Cardon Cactus - Echinopsis
I believe this is Echinopsis terscheckii or Echinopsis atacamensis.
These tree-size cactus are growing in the High Desert in North-West Argentina, Chile and Bolivia. I saw this one near Cachi (Argentina). There is another one here
If you can precisely identify the species of cactus, please post a comment!
This is a HDR (High Dynamic Range) image from 3 RAW exposures, processed with tone mapping.