How tall can a tree grow, and why do even the tallest redwoods stop?

A tree stops growing taller when it can no longer lift water to its highest leaves. The tallest living tree known is a coast redwood called Hyperion, in California, about 116 metres high. Trees have a theoretical maximum height of about 120 to 138 metres, and the primary reason is their limited ability to siphon water up the trunk: if a tree grew too tall, it would dry out and die. For coast redwoods, a 2004 study in Nature led by George Koch, who climbed five of the tallest trees on Earth with the botanist Steve Sillett, put the ceiling at about 122 to 130 metres: beyond that, evapotranspiration cannot move water to the leaves.

Why is it so hard to lift water that far? As a tree gets taller, moving water to the leaves becomes harder because of gravity, so the leaves at the very top of a redwood are perpetually short of water, even in a rainy region. Water is pulled up in thin columns held together by cohesion, and a bubble of air in one of those vessels breaks the bonds between the water molecules, so they can no longer pull more water up.

Redwoods have a trick: fog. Their leaves take in fog water directly from the air, bypassing the xylem, and they absorb water through their bark. Water from fog drip may supply 30 percent of what a redwood uses in a year, and the height of coast redwoods is closely tied to fog availability: taller trees become less abundant as fog becomes less frequent.

Hyperion was discovered on 25 August 2006 by two naturalists, Chris Atkins and Michael Taylor, and Stephen Sillett verified its height with a laser range finder and a fibreglass tape. It is estimated to be 600 to 800 years old, so it sprouted when the Chilula people lived in the region. Its popularity became its problem: in July 2022 the park superintendent closed the area around it, citing devastation of the habitat caused by visitors.