The long-term decrease in extreme cold events is a key factor in the increased mortality of Norway spruce and silver fir
Résumé
Temperate forests are facing increasing threats due to climate change, which is affecting their functionality and putting ecological services at risk. While the long-term impacts of climate trends on tree mortality have been extensively studied, the role of extreme events, especially extreme cold events, remains understudied. We analysed tree mortality in 19,056 silver fir or Norway spruce trees throughout France between 2018 and 2023. We created mortality models based on tree and stand characteristics, and included variables describing extreme climatic events related to drought, heat and cold, and their frequency, intensity and duration. The models showed an area under the curve (AUC) of 0.83 for silver fir and 0.81 for Norway spruce, with extreme climatic events explaining 70% and 29% of mortality for silver fir and Norway spruce, respectively. Extreme cold was a major contributor for both species, explaining 33 % and 5.3 % of tree mortality, for silver fir and Norway spruce, respectively.
For both species, the effect exceeded that of heat (8.7% and 4.4% for silver fir and Norway spruce, respectively) and of drought for fir (28.5 % and 18.9 % for silver fir and Norway spruce, respectively). Surprisingly, most cold extremes had a positive effect on tree vitality, thus decreasing mortality. This was probably due to the favourable impact of reduced pathogen activity, which is strongly influenced by autumn and winter temperatures. Extreme cold events did have negative effects on silver fir during winter, probably because the species is highly sensitivity to hard frosts. Our findings reveal the significantand complexrole of extreme cold events in modulating tree mortality, with impacts varying by season and species. As extreme cold events become less frequent due to climate change, some species may become more vulnerable, highlighting the need to better integrate cold extremes into forest vulnerability assessments.
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Est une version de 10.2139/ssrn.5608810 Autre Carletti, H., Caloin, M., Joetzjer, E., Marçais, B., Gégout, J.-C., & Piedallu, C. (2025). The long-term decrease in extreme cold events is a key factor in the increased mortality of Norway spruce and silver fir. Elsevier BV. https://doi.org/10.2139/ssrn.5608810