without the express prior consent of the adjoining property owner. My
neighbour started installing a fence without consulting me, and I was
informed by the city that I could apply for an injunction to stop all
work.
This is in spite of the fact that the fence was going on a recently
surveyed
property line. I, on the other hand have planted a hedge for privacy along
the same line, and because it's well on my side of the line, the neighbour
has no right to touch it, even though I plan on growing the cedars to
about
10 feet high. (which should change their sunny flower beds to nice shady
beds) Their fence, on the other hand, being right on the line, is now half
mine, and I have the right to affix anything I wish to my side. (including
Day-Glo pink plywood if I want) Property standards also advised me that if
my hedge grows through the fence, the neighbour can trim it, but that they
will be responsible for damages if they cause damage to my hedge from
improper or negligent pruning. The same applies to their maples which
overhang my yard - if I want to trim them back, I can do so, but I am
liable
for damages if it's done improperly. Of course, improper or negligent
pruning becomes a matter of opinion, and becomes something contentious
that
might have to be settled by a court.

you're right--from what I've gathered so far, it's pretty much the same
here. In our case, the neighbor did have our consent to put the fence up,
but unlike *our* last fence (which was taken down)--this new one is more
true to the actual property line--which is also why the tree is in jeopardy.
Of course we didn't know this new fence would have endangered the tree; I
guess we were just being naive in assuming that no one in their right-mind
would want to harm a beautiful mature tree for the sake of some ugly
synthetic fence. We also had various bushes along the fence and they are now
slightly crushed, but we're not giving them a hard time about the bushes,
seeing as how we have our hands full with the tree, and the bushes are
easily replaceable.

The day-glo plywood is a good idea , but it's like cutting off your nose
to spite your face.