dendrochronology

a dating technique based on the number and variation in tree rings; usually, there is one ring for each year of growth and specific climatic changes are evident in thickness of ring

Dendrochronologists compare the growth rings from many trees or pieces of wood found on archaeological sites to make a combined plot of ring thickness that stretches back many centuries. By comparing tree-ring dates with radiocarbon dates, scientists realized that radiocarbon dates did not quite match calendar dates, and needed to be calibrated.