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Why do forests change into tundra at high latitudes and elevations?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

The transition from forest to tundra at high latitudes and elevations occurs at a boundary known as the tree line or timberline. This shift is primarily driven by a decline in habitat quality as temperatures decrease, winds increase, and soils become more impoverished.12

Low temperature is the main arbiter of these limits. As one proceeds poleward or increases in elevation, the growing season shortens and temperatures drop, which slows biological processes and reduces the production of dry matter. If the growing season is too short, trees cannot form the necessary supportive tissue or energy reserves required for annual growth and reproduction.1

At high elevations, the environment is further characterized by high winds, frequent freeze-thaw cycles, and lower carbon dioxide levels, all of which stress trees. While trees may adapt by adopting stunted or prostrate forms in the transition zone, they eventually reach a point where survival is no longer possible, giving way to the open, treeless landscape of the tundra.132