system
You are a chatbot acting as a human, specialising in Generation. Give a response that a human will want to hear.
human
Hi, can you tell me about why treelines exist? My friend who lives in the mountains says trees just don’t grow above a certain altitude. Is that true? What makes it happen?
gpt
A tree line marks the edge of the conditions in which trees are reliably capable of growing. This is often a function of elevation, but it is more specifically defined by temperature, snowpack, wind, and/or available moisture. In mountainous regions that are tall enough to maintain permanent snowpack, the tree line usually occurs between 300 and 1000 meters below the permanent snow and runs roughly parallel to it.

In most cases, the tree line is defined by the mean seasonal temperature. Even conifer trees will cease to grow where the seasonal mean temperature is less than 6 degrees Celsius (43 Fahrenheit). They need roughly 94 days’ worth of growth time above that temperature to grow successfully.

Some regions of Earth are close enough to the poles that they don’t spend enough time at a growth-sustaining temperature even at sea-level elevations. These regions (the tundra and Antarctic, for example) can be considered to be above the tree line as well.

Other factors can create tree lines, too. Wind can prevent seedlings from growing to a mature height and thus preclude the growth of forests. Some windy areas (especially along coastlines) get stripped of usable soil, so they can’t support tree growth. If there's too little precipitation that will keep trees from growing as well.

Human activities like logging create artificial tree lines, though these lines are not permanent. The only persistent anthropogenic (human-caused) tree line changes come through the effects of climate change.
