The tallest trees can get thirsty above wet ground

A tree can have wet soil around its roots and still struggle for water at its top. That’s the problem facing the tallest coast redwoods, whose upper leaves live at the far end of an enormous water-delivery system. Hyperion, a coast redwood in California’s Redwood National Park, holds the record for the tallest known living tree, reaching roughly 116 meters, or 381 feet. Its height is more than a spectacular measurement. It brings a living organism close to a constraint that extra sunlight and moist ground can’t simply remove: getting enough water all the way upstairs.

The higher a redwood grows, the harder that delivery becomes. Gravity pulls against the water, and the long route through the tree adds resistance to its flow. At the top, increasing water stress limits how much leaves can expand and how effectively they can photosynthesize. So a tree doesn’t need to run out of water in the ground to run into trouble above it. Researchers studying giant redwoods estimated a potential height ceiling of about 122 to 130 meters under their study conditions, excluding mechanical damage. The limit emerges from the workings of the leaves, not just the strength of the trunk.

One experiment made that connection especially clear. Researchers took a fallen branch from a redwood’s upper crown, rooted it in wet soil and kept it in bright light. Its new leaves expanded much more than the leaves it had grown high in the tree. The branch hadn’t become a different species. It had escaped the water stress of its former address. That’s a useful way to understand these giants: the same tree can offer very different growing conditions depending on how far above the roots a leaf sits.

Fog helps with the problem from the other end. Coast redwood foliage can absorb fog water directly through its surfaces, and researchers have measured sap briefly moving downward during heavy fog. Water doesn’t always enter this tree through the roots. But direct absorption is only part of fog’s value. Fog also reduces water loss from leaves, easing the demand on the tree’s internal supply. Water caught in the canopy can drip to the ground, where roots can reach it. A foggy day helps both by adding moisture and by slowing its escape. For a tree with thirsty leaves far above the soil, those are two very useful services.

Height also makes a record vulnerable. Redwoods can lose parts of their crowns to wind and lightning, then grow replacement tops. And being enormous doesn’t protect them from damage below. Visitors seeking Hyperion have trampled surrounding vegetation, while repeated footsteps compact soil around shallow redwood roots. Its full height can’t even be appreciated from the ground beside its trunk. There’s a strange mismatch here: people chase a single tree for its measurements, while the thing worth protecting includes the plants and soil around it. You can admire giant redwoods from established trails without putting another pair of boots through that fragile neighborhood. The tallest tree isn’t separate from its forest.