Because of air thinning. At around 12500 feet above sea level there is simply not enough air for the trees to thrive, so they don’t. You may find smaller shrub versions of trees in the transition, or ones that are less hungry for carbon dioxide like blueberry bushes.
Also carbon dioxide is heavier than most other components of air so it may literally fall out of thinned atmosphere, separately from oxygen and nitrogen. Which is how there is still (barely) enough oxygen for a human above the treeline for a few more thousand feet (before you have to bring canned air).
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