Dendrochronologists are experts at reading tree rings. They can learn a great deal of information—including past climate in an area and the age of the tree—by taking a tree core sample and reading between the lines (literally).

But as the BBC reports, one climate researcher was stumped when she discovered that many trees in the Norwegian village of Kåfjord were missing their rings. Extreme weather and invasive insects can cause some degree of damage to trees, but not enough to render them ringless.

Claudia Hartl, from Johannes Gutenberg University in Germany, knew that these trees dated back to 1945, but that alone wasn't enough information. Two other clues that helped Hartl and her colleagues solve the mystery were location and history. During World War II, Nazi soldiers moored the Tirpitz—the largest battleship of Hitler's navy—off the waters of Kåfjord to intercept vessels carrying Allied supplies to the Soviet Union. The Germans released an artificial smoke containing chlorosulphuric acid to conceal the ship's location, and this is believed to be the root of the trees' problem.

Artificial smoke could have damaged the needles of the trees, halting the photosynthesis process and stunting the trees' growth, researchers found. It takes time for the trees to recover, but it is possible. One tree saw no growth at all from 1945 to 1954, but after 30 years its growth had returned to normal. Hartl presented the findings at the European Geosciences Union General Assembly in Vienna this week [PDF].

"I think it's really interesting that the effects of one engagement are still evident in the forests of northern Norway more than 70 years later,” Hartl told BBC News. She believes her "warfare dendrochronology" will unearth similar findings elsewhere in the world.

[h/t BBC News]