Climate: heat stroke in beeches

I do (Fagus sylvatica) is one of the dominant species in Europe. As such, it is among the most studied, especially from the point of view of the impact of climate change on its abundance and productivity. “Until now, studies have focused on small populations or on a regional scale,” says Élodie Magnanou, a CNRS researcher at the Banyuls-sur-Mer Oceanological Observatory, where she studies the reactivity of the Massane beech forest to climate change. . “This is the first time that a growth model has been carried out in the entire beech distribution area. This study also puts into perspective the results of previous studies that seemed contradictory, according to Edurne Martinez del Castillo of the University of Mainz, Germany, and her colleagues, who conducted this work on a large scale.

They collected growth data from 5,800 beeches from 324 sites in Europe over sixty years, from 1955 to 2016. They based their analysis on the evolution of the basal area. (BAI or Basal area increase), that is, the annual gain in area occupied by each tree trunk. It is calculated by measuring the diameter of the trunk at 1.3 meters above the ground, at chest height. These data have the advantage of taking into account both young and fast-growing trees and older ones. In addition, it is available for many sites, and forest managers generally record it annually and in a standardized manner, which allows for comparison of different sites. According to this measure, during these sixty years, beech growth has increased by 10 to 20% in northern Europe and decreased by 20% in the south.

To model climate beech growth, the researchers used a classic statistical model in ecology, the GLMM. (Generalized linear model of mixed effects). They adjusted it to the real climatic data (temperature, precipitation) and took into account the altitude and latitude, the latter being a good indicator of the length of the day, and therefore of the capacity for photosynthesis.

They identified two periods of thirty years, very different in terms of temperature (1955-1985 and 1986-2016), over which they checked the relevance of their modeling. Of all the parameters studied, the two most important and most impactful on the evolution of trees are temperature and water availability. It would be the way of considering these two parameters that would explain the sometimes contradictory conclusions of other predictive studies.

What did the researchers observe? Over the last sixty years, growth has been two to three times higher at low altitudes in central and northwestern Europe, including Belgium, the Netherlands, Denmark, and Britain, from east to south. of Europe. From this modeling, Edurne Martínez del Castillo and his team simulated the growth of beeches over the next eighty years under two climate scenarios, an optimist where carbon neutrality will be achieved in 2080, then capturing the dioxide carbon (SSP1-2.6), the other. based on rapid economic growth driven by fossil fuels (SSP5-8.5).

Results: Beech growth could be negatively affected by up to 30% in southern Europe between 2020 and 2050 compared to the period 1986-2016. In contrast, it would be favored by 25% in the mountains of central Europe, the ideal area for beech in the current and future climate, and even by 35% in southern Scandinavia. Projections for the periods 2040-2070 or 2060-2090 are comparable, with the decline in growth possibly reaching 50% in certain areas of the Balkans, the Apennines, Greece, Romania or Spain by the end of this century.

“These results have the merit of giving a probable inventory regarding the possible maintenance of beeches,” says Élodie Magnanou, the corollary of zero growth being the long-term mortality of trees. This global modeling is complementary to the fine physiological analyzes performed in local areas. Also, the factors that are taken into account have the advantage of being available for all the sites studied; but this ignores the impact of competition with other species, the nature of the soil, the genetic family of each beech population, or their phenological characteristics. »

So there are still many things to find out! Meanwhile, this work provides valuable information for forest managers to choose where and how to act, where to plant, where to promote beech migration with ecological corridors, and even where to leave this species to plant others. Knowing that only healthy forests, that is, growing, therefore in northern Europe in the case of beech, are carbon sinks.

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