Global warming, class warfare and tree mortality in French forests

50 shades of mistletoe: when trees cultivate dominant/dominated relationships [Thesis].

Social relationships of trees in French forests

Forest ecosystems are the main providers of terrestrial ecosystem services such as pollination, natural water purification and oxygen production, which can be vital for many species, including mankind.

The toll of human activity on the planet is easily observable, with 60% of forests lost within a few centuries, 60% of wild animals within a few decades, and 60% of insects within a few years. Ongoing climate change is one of many causes affecting forests. Increased rates of local or mass tree mortality, which can be associated with events combining heat and water stress, are being observed with increasing frequency and intensity.

Climate hangover

Tree mortality, like human mortality, is a natural phenomenon that can be associated with a number of intrinsic and extrinsic factors. It is therefore difficult to estimate the specific proportions to which each of these different factors contributes to the process.

In his thesis entitled "Determining the potential impact of climate change on the mortality of the main European forest species", Adrien Taccoen explores some of the determinants of background mortality, which is the mortality observed in a stand in the absence of extreme disturbances, for the main European forest species through three issues:

What is the significance of climate change on the mortality of Europe's main forest species?
Determining the link between the average climatic conditions to which trees are subjected and their sensitivity to climate change.
Importance of the tree's social status and stage of development on its sensitivity to climate change.

The main aim of this thesis is to assess the relative importance of different factors associated with increased mortality rates that may be linked to global warming, using a European forestry model.

The author's research is based on the analysis of forest inventory data from the Institut national de l'information géographique et forestière (IGN) in association with climatic data from Météo France from 1961 to 2015. Based on a population of 372,974 trees, including 7,312 surveyed dead trees, the student determined by modeling the determinants of background mortality for 43 tree species representative of European temperate forests.

Why read this thesis

This thesis is a compilation of research work, presented in the form of argued and discussed articles, in which climatological, geographical and forestry information is interwoven. It contains the usual data on temperature increases linked to human activity, but above all it reveals the forest, its nature, composition, evolution and characteristics, and more specifically the mortality of the trees that make it up.

This manuscript is bilingual, but the comments and descriptions associated with each chapter/article, as well as the final discussion, are written in French, so anglophobic friends, don't run away. The author is self-critical when addressing the limitations of his research work, which is always appreciated.

This thesis explores data on certain tree species that have never been analyzed before, and confronts the reader with the effect of climate change over the last 60 years. We are reminded that climate change is no longer just an abstraction of projections for a more or less near future, debatable by climatoskeptics, but an ongoing phenomenon that can be observed locally.

Showing what we're made of - Extract

"Dominant or dominated trees, big or small: different issues at stake
It's important to focus on large trees, as they are a major provider of ecosystem services. They influence biogeochemical cycles, mobilize deep-water resources, alter climatic and microclimatic conditions, and provide habitat for many species.

Large trees are also of great importance for carbon sequestration and biomass production. A global study based on the analysis of 5 million trees showed that the 1% largest trees in diameter accounted for almost 50% of the world's forest biomass. Finally, large trees play a very important social and cultural role.

As far as the social status of trees is concerned, in irregular management, trees of all diameter classes and social status must be present in the stand. Small and dominant trees are those that will form the regeneration and mature tree populations of the future. So, for different reasons, it's important to understand the response of both dominant and dominated trees to ongoing climate change. Despite the stakes, little is known about the relative importance of social status and tree size in their sensitivity to climate change, in terms of mortality."

Conclusion

Among the factors associated with mortality, 80% are related to competition, development stage, stand structure and composition, and silvicultural intensity. Environmental mortality factors, which can be linked directly to rising temperatures and falling precipitation within the observation window of the data studied, account for only 6% of these factors and affect only 45% of the species in the sample.

Adrien Taccoen's in-depth study of the possible links between the position of trees along environmental gradients and sensitivity to changes in temperature and precipitation regimes revealed that, for average hot and dry climatic conditions, 9 out of 12 species showed an increase in mortality caused by rising temperatures and falling precipitation.

This analysis highlights the fact that trees moving towards the hot, dry limits of their range are more vulnerable.

Assessment of the potential links between sensitivity to climate change and social status, as well as average tree size, has shown that :

Dominated trees are more sensitive to temperature increases than dominant trees;
Dominant trees are more sensitive to decreases in precipitation, with a maximum effect reached for large-diameter dominants;
Mortality due to temperature increases or decreases in precipitation is significantly higher for dominant trees than for dominant trees;
The social status of the tree plays a more important role than tree size.

Big woodcutter

In addition to revealing a wealth of hitherto unknown information about certain tree species, this research and analysis work has led to a triple finding. Firstly, it validates the existence of a link between increases in temperature, decreases in precipitation and mortality rates observed in 45% of tree species in French forests. Secondly, it demonstrates an increase in the effects of these factors for trees growing near the warmer limits of their range. And thirdly, it suggests that the variations in sensitivity to climate change observed may be correlated with the social status and size of the trees studied.

In short, touch wood!

Adrien Taccoen's work, put in perspective with that of his community, provides a better understanding of the effect of future climate change on French and European forests. The projections proposed help us to better anticipate the effects of these changes by developing and implementing appropriate silvicultural management strategies. The author adds another brick to our knowledge by reminding us of the complexity of the interconnections between abiotic factors such as temperature, and biotic factors such as social relationships in the dynamics of the tree populations that make up a forest.

This work was defended on 07-07-2020 in Paris, as part of the doctorate awarded by the Institut des Sciences et Industries du Vivant et de l'Environnement (AgroParisTech) and was carried out in collaboration with AgroParisTech, the University of Lorraine and INRA: UMR1434.

Sources

Adrien Taccoen. Determination of the potential impact of climate change on the mortality of the main European forest species. Silviculture, forestry. AgroParisTech, 2019. French.⟨NNT: 2019AGPT0004⟩. ⟨tel-02433527v2⟩

Open access thesis:

https://tel.archives-ouvertes.fr/tel-02433527

Publication associated with this thesis:

Taccoen Adrien, Piedallu Christian , Seynave Ingrid, Perez Vincent , Gégout-Petit Anne, Nageleisen Louis-Michel, Bontemps Jean-Daniel and Gégout Jean-Claude (2019) Background mortality drivers of European tree species: climate change mattersProc. R. Soc. B.286 :20190386 - https://royalsocietypublishing.org/doi/10.1098/rspb.2019.0386