What can they possibly understand, the trees, about the weather of the last few weeks? September was marked by temperatures between 3.5 and 3.6°C above normal, Météo France noted last Friday. And the beginning of October is off to a flying start, with Monday ranking among the three hottest days on record.
This particular weather is reflected in the still very green color of the vegetation. Sylvain Delzon, research director in plant ecophysiology at the National Institute of Agricultural Research (INRAE), does not only mean high temperatures. “The big difference with last year is that it is not only hot, it also rained a lot over a large part of the territory, sometimes heavily,” begins the researcher.
Are French forests sure to be carbon sinks in 2023?
These conditions are almost ideal for the proper functioning of gas exchanges between trees and the ambient air. “Evaporation on one side, the entry of CO2, via the photosynthesis, on the other, continues Sylvain Delzon. Trees continue to grow at this time. »
If these high temperatures are alarming, they can also, paradoxically, have benefits, including for the climate. “Since the growing period is longer this year, the trees will fix more CO2 and thus contribute more strongly to reducing the greenhouse effect,” indicates the INRAE researcher. In other words, French forests should fully play their role as carbon sinks in 2023, a key mission in the fight against climate change but which has recently been undermined. “Last year, for example, tree growth was completely stopped at the end of July in many cases due to drought,” recalls Sylvain Delzon. The leaves began to yellow or even fall. From then on, trees no longer stored carbon but sometimes even became sources of emissions. »
Less complicated harvests
On the side of arborists – farmers who grow trees for their fruit – morale is also generally good according to Daniel Sauvaitre, general secretary of‘Interfel, the inter-professional association of the French fresh fruit and vegetable sector and producer of apples and pears in Charente. “Better than last year, when the drought had a major impact on the harvests of many producers,” he explains. This late heat wave hasn’t hurt us so far. » Not only is the quality of the fruit that goes into the cold rooms good, he notes, but the temperatures of recent weeks have allowed certain fruits to play overtime at the French table. This is the case for peaches, apricots and melons, which had a bad start to the season.
Forced to water like in the middle of summer?
While many pickings are still underway, Françoise Roch, president of the National Federation of Fruit Producers (FNPF), evokes “this advantage, not so negligible, of being able to work in mild weather”. But this arborist based in Moissac, in Tarn-et-Garonne, also notes negative points and paints a more nuanced picture. “In my region, for example, it hasn’t rained for a long time. For many fruits, we are in full harvest. However, without water and with this late heat, the risk is of not being able to complete the harvest. Let the trees give up before and no longer nourish their fruit. The latter then fall prematurely. » The fear for an arborist: “a fruit on the ground is lost,” say Françoise Roch and Daniel Sauvaitre. The fall damages it, and this fruit will not be preserved. »
To avoid this outcome, in regions where there has been little rain, “orchardists have sometimes been forced to water in September as in the middle of summer,” indicates Françoise Roch, who fears that this late watering will become the norm with climate change. .
Disturbed dormancy?
This is not the only concern posed by these very mild beginnings to autumn in recent years, and potentially for those to come. What impacts, for example, on dormancy ? This period of the tree cycle corresponds to the moment when they will enter a phase of slowed down activities, allowing them to conserve their energy as best as possible. Normally, it corresponds to the arrival of the first low temperatures of autumn. “For the vast majority of species, the trees prepare to go dormant between October and November,” indicates Sylvain Delzon. And they are deciduous, that is to say without leaves, from the beginning of December. » A late heatwave can disrupt this cycle. “The risk is to have buds that hatch [on parle de débourrement] erratically, in the middle of January for example, continues Sylvain Deloze. So much too early. The low temperatures of winter will roast these young leaves and flowers. There will be so many buds which will not bear fruit in spring. »
We are not there yet, slip Daniel Sauvaitre and Françoise Roch, who hope that the temperatures will end up falling by November, to definitively rule out this risk. In the meantime, the president of the FNPF remains on alert regarding certain fruit trees such as cherry trees, whose harvest takes place early in the year. Between May and early July. “In areas where water is lacking, arborists stop watering their cherry trees once the harvest is finished, to save it primarily for their trees still in fruit,” explains Florence Roch. Very quickly, these cherry trees lose their leaves and go dormant. But this late heat wave sometimes combined with precipitation can completely disorient the cherry trees to the point that they bud in the coming weeks,” continues the arborist. Here again, these would be so many buds which will not bear fruit next spring.