willie,
Generally, if a tree grows straight up then any half when split would be similar in strength than the other half. If the tree doesn't grow straight and is leaning, or in the case of a horizontal branch, the wood density and characteristic are different from the upper half compared to the lower half--the tree creating differences in order for the tree or branch to have the strength to grow leaning. So there becomes a tension and compression side of the tree itself. So if you make bows from the two halves then theoretically you are now making bows from inherently tension or compression wood, not to be confused with the tension/compression sides of the stave you end up.
Its different for hardwoods and softwoods, exactly what happens, and you can read about it, but theoretically with vinemaple the skyward half, is inherently tensioned wood, and so it will, as it dries, curl, because it has been released from it compression half. It will be the better half for making an efficient bow.
I have not found this to be true with any wood that I've dealt with, yet. But I haven't worked alot of vinemaple. In the case of the stave I'm working now, the tree was growing straight up but at some point in its life steered slightly so that the "straight up" trunk had a natural bend that would yield a reflexed and deflexed half. After drying neither half of the log moved much in any direction.
