Why must radiocarbon c14 dates be calibrated using tree rings?

Calibration of radiocarbon results is needed to account for changes in the atmospheric concentration of carbon-14 over time. These changes were brought about by several factors including, but not limited to, fluctuations in the earths geomagnetic moment, fossil fuel burning, and nuclear testing.

How are dates determined from tree ring samples?

Crossdating is the most basic principle of dendrochronology. Crossdating is a technique that ensures each individual tree ring is assigned its exact year of formation. This is accomplished by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations.

Which tree-ring is the oldest?

Each year, a tree adds to its girth, the new growth being called a tree ring. The most recently formed tree ring is the new wood near the outer part of a trees trunk, just beneath the bark. The oldest rings are smaller and near the center.