It's in your nature
Trees are dying from drought
Audio 02:53
The consequences of the heat wave, near Nancy: to save water, the trees close the stomata of their leaves, which dry out under the effect of the sun and the heat.
© Nathalie Breda
By: Florent Guignard Follow
4 mins
It's in your nature
, the chronicle of biodiversity, continues this week to focus on the effects of the historic drought that is hitting France this year.
Repeated heat waves weaken the trees.
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Why do trees lose their leaves in summer?
This is a defense phenomenon against the multiplication of heat waves, which could have consequences on the survival of certain species.
These days, in Paris, the walker who passes under plane trees or chestnut trees would think he is in autumn.
Yellow, dry leaves crunch under his feet.
The passer-by then hums Prévert's song, immortalized by Montand: "
The dead leaves are picked up by the shovel...
".
It's an autumn song, but it's the summer hit this year in parched France.
The trees lose their leaves, thirsty.
The gaze of the passer-by then falls on a few women and men in green responsible for the cleanliness of the city of Paris.
Crushed by the heat, they take their break under the imperfect shade of the chestnut trees.
They don't have a shovel to pick up the leaves, but a big motorized vacuum cleaner.
The times are changing.
It is not the first Parisian summer where the leaves of the trees already have the color of autumn, but the drought which is hitting the whole of France this year is exceptional, unprecedented.
Almost everywhere, russet spots dot the landscape;
suffering trees.
An oak tree, for example, needs "
several hundred liters of water per day, depending on its size
", specifies Nathalie Bréda, research director at INRAE, the National Institute of Research for Agriculture. , food and the environment.
For lack of water, the trees then trigger a defensive response which leads to the loss of their leaves;
for them too, it is a question of saving water.
When the trees no longer sweat
"
The main mechanism is for trees to close their stomata, the small holes in the leaves, so as to save the water transpired by the tree
", explains Nathalie Bréda.
Normally, a tree rejects 90% of the water it has drawn through its roots, and still in the oak, according to the National Forestry Office, transpiration can reach 1,000 liters per day in the largest individuals. .
It is this phenomenon that provides this feeling of freshness under a tree: evapotranspiration lowers the air temperature by 2 to 8 degrees Celsius.
But what happens during a heat wave, when the trees stop perspiration?
Nathalie Bréda recalls that “
perspiration, in trees as in most living organisms, serves to cool the organism.
With an episode of heat wave, the cooling usually provided by evapotranspiration no longer works.
The temperature of the leaves will then exceed 40
degrees, which will cause irreversible damage to the leaves which will dry up and fall.
»
It's fall in summer.
Drought in summer, weakened trees in winter
So much for the immediate and visible effects.
But heat waves and droughts also have longer-term consequences.
High temperatures have an effect on photosynthesis, this phenomenon specific to plants, which transforms sunlight and CO2 in the atmosphere into sugar, into energy.
"
Photosynthesis is optimal between 20 and 30 degrees
"
,
specifies Nathalie Bréda.
But when it's too hot, since the trees close the stomata of their leaves, there's no more photosynthesis.
The tree no longer feeds.
"
If photosynthesis is stopped too early in the season, for example on July 14 because it is too dry, the tree will not reconstitute its stock of sufficient reserves to attack winter in good conditions, to resist the cold. and insect attacks
.
Several months after the drought, the trees suffer the effects.
Heat waves weaken trees in summer, which will be more vulnerable in winter.
A phenomenon observed in particular on spruces, the most common softwoods in Europe, by Nathalie Bréda, who works in the Silva unit of INRAE in Nancy, in the east of France.
“
In 2018, after a very severe drought, many spruces died, particularly in the Grand Est, attacked by bark beetles, secondary insects that can only attack trees weakened by this dysfunction of their defense metabolism
.
Deadly Heat
Trees suffer in summer and may die in winter.
The question now arises of their survival in the face of climate change: “
When will the trees be able to recover, replenish their carbohydrate reserves and produce foliage and roots again if the droughts follow one another too quickly?
We no longer have a year allowing the recovery of all these vital functions for the tree
”.
The trees are suffering, and the whole planet.
It has been shown that with global warming, plants absorb less CO2, the main greenhouse gas.
The vicious circle is set in motion.
THE QUESTION OF THE WEEK
“What is this international black cat day, August 17?
Ten days after World Cat Day, the aim is to make black cats, accused of bringing bad luck since ancient Egypt and associated with witches in the Middle Ages, sympathetic
.
Superstitions that endure;
in animal shelters, it is the black cat that is adopted last... Whereas a black cat is just suffering from melanism: a genetic mutation that makes the skin and hair black (and feathers in birds) .
As in another feline, the black panther, which is just a panther (or a leopard).
Melanism is not bad luck, quite the contrary.
According to a scientific study, black cats live longer.
Because their coat is camouflage against predators.
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