My guess would be shadows. And I'd be correct. On a smooth surface the shadows of the small tress might have been sharper; however, against a rough concrete wall the edges diffused into the peaks and valleys softening the effect. Lines from the seams of the plywood forms are still clear even in the photo as are the quarter size indentations left from the form ties. All I had to remember was to keep my own shadow from intruding into the scene. Often times when composing the shot, the fact that the black blob towards the bottom is yourself just does not register.