Clone Stamp Issues with Masked Photo

Hi!  I'm working on an image for the back of a CD tray.  I created one image with snow using a photo and some adjustment layers.  Then I used another photo of trees to mask off one tree that I wanted to use.  I placed that tree on the snow.

I want to use the clone stamp to "fill out" the tree a little more. This would require that I select part of the tree and stamp it on part of the image that has part tree mask and part snow background.  But whenever I use the clone stamp and try to place it partly where the masked tree is and partly on the snow, it only stamps on the part of the tree that I masked.  The snow remains untouched.

I don't want to do anything "permanent" if I don't have to because I'm not doing this for me - so I want to be able to still move the tree around, etc....if the person I'm doing this for wants this moved.

Or...now that I think about it more, if there is a way to alter the file where I masked the tree so I could fill out the tree in that file *before* even placing it anywhere, that would be great...probably optimal since I could use the tree elsewhere.  But I would still need a transparent background so I could place it on any background.

Any advice on how to do this efficiently would be very, very welcome.  Thank you!