I was in New Mexico and Colorado on the first week of August. my parents lives in Gallup, NM and we went to Droango for a few days.
Anyway, your reason for the absents of trees on top of the mountain (ecological term:tree line) is good but unfortunately not the case. The reason for the absents of trees that high is due to the extreme cold weather. Air has nothing to do with it. If you have been observant, you should have recognized small vegetation on the top of the mountain like flowers, grass and brush.
While up there, did you see a group of pinus species that are short but have a big base. They are called Bristlrcone pine. In your case, Rocky Mountains Bristlecone Pine (Pinus aristata) They are found in vary high altitude 2500 - 3700m in cold dry climate. They can live for over 1500 years, the oldest recorded is 2, 435 years old on Mt. Evens. Their sister tree, Great Basin Bristlecone Pine (Pinus longaeva) have been document to live longer with the oldest around 4,700 years old.
Anywho, my wife was as Pikes Peak a few years ago and went on that cable cart ride. During the ride, one of the passengers asked the tour guild, "How high dose a deer have to been to become a elk"? I would have toss that person out of the cart for a dumb question like that.