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Many tree problems are associated with the following:

Troubles in the Rhizosphere
http://www.treedictionary.com/DICT2003/shigo/RHIZO.html

Unhealthy Trees from the Nursery / Improper Planting
http://home.ccil.org/~treeman/sub1.html and
Look up "Tree Planting"
http://www.treedictionary.com/DICT2003/shigo/RHIZO.html

Improper Mulching - http://home.ccil.org/~treeman/sub3.html and
http://www.treedictionary.com/DICT2003/M/index.html Look up "Mulch"

Improper Pruning
http://www.treedictionary.com/DICT2003/tree_pruning/

Improper Fertilization (See A Touch of Chemistry)
http://www.treedictionary.com/DICT2003/shigo/CHEM.html

Sincerely,
John A. Keslick, Jr.
Arborist
http://home.ccil.org/~treeman
and www.treedictionary.com
Beware of so-called tree experts who do not understand tree biology.
Storms, fires, floods, earthquakes, and volcanic eruptions keep reminding us
that we are not the boss.

wrote in message
oups.com...
I have a cedar hedge in Gander, Newfoundland, Canada.
The trees have begun to lose their foliage in the lower half and the
process seems to be extending progressively upwards. Our soil is a
shallow, stony and damp clay and my back garden is particularly poorly
drained. I cannot see any bugs or fungi. Any ideas?