I read that after a tree is three years old or so, it can survive colder temperatures. How true is that?
Does this mean, if I can get a Zone 8 tree to survive the first two winters in my Zone 6A, by creating a temporary micro-climate around it, it might be able to survive on its own in the third winter?
Obviously that's not a guarantee, but am I thinking about this the wrong way?
Depends entirely on the tree. Most trees are "suitable" for a particular climate because that's where they can survive (or fruit), not because that's what they adapt to. A fig in zone 4 won't survive, and if the roots do survive through whatever quirk of environment, taking away that protection after a few years isn't going to make much difference. The tree is still dead.
On the other hand, a seedling is incredibly fragile. The first year most die. The second year they're stronger and most survive. The third year they're swearing at you as you try to keep them from taking over the world.
What you're talking about is adaptation, which I think is far more likely to happen with seedlings than a mature tree. Let a mature tree get used to its sheltered environment, it probably wouldn't adapt well when that shelter is removed.
Zone 5b/6a, alkaline soil, 12 inches of water per year. For now the goal is a water independent urban homestead with edible landscaping and food forest.
I am working on selecting for walnut trees that are more cold-hardy. I do that by planting (genetically diverse) seeds. A lot of seedlings die during the first few winters. That's due to them not being genetically suitable for my climate. But once a tree survives it's infancy, they tend to remain winter-hardy.
Sometimes a particularly cold winter will more or less kill the peach trees, regardless of how mature they are.
When it comes to cold hardiness in trees it is all about the bark thickness and cambium layer thickness, the thicker, the more cold hardy the tree will be.
Bark serves several purposes, injury protection, draught protection, fire protection and insulation.
The cambium layer is the living, growing part of the tree, it carries the tree's blood up and down from roots to leaves and back again, warmth is the "heart" muscle of the tree, it is what signals the sap flow.
If the cambium layer is not insulated well enough to prevent Ice crystals from forming, then the tree will go through the same signs of freezing that humans go through, starting with hypothermia moving into frost bite and ending in freezing to death.
Most trees can survive a pretty good range of temperature change from deep winter to hottest summer, this allows us to plant a sub tropical tree in a "border line" temperate zone (usually trees will do ok 2 zones away from their ideal range).
One of the purposes of trees going dormant is to drain the sap from the phylum in the cambium layer, no liquid equals no ice crystal formation thus no freezing to death.
The caveat to this rule is the "pith" trees, such as ficus species, unless the trunks have been able to harden up (thickening of the bark layer) sufficiently they will be subject to freeze damage, these trees go dormant with fully formed leaf buds partly swollen for spring opening.
When the weather gets cold enough to give these buds frost bite, the limb will also become frost bitten and that leads to the death of the limb, the root system is fairly freeze resistant though and once spring arrives, new growth will begin from the roots.
This is why so many fig trees are multi trunked after a few years of growing in most of the USA and Europe while in the Middle East fig trees tend to be single trunk trees.