Alpine Tree Line

The maximum elevation where trees survive in an alpine environment is called the tree line. Viewed from a distance the tree line appears abrupt.

Tree line below Wheeler Peak

One forgets that there are environments which do not respond to the flick of a switch or the twist of a dial, and which have their own rhythms and orders of existence. Mountains correct this amnesia.
— Robert Macfarlane

What accounts for an alpine tree line?

High-elevation snow lingers before melting. Lingering snow shortens the growing season. New growth is inhibited because saplings do not have enough time to harden before the impending frost.

By speaking of greater forces than we can possibly invoke, and by confronting us with greater spans of time than we can possibly envisage, mountains refute our excessive trust in the man-made. They pose profound questions about our durability and the importance of our schemes. They induce, I suppose, a modesty in us.
— Robert Macfarlane

High-elevation winds are another existential threat to new growth. The survival of a sapling depends its ability to bend and buttress itself against wind.

Those who travel to mountain-tops are half in love with themselves, and half in love with oblivion.
— Robert Macfarlane

Alpine microclimates are determined by elevation and exposure. The higher the elevation the lower the temperature.

Tree line transition on the
Wheeler Peak summit trail

Ascending a peak in the Rocky Mountains, a traveler will lose about 3.5°F in temperature for every 1000 foot gain in elevation.

On the trail, there is a transition zone of scrub conifers between the full canopy forest below and the treeless tundra above.

The clearest way into the Universe is through a forest wilderness.
— John Muir

REFERENCES

• Mountains of the Mind: A History of a Fascination by Robert Macfarlane, Granta Books, 13 May 2004.
• Tree line, Wikipedia.