I think the "tilt shift" effect is a bit to strong and it needs a painted mask to work properly.
You are applying a "3D" lens effect to a 2D image, so you are not considering the camera distance in "3D" to the objects.
Look at this image.
The dome is in focus as well as the other objects in that "distance plane" in 3D...even tho' the background buildings are in the same "line" (as in XY coordinates of the image) they get out of focus because they are indeed farther away in space...
So the closest tree in your image, will probably be all out of focus, not just the bottom part.
The gradient mask works well, but you can paint with a soft brush the elements wich are not in the same distance with your object of focus...
I hope I've explain it well, pardon my english :)