Warfare dendrochronology: Trees witness the deployment of the German battleship Tirpitz in Norway

War has an immediate and obvious effect on people and communities, but its impacts on local ecology can be more subtle. This paper shows how one military encounter in the Second World War has left a clear legacy in the northern forests of Norway, trackable more than seventy years later. We used annual growth rings of ∼180 pine and ∼30 birch trees as witnesses of the deployment of the German battleship Tirpitz at the Kafjord. The Tirpitz was the target of several Allied air attacks, but the Kriegsmarine (German navy from 1935 to 1945) used artificial smoke, consisting of chlorosulfonic acid and zinc/hexachloroethane, to hide the ship. These smoke-screen actions throughout 1944 caused pine forests surrounding the Kafjord to exhibit a strong and unusual growth decline during the following year. Severe defoliation and limited photosynthetic activity likely triggered this decline. The tree damage extended up to 4 km away from the Tirpitz. In the most extreme case, growth was interrupted for nine years. ‘Warfare dendrochronology’ could help to evaluate potential environmental impacts of the Second World War on forest health and composition elsewhere in the European theatre.