The tree and its annual rings

 How are the annual rings formed?

In spring, when the tree resumes photosynthesis after winter dormancy, the new annual ring forms just under the bark.

More precisely, cells multiply in the cambium, an alternating tissue between the wood and the bark (bark consists of the outwardly protecting bark and the bast). Some cells are added to the bark, the larger part goes inward to the wood. The cambium also forms the callus tissue, which forms the wound closure when injured.

In spring, the light-colored early wood is formed. The cells here are large and loose. Water can be transported well.

Later in the year, the dark late wood is formed. The cells are smaller and firmer and give the tree stability.

This is followed by the winter dormancy, during which there is also no wood growth.

Thus, a light earlywood ring and a dark latewood ring together make up the wood increment of a year.

What do the annual rings tell us?

Not only the age can be read by counting the rings. Depending on the thickness of the rings, you can tell whether (weather) conditions were favorable or unfavorable for growth, such as drought. The location of a tree can also be reflected in the annual rings; the annual rings on the sunny side are thicker.

In the science of dendrochronology (see also the panel on the „Wall of Records“), the respective „annual ring patterns“ of successive years are recorded and compared. This makes it possible to determine when a tree grew. Meanwhile, the annual ring patterns for Central European oaks of the last 12,000 years are known.

Other disturbances – such as a neighboring tree that took away light or a fence that was leaned against the tree for a long time – also show up with „dents“ in the annual rings.

 

The sketch shows that growth conditions were better on the lower right, where the annual rings are much wider.

Sources: Prof. Michael Hohla, TIME-LIFE Book Der Planet Erde „Wälder“, Reader´s Digest „Wunderwelt der Pflanzen“, Erwin Thoma „Die geheime Sprache der Bäume“.