The French military was the first to incorporate trees into its arsenals, using hollowed-out trunks as observation posts or gun turrets as early as 1915. The British and the Germans also “recycled” trees during World War I in order to secretly place soldiers in strategic positions along the front lines. These “Observation Post” (O.P.) Trees — or Baumbeobachter, as the Germans called them — were time-consuming to make. The ideal tree was dead, often a casualty of bombing. After finding the right specimen, a replica would be constructed and reinforced inside with steel. At night, the original tree would be taken down to its roots and replaced with the O.P. Tree.
I think that I shall never see/A poem lovely as a tree:
The tree replicas had the same dead and broken limbs, with expertly crafted “bark” made from wrinkled, painted iron.
To make the bark appear more real, artists would often cover the tree with a rough-textured concoction made from materials such as pulverized seashells.
Soldiers would climb up a narrow rope ladder through the middle of the tree and sit near the top. Sections of the outer bark were cut away and replaced with mesh to disguise viewing holes.