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: