Cloud forest trees drink water down

By John Upton

One of the first things that every botany student learns is the simple process by which trees drink water. The water enters the roots from moisture in the soil and is sucked up the trunk through straw-like xylem to the leaves, where some evaporates. The combined effects of water tension and water cohesion inside the xylem and evaporation from the leaves keeps the water flowing against the force of gravity.

Researcher Greg Goldsmith simulates clouds in a laboratory to study foliar water uptake / Courtesy: Drew Fulton (Canopy in the Clouds)

A study of trees growing on Costa Rican mountains revealed that some high-altitude species can pull switcheroos on this widespread drinking system. When the soil is parched and their canopies are saturated by clouds, these trees use their leaves to suck water out of the air and then send the moisture back toward their trunks.

“Water is still moving along a gradient from areas with more water to areas with less water,” Greg Goldsmith, a tropical plant ecologist at the University of California, Berkeley and lead author of the study, which appeared this month in Ecology Letters, told me. “It’s just a different gradient.”

The clouds that nurture these cloud forests are evaporating as the planet warms, meaning the cloud-drinking strategy could doom those trees that rely upon it. That would be bad news for the birds and other wildlife that live in cloud forests, which are some of the world’s most striking and biodiverse ecosystems.

“The phenomenon of water from clouds entering leaves — foliar water uptake — indicates a much tighter relationship between clouds and cloud forest plants than previously known,” Goldsmith said.

Tropical mountain cloud forests are renowned for their high biodiversity but are threatened by climate change / Courtesy: Drew Fulton (Canopy in the Clouds)