Team : MEA

PhD student : VAN ROOIJ Mahaut

Thesis supervisors :  AMEGLIO Thierry ,  CHARRIER Guillaume

Dates : September 2020 - December 2023

Description

Douglas fir, the leading reforestation species in the Auvergne-Rhône-Alpes region and second at the national level, is of major economic interest in France with 13 million plants produced per year. A physiological reddening phenomenon affecting young Douglas firs (< 15 years old) has been observed on average one winter out of three in France over the last 30 years and can affect up to 80% of the plantation. A reddening tree has no forestry future and generally dies within the year following the reddening. The objectives of the thesis were to understand the winter reddening phenomenon through the identification of the triggering climatic parameters and the physiological mechanism inducing needle reddening.

In order to identify the critical climatic hazard, an in-depth synthesis based on the literature, as well as a bioclimatic analysis were conducted. The literature synthesis made it possible to identify certain climatic conditions characteristic of reddening years, namely anticyclonic periods at the end of winter and/or the succession of cold and warm periods. Both the synthesis and the bioclimatic analysis together identified a combination of climatic variables, namely warm daytime temperatures, a high daily temperature range, abnormal wind speed and relative dryness of the air. Freeze-thaw cycles with cold night temperatures did not emerge from the bioclimatic analysis but were mentioned in the literature.

In order to understand how a Douglas fir reddens, the author first highlighted the knowledge gaps on winter reddening and proposed potential mechanisms that could, alone or in interaction, induce this physiological disorder, namely: winter drought inducing hydraulic failures, photo-oxidative stress and early deacclimation. Under controlled conditions, we exposed young Douglas firs to winter drought through a thermal differential between the roots and the tree crown (cold soil < 5 C and warm air ~ 14 C). Some of the trees were exposed to light intensities likely to induce photo-oxidative stress (> 1,800 PPFD). Cold soil temperatures induced moderate water stress by limiting root water absorption while warm air temperatures promoted water loss at the needle level. However, the Douglas fir was able to acclimatize to this new environment until they resumed growth. Exposure to high light intensity did not generate irreversible damage to PSII, nor photo-oxidative stress. No reddening of the Douglas fir was observed, suggesting that the physiological mechanism inducing reddening is not winter drought or photo-oxidative stress. In the field, we continuously measured the diameter variations of young Douglas firs as well as air temperatures and humidity within four plots in the Massif Central from December 2020 to June 2023. Spring frosts in April 2021 on deacclimated Douglas firs were not followed by needle reddening or cambial damage, which is in contradiction with the hypothesis of early deacclimation. Nevertheless, the comparison of a winter without reddening (2021) with a winter with reddening (2022) highlighted a strong water stress generated from the apex, linked to an anticyclonic period in January 2022. The hydraulic failure would be favored by daily transpiration to which are added freeze-thaw cycles which amplify the water stress and create hydraulic failures in the crown, which could explain the drying and reddening of the needle. Thus, the hypothesis of winter drought is the most plausible and would deserve to be tested under controlled conditions.

Links

https://theses.fr/279236905

Mahaut van Rooij, Thierry Améglio, Olivier Baubet, Nathalie Bréda, Guillaume Charrier. Potential processes leading to winter reddening of young Douglas-fir Pseudotsuga menziesii in Europe. Annals of Forest Science, 2024, 81 (1), pp.30. ⟨10.1186/s13595-024-01242-x⟩. ⟨hal-04690288⟩