The tree line is not a sharply defined line. It's actually more of a gradual, irregular fade out as you go up in elevation. Here is an example from an area I visit a lot: https://en.wikipedia.org/wiki/Mount_San_Antonio#/media/File:Mt.Baldy_Looking_NorthWestAtSummit.JPG The person in the yellow shirt is standing on a trail that is very close to tree line at about 9500' (2900 m). There are trees on the downhill side of the trail, but not many on the uphill side. But there are some trees on the uphill side. And if you look at the summit in the distance, which is at 10,060' (3070 m), you can see that there are trees very nearly up to the summit, i.e., about 500' (170 m) higher.
What happens as you get close to tree line is that the trees get stunted and krummholtzed, and they get more sparse. However, some will survive higher up, for instance if they're in some little drainage that gets more water, or have better shelter from the prevailing winds. In the photo, the saddle in the middle distance is a wind tunnel, so almost no trees are able to survive there. But the southern (left) shoulder of the summit usually doesn't have much wind, so you have trees there, even though it's higher than the saddle.
USGS maps do not show the tree line as a line. They do show forested versus non-forested areas, so you can usually get a pretty good idea of where the tree line is. But the threshold for marking something as forested is some kind of subjective judgment about the density of trees, not a complete absence of trees. (Legend has it that this was originally defined as trees dense enough to provide cover for a squad of infantry.)
