When rainfall increases each spring, trees devote their energy to growing new cells. New cells get smaller as summer progress until the fall when they stop growing for the winter. The contrast between the older autumn cells and the larger spring cells establishes rings that are visible when a tree is cut in half.
Studying Tree Rings
In the 1890s, Andrew Ellicott Douglass, an astronomer at the Lowell Observatory in Flagstaff, Arizona, noticed that Douglas firs grew thicker rings in periods of heavy rain than they did in years with sparse rainfall. Douglass studied rings of living trees and those found in wood preserved in villages of Native Americans to construct a calendar of climate change dating to 700. This was the beginning of dendrochronology, studying the growth of tree rings to date climate changes including rainfall.
Xylem Cells and Tree Rings
The xylem cells, the vessels that transport plant nutrients, are found in the cambrium, the growing tree tissue just under the bark. It is easier to read the rings of trees that have porous, concentrated xylem cells than species where these cells are smaller and more diffuse.
Effects of Rainfall
Abundant rainfall ordinarily increases the growth of trees, producing wider rings; drought suppresses growth, producing narrower rings. One to three narrow rings in a row suggests drought.
Response of Different Species
Not all trees respond to rainfall in the same way. For most species, more rain results in thicker rings, however some species produce narrow rings in years with heavy rain.
Some species of trees grow more in periods of drought than others.
Rain usually contributes more to the growth of trees in the spring than it does in the late summer.
There is no judging with certainty the effect of rainfall on a tree ring because other conditions of climate are involved.
Changes in temperature, including global cooling and warming can affect the growth of trees.
An early spring with warm temperatures results in a longer growing season that will cause a tree to have a wider ring. A spring delayed by a late frost can result in a shorter growing season and a narrower ring.
Crowding by other trees results in increased competition for soil nutrients and can result in slower growth and narrower rings. When there are more than three rings in a row, crowding is a possibility.
Plant disease can slow the growth of a tree and result in narrow rings.