Lisa,
It's funny how nature sometimes refuses to give us trees that stand up straight or insists on introducing other "flaws" in our photos. In Death Valley National Park, the sides of the valley have broad, gently sloping planes running down to the valley floor several miles away. I have several photos taken with a level camera in which the photo looks as if the camera was tilted because the main visual reference in the photo is sloping. In some cases, I have elected to rotate the photo a little bit to make the "horizon" appear level because the more realistic presentation is just too visually disturbing.
John