While I can't find an on-line link, it was reported in my local paper today
(Marietta Daily Journal - GA) that a recent paper by Dr. Hans Van Storch,
Institute for Coastal Research GKSS Research Center, Germany, has indicated
tree growth rings are not a good indicator of previous climatological
conditions.

By studying the rings it has been taken for granted that they indicate what
the weather conditions were like for any time in the past. Fossilized trees
has given us insight into conditions going back for centuries. Well the
professor believes this may not be very accurate. This could have a
significant impact on our understanding of global warming.

The most recent abstract of his study, but no details is at,
http://www.atmos.colostate.edu/dept/...F04seminar.htm

Alf