On Tue, 13 May 2003 15:56:03 GMT, (Babberney)
wrote:

Trees probably get a bad rap for a lot of things that are equally
explained by shoddy construction and bad engineering, but I think you
are going too far.

[...]
Actually, this is one of those unbeatable myths, largely started by
engineers in England where clay soils are apparently common (search
for "subsidence" if you want to see a ton of stuff about it). In
fact, shaded clay is less likely to lose water to evaporation than
sunny spots, so trees moderate this effect rather than causing it.

Would that not depend on the distance between the tree and the "spot"?
If the tree's roots are in the soil in question, it seems reasonable
to assume that the moisture the tree transpires does not come from
next door. We too, have ample clay soil (where there is soil at all)
and for "subsidence" see "Taylor Clay". (for a practical application,
just about any construction in Tarrytown) The clay itself swells and
shrinks markedly, depending on moisture, so sure, it is not all the
fault of a rogue tree.

We tree guys are continually trying to counter the arguments of
construction guys because theya re used to saying "the tree did it"
and getting away with it. They have years of advance propaganda,
which has led to the "common sense" notion that these things must be
true. Another biggie is the "tree heaves sidewalk" angle. Tree roots
don't push outward, they flow into spaces. They are no doubt
contributory--they fill a space, water comes in and freezes, space
expands, root fills it, etc.


We don't have a "frost heave" problem in Austin. I doubt that the
temperature at the depth of the average sidewalk sandbed has ever
gotten to 32, much less lower. If you insert a small root into the
retained moisture beneath a sidewalk or drive, then grow that root to
8' in diameter, my guess is that something is going to give. Most
likely, that something will be the slab above rather than the earth
below. Your thesis would be a lot more plausible were the lumps in
my driveway not perfectly correllated with the fast growing "trash
trees" (silver maple, Chinaberry, hackberry) that previous owners
either planted or failed to suppress. FWIW, I also have a cracked and
displaced brick retaining wall, caused, IMO, by the growth of a
hackberry in my neighbor's yard. The displacement is away from the
tree (now gone) and upslope, so it is unlikely to be any hydraulic
effects.

But studies have found just as many heaved sidewalks where there are
no trees around to blame. Next time you see this, look at the
concrete. Do you see reinforcements (rebar)? My guess is, you don't.
Bad sidewalks crumble, the installer gets a call, hmm, must be the
tree, cuase we do only the best work.

Frost heaving is real enough. That's why one has building codes that
specify foundations in very cold climates. We don't have that
problem. When the slab heaves as a piece, at the expansion joints,
the wire (rebar is pretty uncommon in a sidewalk) is not going to
matter very much. Crumbling sidewalks don't heave. They are usually
not strong enough to be levered in that way, that is they are
crumbling.

As for mulberry trees, I like the small ones in my back yard for the
berries/birds, but I won't ever let them get out of my control. they
are prone to poor structure, they sprout profusely, and they are
fairly short-lived. If you are looking at a spot that is wide open a
spacious, you might go for it, but if this is going to be the one tree
your yard can hold, I'd pick something else.


As long as one is fond of having everything in about a 200 foot radius
"painted" purple red with bird crap, they are a treat. I have never
found that I could pick nearly as fast and efficiently as a flock of
birds. You are probably younger and quicker.


Keith Babberney
ISA Certified Arborist
For more info about the International Society of Arboriculture, please visit http://www2.champaign.isa-arbor.com/.
For consumer info about tree care, visit http://www2.champaign.isa-arbor.com/.../consumer.html