Here’s a beautiful picture of a foxtail pine from the MWP, illustrating eloquently the change in treelines: 
Original Caption: A dead trunk above current treeline from a foxtail pine that lived about 1000 years ago near Bighorn Plateau in Sequoia National Park.
This is taken from an article here by Andrew Bunn. Bunn has written a terrific article on stripbark trees, cited in our E&E article. (He also posts occasionally on the R-help line.) I mentioned in connection with the Polar Urals that the need for an altitude adjustment seems pretty obvious for tree rings, if there are material changes in altitude. I’ve seen some very important references to changes in bristlecone pine altitude (with a dramatic lowering since the MWP). If the effect went the other way, you can be sure that the Hockey Team would adjust like crazy for it.
Climate Audit 
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But the MWP never happened. You’re just in denial, Steve. 😉
so does noone use elevation as a formal proxy?
What’s even more interesting is that some of these sites are the very ones that are ESSENTIAL to all hockey stick diagrams. I’ve been meaning to post up an elevation diagram on bristlecones and I’ll do it today when I find it. Actually, it’s interesting to carry the elevation idea along together with the data-mining/non-robustness idea. I’ve got two locations in the entire world right now where I have quantified estimates of treeline over the past millennium (neither done by core Hockey Team). One is bristlecone; I think that there’s one for foxtails and one is Polar Urals. All have depressed modern treelines relative to the MWP. So aside from CO2 fertilization in the bristlecones, you have elevation changes.
I understand the argument that treeline changes are delayed i.e. that treelines would not move in 15 years. But the bristlecones were not reproducing in the mid-1950s so I find it hard to believe that the mid-1950s were warmer than when these foxtails were established: which is part of the proxy syllogism. (The syllogism is: they can “show” by proxies that the mid-20th century was “warmer” than the MWP; they can show by instruments that the late 20th century is warmer than the mid-20th; ergo the late 20th warmth is unprecedented without need to check to see whether the proxies work in warm decades like the 1990s.
regardless of the lack of these studies, that sylogism you mentioned is inadequate, because the acid test of a proxy is performance going forward. Especially given all the possibilities to cherry pick. Admitted to on RC.
Karl Rove, in addition to having a hurricane machine, has a time
machine. He went back in time and dug up a tree from a lower
altitude and re-planted it where it is today.
He did this thousands of times, and also changed the course of rivers,
stopped inlets and harbors from icing over, left false evidence of what
grew when and where, caused increased drift ice in the North Atlantic
in the 1200s, and forged hundreds of writings in which some of these
events were observed and explained as having resulted from climate change.
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[…] bristlecone-foxtail areas. Medieval treelines in California were higher than at present, discussed here and here. Post-medieval lakes have even submerged medieval trees. Miller (2006) discussed here and […]