Dennis:
Deerlope is correct about the rule of thumb of 5hp/ft. My practical experience indicates that you may be able to get by with a little less if you have a "light duty" mower like the Bush Hog Squealer that uses 3/8" thick blades rather than 1/2" (or even 5/8") thick blades like the medium and heavy duty Bush Hogs.
I have both a light duty Bush Hog Squealer 480 (4' with 3/8" thick blades) and a medium duty Bush Hog 285 (5' with 1/2" thick blades). I was surprised when mowing side by side in 3' grass/weeds (an overgrown pasture) that the light duty Squealer on an 18hp (pto) tractor could travel significantly faster than the medium duty 285 on an 29hp(pto) tractor, yet the Squealer had only 4.5hp/ft and the 285 had 5.8hp/ft. I could attribute the difference only to four things: 1. The light duty mower had 3/8" x 3" blades while the medium mower had 1/2 x 4" blades; 2. the light duty blades were "flat" with no wing to create an uplift while the medium duty blades had the wing at the back of the blade ends; 3. the light duty mower had a 40hp gearbox while the medium duty had a 70hp gearbox, which probably had slightly more friction losses; and 4. with a 6" overlap on the prior cut, the actual cutting widths were 3 1/2ft and 4 1/2 ft, which changes the hp/cut ratio to 5.14 and 6.44, respectively.
If there is a lesson in my experience it is that there are disadvantages (in addition to higher cost) in buying a heavier duty mower than is required. The light duty Squealer is rated for 1" brush, but I have used it on 2" softwoods for years and it seems to have suffered no damage. If your saplings are no larger than 2" softwood (pine, etc.) or 1 1/2" hardwood (oak, etc.), consider a "light duty" mower like the Bush Hog Squealer. You will save on the original cost, it will cut hay, grass, etc. faster than the heavier machine, and you can use a smaller tractor.