It didn't "cut" your photo, as such, it's simply that this part of the sensor is basically blown out on all channels and so has become pure white. If you think of each spot on the sensor as basically a bucket that can contain some amount a light, what has happened is that the sun, being such an intense source of light, has filled all the buckets in the area and so it appears that there is now nothing there.
Now, the way to avoid the problem is through the use of neutral density and graduated neutral density filters or not shooting directly at the sun when it is in a more peak position. For a photo like this one, however, you'd be pretty hard pressed to avoid this happening while still maintaining proper exposure of the tree. In fact, if the tree was more correctly exposed, the effect would have been worse. So your best bet, in this circumstance is to try and make the effect more artistic as best you can.