Obviously the German soldiers might notice a tree where there had not been one before. That's why the British Empire's Royal Engineers built fake trees to resemble specific trees in no man's land. During the night, they'd cut down the original and replace it with the fake:
To develop the O.P. Tree, Royal Engineers representatives selected, measured, and photographed the original tree, in situ, extensively. The ideal tree was dead; often it was bomb blasted. The photographs and sketches were brought back to the workshop, where artists constructed an artificial tree of hollow steel cylinders, but containing 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. When the sun rose over the field, what looked like a tree was a tree no longer; rather, it was an exquisitely crafted hunting blind, maximizing personal concealment and observational capacity simultaneously.
Link -via io9 | Photo: Australian War Memorial