Dendrochronology cross dating
For each core that we have, we need to gain a high confidence in the actual date that we assign to each ring.
Unfortunately, any given tree may sometimes have a missing ring or a false ring present and therefore every ring we count thereafter is thrown off by one or two years.
The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" (or "spring wood", or "late-spring wood" Many trees in temperate zones make one growth ring each year, with the newest adjacent to the bark.
During the first half of the 20th century, the astronomer A. Douglass founded the Laboratory of Tree-Ring Research at the University of Arizona.
Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth, which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → tree rings).
New growth in trees occurs in a layer of cells near the bark.
A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings.