I am getting more fruit tree customers as time goes by, and have been working on some multiple times now. Many are decades old and producing well. I've done research and observed the trees.
We've started an orchard at home, and we have had a few fruit trees established and producing. Some of the following can apply to more than apple trees, but I will try to stick to what I believe to be true for apples.
You can skip to the OR state extension for some good pruning and training info
http://extension.oregonstate.edu/catalog/html/ec/ec1005/ , starting with restoring a neglected apple tree. I think that various fruit trees can be found on the OR Extension site.
Somewhat counter-intuitively, we tree apple trees a bit like shrubs-ish, and a bit like bonsai. Think of the pruning as significant crown reduction. As Winch said, 1/3, can be in the ball park, unlike what people frequently hear for most trees, 20-25% on vigorous trees.
As well, bending/ training limbs horizontally can reduce age to production, and increase yield. These are being by tying down branches or placing spreader sticks.
Production apple trees are headed/ topped, severely pruned, and often forced away from a central leader and apical dominance.
Apple trees are shaped for convenience of pruning (accessible with an orchard ladder), picking, and producing smaller numbers of higher quality fruits. Some people train a central leader, others use an open vase for apples, and it also varies by species.
A customer has apple trees with a 30-35' spread on three main branches, and only 8' high.
We have to keep ours at home, and some customers a bit taller because of deer, and we don't want to put plastic deer netting over the whole tree.
Light penetration and air flow are important for us in western Washington due to the climate, so we avoid more fungal problems.
Neglected apple trees will often have columns of decay from the heading cuts. If they are not trained right, from the beginning, or at least a young age, it is a bit unavoidable. They still live on quite well.
Trees trained from a young age can be central leader, open vased, or espalier-ed (grown along a wall or trellis. This is cool, and provided fresh fruit in small spaces, which is good for urban areas, maybe even in a huge pot on a balcony.
Grafting is another aspect of apple/ fruit trees that is an interesting thing. I always have heard that the Chinese developed grafting, maybe 5000 years ago--or some old time ago.
You can one hardy rootstock, with a head stock grafted onto it, say Gravenstein, then graft on Johnagold, Red Delicious, and whatever else, kinda like connecting plumbing-- get the sizes of PVC connectors right, glue together, then the tree takes over from there.
Usually a short branch of one tree is cut and shaped like an ax-head on the end, and the parent tree is notched to accept the ax-head shaped "scion" (I believe). Some "glue"/ grafting compound is wiped on , and the grafts are sometimes splinted, maybe tied a bit, too.
I haven't done this yet, but I will hopefully in the future.
Something that I recently read is that you can cross-species graft. Stone fruits like cherry, apricot, and peach can all be grown on one tree (I believe).
Apples, pears, and that type with a central cluster of seeds can be grown together on another tree (I think).
I really don't know all the details on this, so take it as a research jumping-off-point, not scholarly publication.
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