Some Examples:
Derek Mohr
Mohr used an extremely shallow depth of field, and it looks like he used free lensing to create an even shallower depth of field and distortion. The shallow depth of field and distortion makes this tree a dynamic and interesting subject.
Bill Vaccaro
Vaccaro used a shallow depth and a vignette to emphasize the center of the photo, drawing the viewers' eyes to the tree's branches and trunk.