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Studying the resistance of the Floirac forest to drought

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As part of a research programme led by scientists from the Biodiversity, Genes and Communities (BIOGECO) laboratory, 336 maritime pines were planted at the beginning of February on the grounds of the Bordeaux astronomical observatory. A wooden rain exclusion structure will simulate drought, marked by the decrease in precipitation predicted for 2100, and will allow the study of the resistance of these new plants from the Floirac experimental forest to extreme stresses.

Photo : The planting of maritime pines took place on 13th and 14th February 2023 © université de Bordeaux
The planting of maritime pines took place on 13th and 14th February 2023 © université de Bordeaux

In front of the Bordeaux astronomical observatory (in French), ground holes have multiplied in the plain. Agents from the National Forestry Office (ONF - in French), armed with spades and rakes, are busy filling them in. Intended to aerate the soil, these holes allow the future trees of the experimental forest to develop properly. The 336 maritime pines are waiting in crates labelled: Spain, Portugal, Corsica, etc. The plants have different geographical origins or have been selected in nurseries because of their resistance to drought.


"The plants are distributed according to a very precise cartography that was thought out in advance," explains Déborah Corso, a post-doctoral student at the Biodiversity, Genes and Communities Laboratory (BIOGECO - University of Bordeaux and INRAE). "Each pine tree must necessarily be placed next to a different one from another source," adds Thomas Caignard, a post-doctoral fellow in the same laboratory. The plants are placed at equal distances to "form a grid along the axes". The grid thus formed makes it possible to provide quantitative and rigorous data for a research programme conducted by INRAE researcher Sylvain Delzon, also at the Biodiversity, Genes and Communities (BIOGECO) laboratory.

S​​​imulating drought in the communal garden

Why this particular arrangement? The scientists are interested in a long-term "provenance test", also known as the "common garden". A rain exclusion structure is to be installed on part of the pine trees to simulate the climate at the end of the century, i.e. a reduction in the frequency of rainfall of 30 to 50%. A removable roof will cover the plantations, only during rainy periods, so as not to influence the microclimate of the ecosystem. The water content of the soil, its evaporation, the hydraulic functioning and the transpiration of the trees will be monitored continuously for 10 years by researchers from the Biodiversity, Genes and Communities (BIOGECO) laboratory. By comparing the different data, the scientists will be able to determine which maritime pine provenances have the best capacity to cope with climate change.

 With the aridity of last summer in New Aquitaine, and global climate models predicting an increase in these extreme episodes, this research programme must provide answers on the adaptation of forest ecosystems to drought. The results will provide future recommendations for the adaptation of plantation forests in our region. In the autumn, almost 400 oak trees will also be planted. This time, the test will consist of comparing different species from temperate and Mediterranean environments within the same climate, to determine which species is more resistant to drought.

The rain exclusion structure will be installed above the new maritime pine trees © Emma Penot
The rain exclusion structure will be installed above the new maritime pine trees © Emma Penot

Other studies are being carried out in the experimental forest, which is already well established in the field with its century-old trees. Thomas Caignard is working on the forest's response to climate change and in particular on phenology. "Phenology refers to the seasonality of trees, such as fruiting and leaf budding, two stages in the life cycle of trees that are sensitive to climate variations". The scientist is particularly interested in the variation in acorn production in oak trees, which varies greatly over time. This phenomenon, which has not yet been elucidated, makes it possible to optimise forest regeneration in response to pressure from acorn consumers (squirrels, wild boars, etc.) and could be influenced by the climate and the availability of resources and water in the soil. "The interest is in understanding how the trees synchronise and why," he explains. To do this, he needs to know how many acorns are produced by placing nets under the oaks in the forest. "If the oaks produce fruit, they are well adapted to their environment!

Counting acorns and observing them... a quiet, but long and tedious job, necessary to understand how the forest regenerates itself! In French, acorn translates as "gland". "The origin of the term "glander", meaning to idle, comes from a habit that is symptomatic of the periods of good fruiting of the oaks. Farmers used to bring their pigs to eat the acorns and they had to wait patiently until the pigs had eaten well," says the post-doctoral researcher.

A forest at the heart of several issues

These experiments are complemented by the observation of the Floirac urban forest. Studied since 2021, the scientists have installed sensors to measure variables such as growth and sap flow on 26 trees corresponding to 6 different species. Although endemic species such as oak, hornbeam and ash predominate, species from gardens (bamboo, prunus, etc.), invasive species (false acacia, laurel, etc.), make this forest a complex and diverse ecosystem. In this context, the scientists' objective is to understand how the forest responds to climate change, impacts the city and vice versa. Some answers show what the forest brings to the surrounding dwellings: cooling corridors made up of several islands of trees could help to bear the heat in the city during major heat waves!

Support the project

The Bordeaux University Foundation FORLand project - Experimental forest and agroecological transition, forest-based solutions - aims to propose levers to accelerate the agroecological transition via forest-based solutions. The main objectives are to adapt forestry to climate change, to green cities (in order to promote biodiversity and mitigate heat islands), to drastically reduce inputs (European Green Deal) and to increase carbon storage in agricultural soils.

The project aims to characterise the response of forests to global changes through in situ monitoring and rain exclusion experiments, but also to quantify the feedback effect of forests on the microclimate and well-being of the city.

The implementation of agroecological transitions and transformations in field crops, with solutions based on the forest, could eventually make it possible to produce healthier and more sustainable food, particularly on peri-urban farms in the metropolis.

Support the project (in French)

Researcher contact

  • Déborah Corso

    Researcher at the EPOC laboratory


Scientific communication

  • Lou Deny

    Scientific communication officer at the University of Bordeaux