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Botany 

The reverse path

A study indicates that trees in the Mantiqueira Range capture water through their leaves and transport it downwards to the ground

Between clouds: mist thickens over a section of Atlantic forest on the Mantiqueira Range, upstate São Paulo

Rafael Oliveira/UnicampBetween clouds: mist thickens over a section of Atlantic forest on the Mantiqueira Range, upstate São PauloRafael Oliveira/Unicamp

in Campos do Jordão

During an expedition in early May on the Mantiqueira range, the biologist Paulo Bittencourt stopped at a stream of cold, crystalline water on a dirt road between sheep farms near the Campos do Jordão State Park. “You can drink the water. It cannot be polluted. This water comes from up there,” he said, pointing to where the creek starts, about 2,000 meters above sea level, on a hill covered by a forest of small trees with small leaves. “It is streams like this that travel down the mountain to feed and maintain larger rivers further down,” he explained. Bittencourt is working on his master’s degree at the University of Campinas (Unicamp) under the guidance of ecologist Rafael Oliveira, who is attempting to quantify the contribution of this little-known type of Atlantic forest to the water supply of the Paraíba Valley.

“There is an important relationship between these forests and the headwaters of the Mantiqueira Range,” says Oliveira. Without this vegetation, known as a tropical mountain cloud forest, the mist that climbs the mountain would continue inland, carrying the moisture it obtains from the evaporation of rivers and the transpiration of plants in the valley along with it. The small cloud forests on the mountain slopes retain the moisture when the water in the mist condenses into droplets on the leaves and flows to the ground. Studies in Costa Rican tropical cloud forests suggest that the water captured by the trees from mist can contribute up to 30% of the volume of the rivers in a region.

Until recently, it was thought that trees could not absorb water through their leaves. After all, the leaf surface is covered with a thin, waterproof layer of wax, the cuticle, which prevents water loss to the environment. But, recently, according to the botanist Gregory Goldsmith at the University of California, Berkeley, 70 species of plants with leaves that can absorb water have been identified.

The botanist Aline Lima confirmed the absorption of water through leaves in her master’s thesis, supervised by Oliveira and part of the Functional Gradient Biota project, funded by FAPESP. She dripped water droplets containing fluorescent crystals on casca-de-anta leaves in a greenhouse, then followed the path taken by the water under a microscope. The study confirmed that the water crosses the cuticle and penetrates into the leaf. According to Oliveira, recent studies in Germany show that the cuticular wax crystals are dynamic. In a very humid atmosphere they rearrange themselves, making the leaves permeable.

Against the flow
These results contradict the contents of biology textbooks. They teach that the flow of water in plants is one-way. According to the classical view, leaves are always transpiring, losing water to the air through the stomata, holes on the underside of leaves that open and close according to the availability of light and water. Like sipping liquid through a straw, the water loss through transpiration exerts a suction force inside the vessels, making the water climb up to the leaves as more water is removed from the soil by the roots. “It’s what most plants do all the time,” says Oliveira. Studies suggest that up to 50% of the moisture circulating in the atmosphere in certain regions comes from forest transpiration.

Microscope image shows that water penetrates through the cuticle and the leaf

Rafael Oliveira / UnicampMicroscope image shows that water penetrates through the cuticle and the leaf…Rafael Oliveira / Unicamp

In recent years, however, some researchers began to notice that this flow can be reversed in situations where the air is more humid than the soil. The biologist Todd Dawson, who advised Oliveira during his doctoral studies at the University of California, Berkeley, described how redwoods transport water in the opposite direction in 2004.

Redwood forests, with trees up to 115 meters tall, occur in regions of California where the quantity of rainfall is comparable to that of the sertão in Brazil’s Northeast. What saves these trees from drought is the mist from the sea, which saturates the air with water vapor. Under these conditions, the redwood leaves absorb water and stop transpiring, cutting off the flow from the soil to the leaves. At the same time, the dryness inside the trunk creates a suction force capable of pulling water from the atmosphere downward, to rehydrate the tree.

Attempting to identify a similar phenomenon in Brazilian trees, Oliveira searched for cloud forests across the country until he found the forests of the Mantiqueira Range, where several rivers begin, although the region suffers frequent droughts. In the forests of the Campos do Jordão State Park, where he has worked since 2009, it rains a bit more than in the Cerrado. The climate is dry from June to August, although often there is mist at the beginning and end of the day.

To understand how the casca-de-anta trees survive under these conditions, Cleiton Eller, a PhD student under Oliveira, cultivated these trees in a greenhouse at Unicamp under three conditions: receiving water from the soil, hydrated through an artificial mist sprayed onto the leaves, or without irrigation. The plants treated only with mist survived for two months.

In order to confirm that the water absorbed by the leaves could be transported to the soil, the researchers conducted a complementary experiment. Taking care not to wet the soil, they sprayed the leaves of the casca-de-anta with heavy water. The heavy water contained atoms of a type of hydrogen that is heavier than normal, called deuterium, which can be detected using a mass spectrometer. According to the reasoning behind this test, deuterium subsequently found in the soil serves as proof that the water was absorbed by the leaves, transported to the roots and expelled into the soil. Based on the numbers obtained in the experiment, Oliveira estimates that if a tree exudes 10 liters of water per day, on the same day it can carry 2.5 liters of water from the atmosphere to the ground through its internal structures.

Saturated atmosphere

Rafael Oliveira / Unicamp…when the atmosphere is saturated.Rafael Oliveira / Unicamp

“This is our most impressive result,” says Oliveira, who just found another report in the scientific literature of water absorbed by leaves reaching the ground. In 1969, the botanist Fusa Sudzuki of the University of Chile demonstrated the same phenomenon in an experiment with the tamarugo, a common tree in the Atacama Desert. “Her work is beautiful, but the results were rejected at the time,” says Oliveira.

Possible and relevant
“The Unicamp group’s study shows that the flow of water from the atmosphere to the soil is not only physically possible, but physiologically relevant,” notes botanist Lúcia Dillenburg of the Federal University of Rio Grande do Sul, who published evidence that the araucária tree also absorbs water through its leaves.

“This is a very original study,” says botanist Marcos Buckeridge, of the University of São Paulo. However, he says that not all of the water containing deuterium detected in the soil corresponds to the water captured by the leaves. According to Buckeridge, the plant could have used the water captured by the leaves to produce organic compounds such as sugars, which are normally released by the roots. “Within seconds, the sugars exchange deuterium with the ground water,” he explains. In his view, a way to answer the question would be to repeat the experiment using heavy water containing oxygen-18, which interacts less with other substances than deuterium does. “It would be a more expensive and complicated experiment,” he says.

Oliveira agrees that there is uncertainty in the amount of water that the roots release into the soil, but emphasizes that his experiments proved the reverse flow of water from leaves to roots. “Because most plants do not have a mechanism which prevents the release of water from the roots to the soil and since there is a large enough water potential gradient to allow movement of water from the leaves to the roots,” says Oliveira, “the most likely probability is that that the roots have released the water into the soil.”

In tests in the Mantiqueira Range, Oliveira’s team traced the flow of water in trees with sensors attached by wires to a machine that stores the information. Now the group is preparing to start monitoring cloud forests with wireless sensors, to be developed by Microsoft engineers with funding from FAPESP. The idea is to monitor the transformations that these environments may suffer due to climate change.

Project
Climate change in Brazilian mountains: functional responses of native plants in rocky and high-altitude fields to extreme drought (nº 2010/17204-0); Grant mechanism Regular Line of Research Project Award; Coord. Rafael Silva Oliveira (IB/Unicamp); Investment R$566,468.84 (FAPESP).

Scientific article
ELLER, C. B. et al. Foliar uptake of fog water and transport belowground alleviates drought effects in the cloud forest tree species, Drimys brasiliensis (Winteraceae). New Phytologist. 2013. In production.

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