There are several factors that can limit] tree growth. Sometimes, low temperature is the bottleneck. So, the tree ring data can in any case be considered a reliable indicator of a floor on the temperature. It isn’t any colder than this point.
They try to pick trees that are more likely to find low temperature the bottleneck. Sometimes it isn’t.
That doesn’t mean that the whole series is useless, even if they happen to be using it wrong (and I don’t know that they are).
There are several factors that can limit] tree growth. Sometimes, low temperature is the bottleneck. So, the tree ring data can in any case be considered a reliable indicator of a floor on the temperature. It isn’t any colder than this point.
They try to pick trees that are more likely to find low temperature the bottleneck. Sometimes it isn’t.
That doesn’t mean that the whole series is useless, even if they happen to be using it wrong (and I don’t know that they are).