In case you missed it: Hockey stick co-author claims that after 1850, critical trees lost their alleged ability to record temperature

Year 2000 ClimateGate email

If you examine my Fig 1 closely you will see that the Campito record and Keith's reconstruction from wood density are extraordinarily similar until 1850. After that they differ not only in the lack of long-term trend in Keith's record, but in every other respect - the decadal-scale correlation breaks down. I tried to imply in my e-mail, but will now say it directly, that although a direct carbon dioxide effect is still the best candidate to explain this effect, it is far from proven. In any case, the relevant point is that there is no meaningful correlation with local temperature. Not all high-elevation tree-ring records from the West that might reflect temperature show this upward trend. It is only clear in the driest parts (western) of the region (the Great Basin), above about 3150 meters elevation, in trees old enough (>~800 years) to have lost most of their bark - 'stripbark' trees. As luck would have it, these are precisely the trees that give the chance to build temperature records for most of the Holocene. I am confident that, before AD1850, they do contain a record of decadal-scale growth season temperature variability. I am equally confident that, after that date, they are recording something else.  [Malcolm Hughes]