text: Churchill described the German battleship Tirpitz as "the beast" Throughout most of World War II, Allied bombers tried repeatedly to sink the Tirpitz, Germany's biggest battleship and a bete noir of Britain's wartime leader Winston Churchill, who took to calling it 'the beast'. 
   
   On Wednesday, tree experts at the annual meeting of the European Geosciences Union showed why they failed to do so until late 1944 
   
   "The story was in the tree rings," said Claudia Hartl, a researcher at Johannes Gutenberg University in Mainz, Germany. 
   
   The unlikely evidence of WWII battles was uncovered during the summer of 2016, when Hartl led students on a routine survey of forests around Kafjord, one of dozens of fjords along the northern coast of Norway. 
   
   "We got back to the lab and measured the tree rings, and saw that they were very narrow—in some cases nearly absent—for 1945," she told AFP. 
   
   The forests, in other words, had been hit by an environmental cataclysm. 
   
   "Of course we wondered, why is that?" 
   
   The first suspect was insect infestation, which can come suddenly and have severe impacts, especially in high-latitude boreal forests. 
   
   Driven north of their historic range by climate change, mountain pine beetles, for example, have recently devastated large swathes of forests in Canada, sometimes in a single year. 
   
   But there were no known insect in northern Scandinavia that could have delivered that kind of environmental shock in the middle of the 20th century. 
   
   "It wasn't until we spoke to a local scientist based in Tromso that we made the connection to the Tirpitz," said Scott St. George, a geographer at The University of Minnesota's Institute on the Environment who took part in the research. 
   
   The Tirpitz and its crew of 2500, it turned out, had retreated into northern Norway's watery labyrinth to escape detection. In the pre-satellite era, even a 250-metre (820-foot) behemoth wasn't that easy to spot. 
   
   But Allied aerial scouts finally found it, and the attacks began. 
   
   The Germans, however, had a counter-plan: producing vast quantities of artificial fog, enough to hide the ship and surrounding area from aerial view. 
   
   And that's where the tree rings come in. 
   
   A 1944 picture shows smoke rising from the Tirpitz following an attack by allied bombers in a Norwegian fjord 
   
   Arboreal casualties 
   
   "The smoke drifted into the forests surrounding the fjord and damaged nearby pine and birch trees, leaving behind a distinctive and unusual 'fingerprint'," St. George told AFP. 
   
   The study of tree rings—called dendrochronology, literally, "timeline of trees"—is used by climate scientists to trace changes in temperature, rainfall or river flows reaching back hundreds, even thousands, of years. 
   
   The concentric circles found in temperate zone tree trunks can also date the age of buildings, shipwrecks, musical instruments, painting frames or anything else made from temperate-zone wood. 
   
   Because trees in the tropics grow continually, they generally do not produce rings, which show growth spurts during spring and summer. 
   
   To investigate further, Hartl returned last summer to the scene of the battle, to see how far the damage had spread. 
   
   She established five test sites ranging in distance from a few hundred meters from the fjord where the Tirpitz was berthed, to about ten kilometres. 
   
   Near where the ship once lay, more than 60 percent of the trees showed virtually no growth in 1945. All of them were affected to some degree. 
   
   Gaps in the forest where young trees sprouted up in the 1950s suggest the chemical fog caused arboreal fatalities too. 
   
   As far as four kilometres away, more than half the tree were severely affected, taking eight years on average to fully recover. 
   
   Pine trees—which keep their needles for up to seven years—were hit hardest. "Being stripped bare would have been a more difficult challenge for that species," St. George explained. 
   
   The artificial fog that denuded the trees was likely made from chlorosulphuric acid which, when mixed with water, produces a thick, white vapour. 
   
   German ships has special teams equipped with gas masks to generate the smoky shroud. 
   
   Despite its firepower, the Tirpitz never saw much action. 
   
   In October 1944, the German naval command moved it to Tromso, where it served as a moble artillery platform until a squadron of 32 British Lancaster bombers sent it to the bottom of the harbour the following month. 
   
   Explore further: Tropical trees maintain high carbon accumulation rates into old age ||||| Media playback is unsupported on your device Media caption Claudia Hartl: "You don't normally see trees in this region without rings" 
   
   The relentless campaign to find and sink Germany's WWII battleship, the Tirpitz, left its mark on the landscape that is evident even today. 
   
   The largest vessel in Hitler's Kriegsmarine, it was stationed for much of the war along the Norwegian coast to deter an Allied invasion. 
   
   The German navy would hide the ship in fjords and screen it with chemical fog. 
   
   This "smoke" did enormous damage to the surrounding trees which is recorded in their growth rings. 
   
   Claudia Hartl, from the Johannes Gutenberg University in Mainz, Germany, stumbled across the impact while examining pines at Kåfjord near Alta. 
   
   The dendrochronologist was collecting wood cores to build up a picture of past climate in the area. Severe cold and even infestation from insects can severely stunt annual growth in a stand, but neither of these causes could explain the total absence of rings seen in some trees dated to 1945. 
   
   Image copyright Claudia Hartl Image caption Some had a ring but you would hardly see it 
   
   A colleague suggested it could have something to do with the Tirpitz, which was anchored the previous year at Kåfjord where it was attacked by Allied bombers. 
   
   Archive documents show the ship released chlorosulphuric acid to camouflage its position. 
   
   "We think this artificial smoke damaged the needles on the trees," Dr Hartl told BBC News. 
   
   "If trees don't have needles they can't photosynthesise and they can't produce biomass. In pine trees, needles usually last from three to seven years because they're evergreens. So, if the trees lose their needles, it can take a very long time for them to recover." 
   
   Image copyright Imperial War Museums Image caption The Tirpitz pictured in Kåfjord with the smokescreen seen drifting across the water 
   
   In one tree, there is no growth seen for nine years from 1945. "Afterwards, it recovered but it took 30 years to get back to normal growth. It's still there; it's still alive, and it's a very impressive tree," Dr Hartl said. 
   
   In other pines, rings are present but they are extremely thin - easy to miss. As expected, sampling shows the impacts falling off with distance. But it is only at 4km that trees start to display no effects. 
   
   The Tirpitz sustained some damage at Kåfjord. However, a continuous seek-and-destroy campaign eventually caught up with the battleship and it was sunk by RAF Lancasters in late 1944 in Tromso fjord further to the west. 
   
   Dr Hartl believes her "warfare dendrochronology" will find similar cases elsewhere. 
   
   "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. In other places in Europe, they also used this artificial smoke and maybe also other chemicals. So perhaps you can find similar patterns and effects from WWII." 
   
   The Mainz researcher presented her research here at the European Geosciences Union (EGU) General Assembly in Vienna, Austria. 
   
   Image copyright Claudia Hartl Image caption Sampling shows the impacts falling off with distance 
   
   Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
target: – A German dendrochronologist stumbled on something puzzling while gathering wood cores on the Norwegian coast in 2016: some trees were missing rings. "We got back to the lab and measured the tree rings, and saw that they were very narrow—in some cases nearly absent—for 1945," Claudia Hartl tells the AFP. A local scientist had a theory, she explained Wednesday at the annual meeting of the European Geosciences Union: that it had something to do with the Tirpitz. The 820-foot battleship was the largest in Hitler's navy, and the BBC reports it spent much of WWII anchored and hiding along Norway's coast, including in Kåfjord, where Hartl observed the tree-ring anomaly. But then Allied forces found it, and that's when the trees suffered, believes Hartl. The Nazi ship released chlorosulphuric acid as a sort of "chemical fog" to try to cloak it from aerial enemy forces, and Hartl now believes the acid caused damage to the trees' needles. "If trees don't have needles they can't photosynthesize and they can't produce biomass," she explains, noting recovery can be a multi-year process as pine trees can retain their needles for as long as seven years. And that jibes with what she found: In one case, a tree had no growth for nine years and took three decades to revert to "normal growth." Near where the ship was stationed all the trees she studied were affected, with more than 60% not growing at all in 1945. She found the impact on the rings diminished with distance, with trees about 2.5 miles away starting to be unaffected. As for the Tirpitz's ploy, it didn't work: It was ultimately sunk. (Using ink and urine, this man recorded Nazi horrors.)