I've never shot my longbow from a stand..

[J. Cook]

I agree with most of the posts -- at short distances, the elevation of the tree doesn't make any difference on arrow flight.  People change their form, then blame it on elevation or physics.  BEND AT THE WAIST.  At short distances, if your top 1/2 is in the proper alignment (just like shooting from the ground) then you'll see NO difference in impact point.  

It's when you simply lower your bow arm or get in odd positions to get the shot off that you have issues.  If you concentrate of bending at the waist and keeping your arms, shoulders, neck, and head in the same position that you'd be in on the ground -- you'll be golden!

[BOHO]

I dont know if Rick Welch's Accuracy Factory is on you tube but he talks about shooting a trad bow from a stand in it. great clip

Roll the bow over until it is nearly flat for close in shots or if the stand is high, follow it with the rest of the form.  I am 5'9" and I will be using a 68" bow out of my right hand placed stand and a 64" out of my left hand placed stand.  I just had to trim one branch in the setups to make clearance for the 64" lefty. Oddly enough out of my Huntmor chair I use one of either two 51 pound 62" r/d bows most of the time when hunting on the ground and sitting, I use the 68" when I am sneaking around.  The choice of bows has nothing to do with the stand, those are just bows I am having the most fun with this year.
If you think about it, if you are 15 feet up and shooting out 15 yards, that is a three to one ratio, who passes on a shot because it is down a 30 degree slope?