Tree lines are usually not light restricted, but temperature. Average summer temps need to be above 5-6 C for even very cold hearty trees. This is actually the theory behind using tree growth rings to try to determine past temperature. The issue with that is that temperature isn't the only factor in tree growth (especially in the varieties of strip-bark trees common at treelines) so the temperature signal is very, very messy resulting in bad resolutions due to error bars. Still, direct observations have pegged the average temp needing to be in the above range.
Light isn't the limit for tree lines because we're talking summertime temps, and summer in the Arctic (or Antarctic) is full of sunlight. On mountains, sunlight is also usually very plentiful during the growing season.
As for
@TheSword's question to find scientific study showing trees can't survive -59C average temps with no sunlight if blanketed in snow for two years, it's a fool's errand: that kind of condition doesn't exist anywhere and isn't of much scientific interest because it doesn't exist. I mean, we're talking burying a tree in snow at the South Pole to achieve these conditions, just to confirm that, yep, it dies. We know it dies because trees, even evergreen cold hearty trees, die without sunlight in much less time, and no tree lives at that average year-long temp. The knowledge about plants and light comes, in part, from the Norse, who would shelter evergreens in caves during the winter for up to six months. These would die if left in the cave much more than six months, and temps in the caves, while cold, were far milder than deep winter temps, so it was the light levels that did it.
I'm still struggling with the argument that normal trees should be expected to live with no light and brutal, arctic cold all the time for two years. The cold does in trees at much milder temps even when there's light. Taking away the tree's ability to create food for two years seems a no brainer as to the result, but here we are.
