system
Given the following post, respond as if you're commenting on Reddit:
human
What makes treeline altitude (highest trees can grow) vary from place to place?

In Colorado it's about 10,800'. In California it's about 8,500' in places.
gpt
Water and heat availability. In general, a you get higher, both become more scarce until it's too cold and dry for any trees to grow. Of course, a mountain in the desert will hit the dryness limit sooner than one in the tropics, and one in Canada will hit the coldness limit sooner than one in Florida.