Why Do Tree Rings Vary in Width?

In regions with distinct seasons, the cambium produces new wood towards the inside of a trunk during each growing season. In many conifers, cells formed early in the season are larger, thinner-walled and lighter in colour; this is earlywood. Cells formed later are smaller, thicker-walled and darker; this is latewood. Together, the two parts usually form one annual ring.

A wide ring indicates more radial growth during that season, while a narrow ring indicates less. It is not, however, a direct rainfall gauge. In moisture-sensitive trees, wetter years may produce wider rings and drought years narrower ones. Near a cold high-elevation limit, growing-season temperature may matter more. Age, soil, competition from neighbouring trees, insects, fire and crown damage can also alter ring width.

Suppose the same pine produces a wide ring followed by a narrow one. That observation can support a hypothesis that the second year was drier, but it cannot establish the cause by itself. Researchers cross-date rings from many trees, then calibrate their patterns against nearby instrumental weather records. Only recurring signals shared across samples are suitable for reconstructing past climate.

Tree rings are therefore both biological structures and environmental records, but they preserve a tree's combined response to several conditions. They contain a signal without automatically supplying a single explanation.

https://www.ncei.noaa.gov/news/how-can-tree-rings-teach-us-about-climate
https://ltrr.arizona.edu/about/treerings