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24-05-2003, 10:21 PM

On Mon, 19 May 2003 21:40:58 GMT, (Babberney)
wrote:

It may be true that it's rare for freezing to occur below sidewalks,
but it's also still true that trees do not exert an outward force as
they grow. Cells form where they have space. If you doubt this,
consider roots near undamaged cement. If you look around, you can see
roots that are flattened, elongated, or otherwise distorted from their
natural shape by obstacles. So, the sidewalk may not develop a crack
due to freezing, but some other factor must come into play. Once a
new crack is developed, the tree will fill it, but the tree cannot
create the space on its own.

Well, further research leads me to back off this as well. I trust the
source who said it's true, but two other sources I'd take as reliable
say roots exert a force of 150 lbs. and 175 lbs., respectively. None
cite sources, so I'm not sure who to believe.

But that still doesn't seem like much force against a well-engineered
slab. I'm not an engineer, so maybe I'm wrong, but I'm trying to
imagine a person moving a sidewalk and I'm having a hard time seeing
it. I have seen studies (such as the one cited earlier in the
mulberry thread) that find equal or greater numbers of damaged
sidewalks where there are no trees around.

As for creating their own spaces in the soil, trees work on a
different time scale than us. A very slight change allows a very
slight growth, and so on. Drying and soaking of soils contributes as
much to this (probably more in Austin) as freezing and thawing. And
this brings us back to subsidence, which is probably the real cause of
much slab failure. And I still say the trees don't contribute much to
subsidence, since they shade the soil and thereby reduce evaporation
while taking out water by transpiration. I'm not staking my
reputation as an arborist on this, but I'm trying to keep the
discussion going because I think there are blind spots in the "common
sense" arguments that should be highlighted and examined.

keith
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

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