The trees, sick from the droughts

More and more hot, more and more dry. With the summer of 2022 not even halfway through, much of Europe is already running out of water. Of 96 departments in France, 93 have restrictions imposed on them. The most spectacular consequence of this lack of water is the multiplication of forest fires in France, just like among our Spanish, Italian or Greek neighbours. By July 21, 5,000 km2 of forests had gone up in smoke in Europe, the equivalent of the surface area of ​​the Bouches-du-Rhône.

“Drought has other, more insidious consequences on trees,” comments Nathalie Bréda, director of research at the National Research Institute for Agriculture, Food and the Environment (Inrae).

tree sweat

Because like humans, trees sweat to cool off, through pores located in the leaves, called “stomata”. They suck up water from the ground via their roots, and this rises upwards, in vessels, under the effect of transpiration. But when water becomes scarce, trees may need to transpire more water than is available. Then occurs the same phenomenon that we notice when we drink with a straw in an empty glass: bubbles of air form in the vessels and lead to embolism in certain organs: the leaves first, then the young twigs and branches.

To limit perspiration – and therefore the risk of embolism – the tree can close its pores. Problem : « A tree is never 100% waterproof, it will always evacuate water, even in small quantitiesexplains Jean-Marc Limousin, researcher at the CNRS in Montpellier. During episodes of severe drought, the tree can find itself in a situation of hydraulic failure and die. However, this remains a very rare phenomenon. »

Less CO2 capture

This strategy also has a perverse effect. It is through the open stomata that photosynthesis takes place – that is, the absorption of CO2. When it closes them, it compromises its role as a carbon sink. « Over certain periods, forests can even emit CO2 rather than absorb it », continues Jean-Marc Limousin. The phenomenon also weakens the tree, affecting its growth and forcing it to draw on its reserves.

With what consequences? « Falling leaves during a season is not a problem in itselfnotes Nathalie Bréda, from Inrae. It is rather the repetition of droughts that is worrying, because it prevents the trees from returning to their initial state”. And therefore to resist the attacks of pests, which are increasing in France. “These insects – like bark beetles – primarily attack very weak trees, which no longer have the energy to defend themselves, continues Nathalie Breda. The effects are cumulative because the presence of dying trees or high temperatures favor the increase of their populations”.

Drought also affects tropical forests. In the Amazon, scientists have found, in a study published in March, that the slightest transpiration from trees limits the humidity of the air, which contributes to increasing… the conditions of drought.