“Where is Tirpitz?”
Britain’s prime minister Winston Churchill, who was hell-bent on seeing Germany’s largest battle cruiser destroyed, asked the question in 1942 in a memorandum that was notable for its shortness. More than 75 years later, German forest ecologist Claudia Hartl is able to give an answer notable for its method: the growth rings of pines on the Kåfjord in the far north of Norway show that the Tirpitz was anchored there in 1944.
Hartl of the Johannes Gutenberg University in Mainz, Germany, wasn’t looking for an answer to the Tirpitz question when she stumbled upon it. She was investigating instead why something was amiss with some trees she and her students happened to be studying on the Kåfjord. In contrast to the normal-seeming tree rings from other locations in northern Norway, a few cores the researchers took in this area showed no growth ring or a ring that was hardly visible for the year 1945.
Today, at the annual General Assembly of the European Geosciences Union in Vienna, Austria, Hartl discussed new findings that German attempts to hide the massive warship from enemy attacks by means of a chemical fog left enduring evidence in the trees. The previously undiscovered evidence consists of varying amounts of damage to long-lived Scotch pine trees on the lands around Kåfjord. This forensic research is, as far as Hartl knows, the first example of “war dendrochronology.” The team has plans to use additional methods to trace that wartime history in the affected trees.
Hartl and her class discovered that something was wrong with the trees at the Kåfjord after a 2016 excursion to Norway. In subsequent years, they followed up and learned that trees nearer to the fjord had skipped even more years. Some nearest the fjord had even stopped growing for as long as 7 years, returning to normal only after 12 years.
Often, such patterns are explained by drought or insect attacks, but the trees that Hartl had analyzed, Scotch pines, are too hardy to stop growing completely in those circumstances. “It is really unusual for Scots pine in Norway to miss a ring,” said Scott St. George, a climate dendrochronologist from the University of Minnesota in Minneapolis who spent a year in Mainz as a Humboldt Fellow and participated in the study. The absent ring indicated that the tree was going all out to survive. “Because it was not forming wood around its circumference, it was taking all of its resources and all of its energy to regrowing a complete crop of needles from top to bottom,” St. George explained.
“Insects don’t affect them to that severity; a cold summer does cause them to form a narrow ring, but to have 60% of all trees not form a ring at all, that’s really odd,” he said. Furthermore, according to Hartl, insect outbreaks that damage trees “have cycles, so you will see missing rings every 7 or 10 years. This is well known for the Alps but not that you have just one single ring, or several, missing over 200 years. That’s really uncommon.”
She and her colleagues wondered if there was something special about the Kåfjord. One day, she asked a Norwegian tree ring specialist, who replied immediately that the Tirpitz was there.
The largest battle cruiser ever built for the German Navy, the Tirpitz saw little action, but it was essential in forcing the British navy to deploy resources to prevent it from attacking convoys. While it was stationed in Norway, it was repeatedly attacked by submarines and especially by bomber aircraft. To make the Tirpitz harder to hit, the Germans released chlorosulfuric acid from the ship and from land. Droplets of this compound attract water, forming an impenetrable mist in a matter of minutes. British bomber pilots said that the fog covered the terrain to a height of 600 meters.
Hartl and her research team got their information about the cloaking fog from American military reports. Chlorosulfuric acid is irritating for people, but it was otherwise described as harmless at the time, St. George said, “because cows exposed to it didn’t die immediately.” But for pine trees the effect was severe: Their tree rings show that they lost most of their needles.
When St. George and Hartl speak about the trees at the Kåfjord, it is with a certain admiration.
“A tree which shuts down over 9 years, but it’s still alive, imagine this,” Hartl told Eos. “That tree is amazing.”
Regarding this new study, “I found it pretty cool,” said Georg von Arx, a tree ring expert at the Swiss Federal Institute for Forest, Snow and Landscape Research in Birmensdorf. “It was very original research, and it shows what a huge diversity of questions we can tackle with tree rings. And scientifically, it was very well done.”
Von Arx agrees that the missing rings are a sign that the trees had lost their needles. And he offers advice: “I think if she has the chance she should go back and sample them a bit higher up. I would expect to see fewer missing rings as you move upwards in the stem.” This is because a growth ring consists of many rings of sapwood, which transports water. “In the sapwood at breast height you still have alternative pathways for the water, but as you move towards the tips of the branches, there are fewer and fewer sapwood rings, eventually only one. And a new needle needs to be connected to a new level of cells: either there is a sapwood ring there, or there can be no needle growth. This difference with height would show that the tree was rebuilding the canopy before—and this is always second priority—growing the stem.”
The Tirpitz was eventually sunk in November 1944, but its legacy remains in the trees, which Hartl said can survive 400–500 years. It’s possible that the acid fog has left not only physical traces in the tree but also chemical ones inside the wood. The team is now looking for evidence of that, analyzing their samples with a mass spectrometer.
Finding out as much as possible is important, according to Hartl, because until now nothing was known about the environmental consequences of battles involving the Tirpitz, and the same goes for many other engagements in wartime. “The Kåfjord was not the only place where the German navy used the smoke. They used it to obscure other ports and other cities. There may be fingerprints in other places that people haven’t paid attention to,” said St. George.
In some sense, the Scotch pines of Norway are also monuments, he added. “We’re losing the generation that remembers the Second World War. It’s easier to forget the lessons after that event. In this case, the trees still keep that evidence alive. Even if you didn’t know anything about the history of the Kåfjord, or the Tirpitz, the information is still preserved, when you have the right person to read it.”
—Bas den Hond (email: [email protected]), Freelance Journalist