Design Elements

 The steady brightening or darkening of the color of the mountains and changing size of the trees give a sense of depth and perspective and a sense of proportion.  The horizontal lines give a sense of calmness and balance.  There is also an implied texture from the trees on the mountains.

Over all it's very geometric.  The large amount of implied circles and lines (I'm not counting them) and the variety of colors create chaos.  The diagonal and parallel lines and arcs and repetition of the circles give a sense of movement and order to the disorder.

P.S.  There are 2 actual circles with no partial overlap or change in color.