If Costa Rican trees could speak, perhaps they'd ask for a cool glass of fog.

A number of plant species in the country's tropical cloud forests quench their thirst by slurping up fog droplets through their leaves, a new study shows.

The forests are already in danger from the changing climate, and the finding raises concerns that they're even more fragile than thought.

For nine months of the year, the lush, mountainside cloud forest of Monteverde in Costa Rica gets plenty of rain to support its roughly 2000 plant species.

During the other three months, February through April, precipitation is scarce. But even during this dry spell, some of the region's forests average 13 hours of fog each day from moisture that drifts in from the Caribbean Sea and condenses under the forest's canopy, forming milky-white threads that weave through the greenery.

Monteverde's cloud forest is also home to a wealth of amphibians and migratory birds. But in 1989, conservationists were alarmed when a renowned bright-orange amphibian called the golden toad went extinct.

Whether the animal died out because of climate change has been a source of debate. But its demise served as a bad omen because a cascade of other amphibians, which are especially sensitive to moisture changes and diseases spread by climate change, disappeared from Monteverde in the following years.

Researchers interested in conserving the cloud forest species have studied the region's animals intensely, but they know much less about the ecosystem's habitat-providing plants, says Greg Goldsmith, a plant ecologist at the University of California, Berkeley.

Because climate change has also been projected to influence where the fog forms, Goldsmith and his colleagues hunted for clues to how the dry season fog influenced the trees by looking for a rarely studied talent among plants called foliar uptake - the ability to absorb water through leaves in addition to the roots.

Since 1950, scientists have discovered 40 species that absorb water through their leaves.

"I think what we're going to see," Goldsmith says, "is the more we look for it the more we'll find it."

And indeed, the researchers found no shortage of foliar uptake in Monteverde.

They picked out the 12 most common tree species in the cloud forest and the 12 most common trees in a far less foggy forest just 2km downhill.

They determined how often the leaves got damp by placing fake, plastic leaves on the tree branches, each with an incomplete electrical circuit that conducted electricity when wet.

They also determined how much the plants drink through their leaves by heating a spot on each plant's branches during clear and foggy days.

By measuring the warmth along the branch, they could track the water moving inside.

If the water travelled toward the leaf, it came from the roots. If it travelled toward the trunk, it came from the leaves.

Goldsmith's team found that all of the trees inside and outside the cloud forest use foliar uptake. But analyses of dehydration levels in the leaves revealed that trees in the cloud forest had a higher capacity to draw water in through their leaves, storing 20 per cent more water for growth and survival using foliar uptake than their downhill counterparts, the researchers report online this month in Ecology Letters.

This is one of the first ecosystems in which scientists have observed the trees drawing in fog water through their leaves, Goldsmith says. The very first was the giant coastal redwood forest of California.

The new study makes a "pretty conclusive case" that cloud forest trees do indeed hydrate using the clouds, says forest ecologist Christopher Still of Oregon State University, Corvallis who was not involved with the study.

"There's always been this notion that this [ecosystem] is tied to the clouds, but it's hard to really show that."

This finding means that a warming climate presents a whole new problem for anyone seeking to preserve cloud forests.

If the moisture from the Caribbean Sea is warmer, Goldsmith says, it takes longer to cool off enough to condense into fog, which means the fog would not appear until the wind has pushed the moisture to a higher elevation than the cloud forest.

If the plant population doesn't migrate fast enough to keep pace, this would leave the trees awfully thirsty.

"The textbooks tell us that plants get their water from their roots," he says, "and here's this system where there's an obvious means that plants can get water from another source: the fog. They're doing it, but that fog is going away."

- SCIENCE NOW