"Built using steel and wrought iron, these are no normal trees. They are camouflaged weapons of war used to devastating effect during World War I on the Western Front. Both sides used these bizarre fake tree observation posts to spy on and shoot at the enemy after switching them under cover of darkness with real battle-scarred stumps left in no-man's land. With a perfect elevated position overlooking the enemy and the element of surprise, they claimed many victims with both sides recording scores of casualties shot by snipers housed in the outposts called 'Baumbeobachter' - or 'tree observer' - by the Germans and 'O.P. Trees' by the British and French. Both sides also used real trees for observation, building ladders up them and sometimes viewing platforms. But the O.P Tree was specially designed, and particularly deadly. On the British side, special squadrons of Royal Engineers were tasked with meticulously selecting a real tree on the battlefield by measuring and photographing it extensively. The ideal tree was dead and often it was bomb blasted. The photographs and sketches were then sent to a workshop where artists constructed an artificial tree of hollow steel cylinders. It contained an internal scaffolding for reinforcement, to allow a sniper or observer to ascend within the structure. Then, under the cover of night, the team cut down the authentic tree and dug a hole in the place of its roots, in which they placed the O.P. Tree." The Army's special branch: How bizarre fake spy trees appeared in no-man's land during WWI | Mail Online
There are one or two fake trees around RAF Brize Norton in Oxfordshire...Always wave to em when passing by.