First, they compared weather records collected over the past 200 years with samples from living trees to see how temperature and moisture affected tree - ring growth. (sciencemag.org)
The ring patterns of old living trees were matched to those of dead trees to pinpoint years when the dead trees had been alive. (scientificamerican.com)
You also need to look at living trees so you can know the years those different rings were put down. (newsdeeply.com)