Depends on the maker of the tree. It will also depend on how the person making the tree defines an Arabian back.
Steele is the largest manufacturer of trees. They say this about the trees they make:
"J - Semi-Quarter Horse (Semi)/Arabian (Arab) - Steeper front and rear rafter angle and closer-spaced bars relative to Standard Quarter Horse fit when positioned at standard spread. In addition, sufficient bow (rocker) enables this fit to conform well to the short Arabian back having a narrow wither dropping off quickly to the shoulder.
AW- Full Wither (FW)/Arabian (Arab) - Flared out front and rear to allow for full or mutton wither with no pockets. The resulting bow enables this fit to conform well to the short Arabian back having wide withers rounding out into the shoulder. The AW has also been found to work for more dipped-back draft horses."
See here: Steele Saddle Tree LLC - Fit To The Horse