We usually think of plants sucking water up through their roots, but UC Berkeley professor Todd Dawson can attest that this isn’t the only way that plants can collect water. Trees immersed in clouds or fog can absorb moisture through their foliage. This little-studied phenomenon, known as foliar water uptake, is an important part of the water cycle in many ecosystems and may be affected by changes in cloud cover due to global warming.

Credit: Liberty Hamilton

Greg Goldsmith, a recent graduate of Dawson’s group in the Department of Integrative Biology, has measured foliar water uptake in one of the world’s most delicate and diverse environments: the tropical montane cloud forest of Costa Rica. His work has shown that plants in higher, cloud-inundated parts of the forest are more efficient leaf-drinkers than plants in lower, drier parts of the forest. “You have this dramatic gradient in the severity of the dry season across this mountainside, and that appears to be showing up in the physiology of the plants,” says Goldsmith. “That’s a really striking result because a small change in climate might have strong effects on plant functional response.”

Surface temperatures are rising in the tropics, causing clouds to form at higher altitudes. This means that the fraction of the forest with access to cloud water is shrinking. Ecologists are concerned about what this might mean for the health of the forests. “If you change the water balance, plants become water-stressed a lot more, which means less photosynthesis and less overall productivity. How does that cascade through the entire ecosystem?” asks Dawson. “We don’t know.”

“Our understanding of these processes is at its infancy,” Goldsmith explains. He stresses the need to investigate the physiology of foliar water uptake, and to combine long-term studies of forest function with improved climate data so that we can better predict how human activity affects this “incredibly charismatic place.”