Why did trees allegedly stop functioning as thermometers last century? Let me count the explanations (three in this ClimateGate email alone)

Email 19

1. There are few tree-core series that extend beyond the early 1980s. This is because
many of the sites we're using were cored before the early 1980s. So most tree-ring records
just don't exist post 1980. [Phil Jones]
...
If you look at the figure in the attached article in Science by Briffa and
Osborn, you will note that tree-ring temperature reconstructions are flat
from 1950 onward. I asked Mike Mann about this discrepancy at a meeting
recently, and he said he didn't have an explanation. It sounded like it is
an embarrassment to the tree ring community that their indicator does not
seem to be responding to the pronounced warming of the past 50 years. Ed
Cook of the Lamont Tree-Ring Lab tells me that there is some speculation
that stratospheric ozone depletion may have affected the trees, in which
case the pre-1950 record is OK. But alternatively, he says it is possible
that the trees have exceeded the linear part of their temperature-sensitive
range, and they no longer are stimulated by temperature. In this case
there is trouble for the paleo record. Kieth Briffa first documented this
late 20th century loss of response.

Personally, I think that the tree ring records should be able to reproduce
the instrumental record, as a first test of the validity of this proxy. To
me it casts doubt on the integrity of this proxy that it fails this test.

Sincerely,
Jeff [Severinghaus]