Daniel,
When you hunt from a treestand there are a couple of things that affect your accuracy that many of us don't think about, the real range and our form.
As mentioned above, make sure that you keep your arms/shoulders at the same angles that you have when you are shooting from the ground. You do this by bending at the waist. It keeps your arms at right angles to your body. This way you are drawing the same amount as on the ground, which puts the same amount of energy into the arrow as on the ground.
The second thing to remember, which is a big part of why you are hitting high, is that the range to your target (the animal) is not how far away it is from where you are but how far away it is from the bottom of your tree. You are actually holding for something farther away which is what raises your point of impact.
And for a final point about treestand hunting, and this is because I want to see you posting here more often, ALWAYS where a full body harness while in the tree. Attach it to the tree and move it up as climb up, while you are on stand, and while you are climbing down. More than half of all bowhunting accidents are related to treestand hunting, so don't become a statistic.
Snakeeater