It's the use of natural shapes or objects as devices to frame the photo.

Creating a frame withing the frame of a photo is an effective an easy way to add interest to compositions. The inner frame focuses on the important aspect of the image, while the outer frame adds a decorative elements. Frames withing frames increase our understanding and appreciation of the subject, can present visual simmetry, or echoes of simmetry- partial simmetry that the viewer's mind completes. The idea is to direct the viewer's attention to the primary subject of your scene.

As with many graphic devices, inner framing elements are often most effective when used with restraint. A framing device that is excessively large, colorful or bright can overwhelm the image and ruin the effect. Used properly, framing devices can add an artistic effect without the viewer even being conscious of their presence.