How tall can a tree grow?

The tallest living tree known is a coast redwood (Sequoia sempervirens) named Hyperion, about 116 m tall (taller than a 35-storey building). Giant sequoias are bulkier but not as tall. Why do trees not grow ever taller?

Part of the answer is the water pull of lesson 2. Every extra 10 m of height costs another 0.098 MPa of pull just to balance gravity, and the longer path adds friction. So:

• the leaves at the top must work at a more negative water potential than leaves lower down;
• the water columns are under more tension, which raises the risk of cavitation;
• the top leaves have a harder time keeping their cells turgid, which they need to expand and to keep stomata open for photosynthesis.

Cavitation happens when air is sucked into a tube of water under tension (or a bubble grows from dissolved gas). The bubble spreads (embolism) and the tube is no longer able to carry water. A tree with many parallel tubes survives some failures, and some trees can refill tubes or grow new xylem, but the more tension, the more tubes fail.

Because of this, the water-pull limit gets tighter with height. In a famous 2004 study in Nature, George Koch and colleagues measured the tops of tall redwoods and predicted a maximum height of about 122 to 130 m for the species, even with plenty of water in the soil. Hyperion is a few metres below that estimate. The same study showed that leaves near the top are smaller and thicker, with more tissue per unit area. That is the physiological signature of a leaf working with little water pressure to spare.

Gravity and friction are not the only limits. Taller trees also have to support their own weight and survive wind, and their roots and leaves must keep pace. In many places a lack of soil water or a storm stops a tree long before it reaches the hydraulic limit. In the rare wet, mild, windless places where redwoods thrive (a coastal strip with frequent fog, which gives extra water to the leaves), trees approach the calculated limit.

Other tall species follow similar rules. The tallest flowering trees, mountain ash (Eucalyptus regnans) in Tasmania, reach about 100 m.