Here are all of the notes I've taken on this topic based on these
discussions... these are not my words... this is largely all copy and
paste from different replies or from Internet searches.

Again, I thank everyone for helping.

Root Barriers

* So, if in 10 years the tree is growing that big and you are still
worried about it, rent a Ditch Witch and trench a 4" wide, 2' deep
trench inside your property line and disconnect any roots running in
your direction. The trench would not have to be long, say 20' as the
major roots will radiate out from the trunk. You could even dig a
smaller test trench with a shovel is a couple of years just to check
things out. You can take an axe to any roots on the surface.

* If you're REALLY worried, in five years, dig a trench inside your
property line to, er, put down PVC for watering - or something. [this
will cut any roots]

* If it really bugs you, spade through any major roots you see when
you build the driveway. The rest aren't worth worrying your pretty
little head over.

* Another slant might be to verify that the tree owner may be libel
for damages to your property and also verify with a local professional
that that tree in that location might damage your property. I would
expect the owner to react.

* I would think it being Aristocrat pear at 17 feet would be more than
a safe distance from your home. If you are going to make a barrier,
24 inches of corrugated tin buried should be sufficient.

* Heavy EPDM would work as a barrier, and be easier than tin to work
with. That is what they use in the bamboo group as a root barrier.
(Heavy black rubber-like material. Used for roofing, and ponds. Can be
cut, and glued if you need to.)

* Another problem that occurs with trees is *reduction* of the
groundwater from evapotranspiration in clay soils, consequent
consolidation of the clay stratum, and settlements of the structure
founded on the stratum. This is usually associated with large trees
like oaks so a relatively small pear tree may not apply... The thing
is, you neighbour by watering may be balancing the equation.

Q: Is this caused by the roots taking moisture out of the soil?

http://www.marinwater.org/evapotranspiration.html

A: Yes, But the tree is also providing shade, thereby reducing the
amount of evaporation directly from the soil, so the likely net effect
is zero.

Check this link for a discussion of the issue from a decidedly
tree-friendly engineer:

http://users.rcn.com/bobw.enteract/UKSubsidence2.2.html

Since you are watering the soil to avoid too much drying out under the
foundation, you probably do not have to worry about this.

For more info about the International Society of Arboriculture, please
visit http://www.isa-arbor.com/home.asp.

For consumer info about tree care, visit http://www.treesaregood.com/

====

Book

A book entitled "Neighbor Law: Fences, Tree, Boundaries, and Noise" by
Cora Jordan, attorney at law, addresses such questions and many other
neighbor-law issues.

Self-Help Law

Property owners in every state have the right to prune branches or
roots of a neighbor's tree that encroach onto their property. This is
known as "self-help."

But there are limitations:

* You can prune only up to the boundary line -- at your own expense.

* If you want, you can go one step further. Your neighbor has a duty
to prevent his tree from harming your property. It seems reasonable,
then, that your neighbor should be required to pay to have the tree
removed or trimmed so that it stops damaging your house. Also, your
neighbor may be liable to you for the damages already caused by the
tree's roots.

* You should be aware of several other facts. Since tree roots usually
take years to cause damage, a judge or jury, or even your insurance
company, may find you were partially or even completely at fault for
not taking steps years ago to prevent the damage.

=====

Q: My neighbor has a non-protected tree that overhangs my yard. What
can I do about this?

A. The City authority does not address trees that are not 'regulated',
such as pines, elms, magnolias etc. when they are on private single
family zoned property. Therefore, you are entitled to trim branches in
any manner you deem necessary on your side of the property line,
providing that substantiated damage does not occur to the tree (to
which you may be held liable), as provided per California case law.
The City will play no role in this scenario.

=====

Q: Can I cut branches off a tree that are in my yard if the trunk of
the tree is in a neighbor's yard? Technically he owns the tree but
the branches affect my property.

A: I am not a lawyer, and cannot advise you as such. Having made this
disclaimer, I can give you an overview of how the law tends to work,
but recommend that you contact your attorney to back it up.

You are allowed to prune branches that extend past the property line
and over your property, but only to the property line itself, and only
if, by doing so, this pruning will not harm the tree.

You are not allowed to enter the neighbor's property (which means
entering their tree, too) without their permission, so the pruning
must be done all from your property.

If the branches are too high up to reach either from the ground or
from an aerial lift truck on your property and it's necessary to climb
the neighbor's tree to do the pruning, you DEFINITELY have to have
their permission, to remain within the law.

Pruning just to the property line rarely results in a good pruning cut
(from the tree's standpoint), so I always recommend that the neighbors
get together to discuss the situation.

It is always better to remove a branch back to the collar at its base,
and rarely is this collar exactly at the property line. If you cut a
branch just to the property line the result is almost always a stub,
which isn't desirable, and possibly harmful.

If the tree is damaging your property (i.e., hitting your roof and
damaging the shingles), you are entitled to self-help, but you may
need to have your attorney contact the neighbors if they're less than
cooperative.
Most neighbors are friendly about these things, so I would imagine
just talking with them will work out fine.

So, the best thing to do is speak with the neighbor and come to a
joint decision.

Ideally, have a qualified arborist present at the time to offer
his/her helpful input on how they would go about both accomplishing
your objective and making proper pruning cuts for the tree's sake at
the same time.

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