What actually sets the tree line
The elevation where trees stop is remarkably consistent when expressed the right way, and the right way turns out not to be temperature at any single moment.
The best global predictor is growing-season mean soil temperature at root depth, sitting near six to seven degrees Celsius across a wide range of latitudes. Not air temperature, not winter minimum, not snow depth. The limitation appears to be on tissue formation rather than on photosynthesis: below that threshold, a tree cannot build new cells fast enough to replace what it loses, regardless of how much sugar it is producing.
This explains a few things that a simple cold-limit model does not. Trees at the line are often carbon-rich rather than starved, which is the opposite of what you would expect if photosynthesis were limiting. And krummholz — the stunted, ground-hugging growth form above the upright line — persists because the microclimate near the ground clears the threshold when the air a metre up does not.
Local tree lines depart from the global pattern for local reasons: wind exposure, avalanche paths, grazing, fire, and human clearance that in some ranges is centuries old and no longer obvious.