On days when there is a heavy mist or when the air is very humid, certain types of trees use a different mechanism to extract the water that they need to stay alive from the atmosphere, to grow and reproduce. Instead of just absorbing soil water through their roots, they also remove water vapor from the atmosphere through their leaves. “This capability may enable the plants to survive for periods when water is scarce”, explains biologist Rafael Silva Oliveira, from the State University of Campinas (Unicamp). He recently identified this capacity to absorb water through the leaves in Amazon Rain Forest trees as well as in trees growing more than 1,000 meters above sea level along the coast of the state of São Paulo in Brazil’s Mata Atlântica (Atlantic Rainforest).
Previously unknown among species of Brazilian flora, this is a not a new phenomenon. In 2004, biologist Todd Dawson, from the University of California at Berkeley, in the United States, had described this hydration strategy in one of the world’s tallest trees: the Coast Redwood (Sequoia Sempervirens), which grows as high as 115 meters and can live for more than 2 thousand years. Although it is still not known exactly how water is absorbed through the leaves, which are not impermeable as had previously been thought, Dawson showed that the leaves capture as much as 30% of the water that the coastal redwoods consume over the course of the year. In California, the redwood forests might not even exist if the tree leaves were unable to extract from the mist the water that they need. “It doesn’t rain much there; the rainfall level is similar to that of those parts of Brazil covered by Caatinga scrub,” states Oliveira, an expert in vegetable ecology from Unicamp who has been working with Dawson for almost a decade.
Leaves that work as sponges are not the only resource that has enabled plants to adapt to the planet’s different environments over thousands of years. In parallel studies conducted in the last few years, Oliveira and biologist Augusto Cesar Franco, from the University of Brasília (UnB), identified other strategies in trees in Brazil’s Cerrado (savannah), Amazon Rainforest and Atlantic Forest that enable them to cope with both a lack of as well as an excess of water. “The Cerrado, for instance, is an ecosystem with a lot of biodiversity. You can find some 60 to 70 species of trees in just a few hectares”, Franco says. “Each species may have developed different strategies to get water.”
It seems that the most interesting strategy developed by trees in environments with scarce rainfall for several months of the year is that of hydraulic redistribution: the roots extract water from the wettest layers of soil and deposit it in the driest layers. First described by Martyn Caldwell and James Richards in the late 1980s in connection with plants from desert regions, this phenomenon was recently observed by Oliveira and Franco in trees found in Brazilian ecosystems.
In the dry season a few tree species in the Cerrado and the Amazon Rainforest absorb water from the deepest and wettest layers of soil – and transfer it closer to the surface. Apart from the trees that perform this task of transporting water, other plants with shorter roots also benefit from gaining access to moisture that they would not otherwise obtain. “During the dry season, the first 50 centimeters of soil become very dry after one month without rain, while the deepest areas remain relatively wetter”, declares Franco, whose field work mainly covers the Cerrado part of Brazil’s Federal District, in areas such as the Ecological Reserve at Brazil’s Institute of Geography and Statistics (IBGE).
Franco explains that, in the Cerrado, where there is an abundance of relatively stone-free deep soils, which plants can penetrate more easily, the roots of certain trees can go down about 10 meters in search of the water leftover from the most recent rainy season. At this depth, the difference in the humidity between the root and the soil is such that the liquid is naturally absorbed by the plant, in the same way as a dry sponge would absorb liquid if placed in a basin of water. In the case of shallow soils the situation is reversed and the earth absorbs water from the roots.
Working like a natural water-pump, this water distribution mechanism depends on two types of roots, which perform complementary tasks. The main root – which, in general, is thicker, with a similar diameter to that of the stem – can grow straight down for several meters in search of the water in deep soil layers. Meanwhile the shallow roots spread out like the arms of an octopus just a few centimeters below the surface.
Sun and rain
During the driest period, the main roots of trees in the Cerrado and Amazon regions go down deep in search of the water left over from the most recent rains and bring it up to the roots close to the surface, which, for their part, deposit it in the shallower layers of soil. With the arrival of the rainy season the opposite occurs: the roots close to the surface absorb the water from the rains and transfer it to the main root, which stores it several meters below the surface. “The tree roots are passive conduits”, explains Oliveira. “They control the transport of water and nutrients, which vary according to the environmental conditions.”
It is relatively easy to determine where the water in the plant sap comes from by measuring the proportions of two forms of hydrogen found in the water: deuterium, the nucleus of which contains one particle with a positive electric charge (proton) and another without any electric charge (neutron), and common hydrogen, the most abundant chemical element in the Universe, made up of a single proton. If the plant shows a preference for water from deep soils, which are poor in deuterium, its sap will contain a smaller proportion of this element.
It is also possible to determine whether the water flows from the soil to the roots or from the roots to the soil using a technique that measures the dispersion of heat by means of sensors installed in the roots of the trees. “We dug down as much as 50 centimeters around the lateral roots or the main root in order to install a heater just a few millimeters below the bark”, explains Oliveira. The heater is placed between two heat sensors, one of them slightly above the vertical roots and the other one slightly below. The way in which the heat pulse spreads through the root (heating either the upper or lower sensor to a greater degree) enables us to determine the predominant direction of the sap flow. By repeating this procedure every half hour we get a picture of the hydraulic redistribution throughout the year.
Although hydraulic redistribution was first explained more than a decade ago, the adaptive advantage that it gives those plants that have these two systems of roots remains something of a mystery. “We are still testing hypotheses”, declares Franco, whose most recent work on this subject was published in January of this year in the journal Tree Physiology. The main hypothesis is that even though transporting water from the deepest areas to the surface causes the plant to lose some moisture, it helps keep the roots that are just a few centimeters below the ground alive and in working order.
It is important to keep these roots healthy because they are the ones that do most of the work of absorbing nutrients, particularly where the soil is relatively poor, as in the Cerrado – where the deeper down you go the less nutrients there are available. “Even during a drought these roots would have access to water and to the activity of soil microorganisms, which are vital to fix nutrients”, says the researcher from UnB. One possible disadvantage is that, by increasing the moisture in the surface soil, the trees may also be helping competing species. “There is evidence that certain plants that lack this double roots system use the moisture brought to the surface soil by hydraulic redistribution. But we can’t yet state that their survival depends on this water”, points out Oliveira.
More efficient water procurement strategies are not only justified in the Cerrado, which is characterized by a dry season that lasts from May to September, during which time there is often no rain for as much as three months. Such strategies are also necessary in the Amazon region. “Almost half of the forests in the Amazon region grow in a climate that has a very clearly defined dry season”, declares the Unicamp biologist.
Five years ago in Tapajós National Forest, in the state of Pará, a region with annual rainfall of 2 thousand millimeters (500 millimeters more than in the Cerrado part of Brazil’s Federal District), Oliveira analyzed the transport of water in three tree species that are representative of the Amazon Rain Forest’s structure: the caferana (Coussarea racemosa), which grows in the shadow of the highest trees; the breu (Protium robustum), which grows to a height of 20 meters and helps to form the middle part of the canopy, where the crowns of the trees are; and the maçaranduba (Manilkara huberi), which grows to a height of more than 40 meters and can break through the canopy. According to a study published in 2005 in the ecological research journal Oecologia, the three species carried out water redistribution in the same way as the trees in the Cerrado – from deep down in the soil to the surface in the dry season and from the surface down to the deep soil in the rainy season. In the Amazon region, hydraulic redistribution enables the trees to eliminate water through their leaves – or transpire, as botanists put it – so fast that it even has an impact on the region’s climate. “During the dry season, hydraulic redistribution causes this transpiration to increase by about 30%. The outcome of this is that the temperature of the air in the Amazon region is much lower at this time of year than one would expect”, explains Oliveira, who described these results in 2005 in an article in the Proceedings of the National Academy of Sciences.
Franco and Oliveira are also helping disprove the myth that plants do not exchange gases at night. They found evidence that, in the dry season, trees in the Cerrado, in the Amazon region and in the Atlantic Forest keep the stomata, microscopic structures in the leaves that are responsible for the absorption of carbon dioxide from the atmosphere and for releasing oxygen into the atmosphere, partially open at night. This is an unexpected observation, given that the open stomata allow water to escape and that the carbon dioxide absorbed is only used for photosynthesis in the presence of light. “Because their stomata are open, they can get down to the work of photosynthesis faster when the day begins”, says Franco. Since the opening of the stomata controls the plant’s water flow, another possible explanation is that keeping them open at night helps the plants obtain nutrients in regions where the soil is poor.
OLIVEIRA, R. S. et al. Hydraulic redistribution in three Amazonian trees. Oecologia. v. 145, n. 3, p. 354-363. set 2005.
SCHOLS, F. G. et al. Hydraulic redistribution of soil water by neotropical savanna trees. Tree Physiology. v. 22, p. 603-612. 2002.