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How do trees handle thirst?

Quercus robur in the Alta Garrotxa. Image: Galdric Mossoll
Quercus robur in the Alta Garrotxa. Image: Galdric Mossoll

All living organisms need water, and plants are no exception. While many animals can move around to find it, most plants are rooted to the spot and have therefore had to develop astonishing ways of obtaining and storing it.

As is well known, plants are characterized by carrying out photosynthesis, a photochemical reaction that takes place in their green parts (leaves and some branches, in most cases). It consists in combining water (absorbed by the roots) with carbon dioxide. In the presence of sunlight, the mixture reacts and produces glucose (which, in turn, can be used to produce wood, leaves and fruit) and oxygen. So, plants play a crucial role in the biosphere: they absorb carbon dioxide, release oxygen, and transfer water from the soil to the atmosphere. In this report, we talk about how plants drink water and how they adapt when their water supply runs out.

How do plants drink water?

Plants act like vacuum pumps: they draw up water from the ground and transport it to their leaves, where a small part of it is used for photosynthesis and most of it evaporates.

When it comes to water movement, plants act like vacuum pumps: they draw up water from the ground and transport it to their leaves, where a small part of it is used for photosynthesis and most of it evaporates through tiny openings called stomata. However, roots cannot suck like we do when we drink through a straw. Interestingly, the suction begins at the other end of the plant, in the leaves! Let’s take a closer look at how that happens.

The water that evaporates from a leaf disappears, creating a vacuum. That vacuum generates tension (as if a string were being pulled from outside), which causes water from slightly lower down to enter the leaf to fill the space left. Owing to the high level of cohesion between water molecules, that small upward movement (mere micrometres) is reproduced throughout a tree’s twigs, branches, trunk and, eventually, roots. In the same way, thanks to the tension generated, the roots draw up water from the subsoil. So, we should think of trees as being full of water. They are like relatively rigid sponges around which water circulates continuously. Depending on its species, its height and the season, a tree’s daily water uptake can vary from a few to hundreds of litres! The water cannot go just anywhere inside the tree, though. Wood is fundamentally composed of xylem, a tissue full of microconduits, which specializes in transporting xylem sap (water containing mineral salts) from the root system to the top of the tree. There is also another kind of tissue, phloem, which transports phloem sap (water containing the sugar made during photosynthesis) to all the tree’s living cells.

“It comes as a surprise when you realize that one of the keys to understanding the biosphere’s response to climate change lies in these tiny conduits hidden inside the wood of trees.”

JORDI MARTÍNEZ-VILALTA, CREAF researcher and professor of ecology at the Autonomous University of Barcelona (UAB).

What happens when the water runs out?

The way trees transport water seems flawless… until the water below ground runs out due to a lack of rain or to an aquifer drying up. What happens then? The water in a tree’s leaves continues to evaporate, but no more is taken up through its roots. At that point, the water column breaks (just like when we are drinking with a straw and reach the end of the drink) and air enters the xylem, drying fragments of it out. This results in conduit blockage, causing the formation of an embolism. Blocked conduits cease to function, usually permanently. According to a study led by CREAF researchers, taller trees are better able to cope with drought (short-lived episodes, at least) because they hold large reserves of water. In the event of a prolonged drought, however, more and more of a tree’s xylem conduits become inoperative, making it difficult for water to reach the top; in extreme cases, this can lead to the tree’s death.

Infographics of water flow through the trees in optimal and dry conditions. Illustration: Laura Fraile.

How do plants adapt to water scarcity?

Water is often in short supply in the Mediterranean, so the region’s plants have developed strategies to survive periods of scarcity. Their most common ‘tricks’ are described below.

“Plants have developed a wide range of solutions to stave off the risk of drought-induced death. Those solutions are all different, but just as successful as one another. Even so, climate change is endangering some of the plants in question.”

MAURIZIO MENCUCCINI, CREAF-based ICREA researcher specializing in plant and ecosystem ecology.

Reduced leaf area. Since plants lose water through their leaves, a simple solution is to have fewer or smaller leaves. Reducing the number of leaves or their surface area also reduces the amount of water lost through evaporation. Some species from arid regions have done away with their leaves entirely, growing spines instead and carrying out photosynthesis in their green branches. The leaves of conifers (pines, firs, cedars and cypresses), meanwhile, are very narrow and pointed. They are called needles because of their shape, which enables the trees to reduce water loss.

In extreme drought conditions, such as those Catalonia has experienced in recent years, some plants (described as summer deciduous) opt to shed their leaves during the hottest months. They sacrifice their ability to photosynthesize but avoid losing water in the summer.

Stomatal closure. Leaves, and their undersides in particular, are full of stomata, the tiny openings through which gases (carbon dioxide, oxygen, and water vapour) pass in and out. Plants can open and close their stomata, enabling them to prevent unnecessary water loss. But while fully closing them stops water loss, it also stops photosynthesis from taking place, as there is no way in for carbon dioxide, halting production and growth. Thus, plants only fully close their stomata on very hot days or when they are under water stress.

Deep roots. Soil acts like a sponge. When rain falls, some of the rainwater filters down into the ground, where it is retained in small cavities (pores) between stones and grains of sand or trickles down more deeply and collects on an impermeable layer — this is how aquifers form. The job of a plant’s roots is to find such points of moisture and absorb their water. Depending on the nature of the soil, water can be found at greater or lesser depths and be more or less easy to absorb. Root capabilities differ from species to species. For example, typical riparian forest trees do not grow deep roots because they usually inhabit areas with very damp soil. In contrast, holly oaks, which inhabit drier regions, grow much deeper roots (taproots), the length of which can exceed the height of the part of the tree above ground!

Dead Pinus sylvestris due to drought in the Prades Mountains. Image: Galdric Mossoll
Climate change is shifting in altitude and latitude some humid climate species, to the point of endangering the survival of some species in the Mediterranean basin.

A region’s floristic communities and their morphological and physiological traits are determined by environmental conditions, especially water availability. As the Mediterranean basin lies in a transition zone between arid and temperate climates, it is home to species typical of humid areas and others typical of dry areas. It is thus a region with a very high level of species richness and a wide variety of strategies for adapting to the climate. However, because of the drop in precipitation and the rise in atmospheric evaporative demand brought about by climate change, some species more typical of humid climates are being displaced to other altitudes and latitudes, to the extent that a number of them are at risk of being unable to survive in the region. The species that will endure despite the increasingly dry climate are the ones that are capable of overcoming embolisms or which have adapted to make it through episodes of drought in ways such as those described above.

The public can help detect drought

Drought causes changes in the landscape. In Catalonia, such changes have been particularly evident in the last few years, which have been characterized by persistent drought. Coordinated by CREAF, AlertaForestal is a citizen science project that has been observing the health of Catalonia’s forests since 2016. The assistance of volunteers, who act as forest ‘sentries’, enables CREAF to help monitor the impact of drought and to track the evolution of forests that lack sufficient water. Anyone can participate in the project; doing so simply involves uploading photos (in landscape orientation) of drought-affected forest scenery via the AlertaForestal app for mobile phones.

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