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https://www.seeker.com/why-trees-dont-grow-on-mountaintops-1792565118.html suggests that the treeline for Humphrey's Peak, in Arizona / Coconino County is at 11,400 feet (3470 metre), but is there a more authoritative source for that? Like is the treeline something that USGS tracks?

Similarly, https://ak.audubon.org/southeast-alaska-birding-trail-town/mount-roberts-alpine suggests the treeline for Robert's Peak and the surrounding mountain range is 1,760 feet (540 metre), but again, a more authoritative source would be good. Not to mention that it'd be cool to have a more consistent resource that doesn't require blind Google searches each time!

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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.)

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    Note that USGS maps are often decades old and are no longer being updated (the new US Topo series is so useless that calling it a map is an insult to mapping). Due climate change, the tree line has likely risen considerably since they were published. – gerrit Nov 17 at 10:01
  • Apparently it's 150 metre in 52 years in the Altai Mountains. At a 15% slope, that would mean a horizontal shift of a kilometre. The trees in the newly forested area are going to be young and small. Although I agree that the shrinking of glaciers and firn is more dramatic, a changing treeline is also something to keep in mind if you use a 40 year old map to follow a tree line. – gerrit Nov 17 at 16:01
  • I think you made an arithmetic error in your comment. If global warming is as bad as 5°C/100 year (it may be in the Arctic, probably a bit less in the temperate mountain zones), it would be 3°C in 40 years (if linear, which it isn't), which would correspond to 3/7th*1km or around 430 metre. In reality it hasn't been quite as fast as that yet, but an order of magnitude of 100-200 metre seems more feasible than 2 metre. – gerrit Nov 17 at 16:04
  • @gerrit: The Altai Mountains are a somewhat cherry-picked example. On a global map of the velocity of climate change, central Asia is an order of magnitude different from most of the globe: news.stanford.edu/news/2013/august/… – Ben Crowell Nov 17 at 16:05
  • The part that's faster on the map looks like Tibet to me; by contract, Altai seems to be firmly in the green parts of the map, which is consistent with the Nature article I linked, which estimates temperatures in the Altai have risen by 1.3–1.7°C in the same period during which they estimated the 150 metre increase in tree line. How typical that is I don't know, it's the first result on climate change impact on tree lines I found. – gerrit Nov 17 at 16:09

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