Why are there no trees above a certain on very tall mountains

Well, it depends on what biome you are in where the trees are trying to grow. In some places the roots cannot go deep enough into the ground, others don't get enough sunlight and in some places, it may be too cold for them to grow, also some may get no rain. 
But usually at tall elevations trees cannot get the oxygen they need at high elevations.

No it doesnt because it can be any number.

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