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Avis de Soutenance

Soutiendra publiquement ses travaux de thèse intitulés
Role of diversity in the stability of forest ecosystem processes under climate change

Soutenance prévue le vendredi 16 novembre à 9h
Lieu : Amphi de la délégation régional

Climate change has a direct impact on the ecosystem processes of forests and on the services they provide, but it also indirectly affects forest ecosystems by changing the composition of communities. However, such changes in bio-diversity are likely to affect ecosystem functioning, since ecosystem processes - such as productivity or decomposition - are particularly sensitive to the species composition of communities. However, while the relationship between diver-sity and the average level achieved by a given process (e.g. productivity or decomposition) has been widely document-ed, few studies have attempted to estimate the effect of diversity on the stability of these ecosystem processes. This stability refers to the ability of a forest ecosystem to maintain its structure and properties after disturbance or stress (resistance), and also its recovery rate (resilience). This lack of knowledge about ecosystem stability is particularly im-portant for forest ecosystems; and the gaps of knowledge are even greater if we consider the interaction between the effects of diversity and the effects of climate change. However, in this context of climate change where stressful events are expected to be more intense and frequent, a better understanding of stand resistance and recovery seems essential, both from the point of view of conserving diversity and maintaining ecosystem processes at the local scale and from the point of view of the forest manager who must adapt stand types and silvicultural practices to new conditions. This thesis thus focused on the effect of species richness and climate on two key ecosystem processes: tree growth and litter decomposition. This involved (i) testing and quantifying the stabilizing effect of tree diversity on forest productivity, (ii) identifying the importance of the effect of litter or stand species composition on decomposition, and (iii) estimating the effect of climate on forest ecosystem processes and the effect of diversity. This thesis focused on stands dominated by three tree species: beech (Fagus sylvatica), fir (Abies alba) and oak (Quercus pubescens) in the French Alps, using empirical (via sampling carried out on a double diversity-climate gradient), experimental and modelling approaches. Through empirical and experimental data, we have shown that the stabilizing effect of diversity can be significant but depends greatly on species identity. Some insights on the underlying mechanisms were highlighted, mainly based on physiological differences and niche complementarity between species. This work also raised the importance of focus-ing on several scales in the study of the relationships between diversity and functioning. Then this work showed that the effect of a stress gradient could significantly, but not systematically, modulate the mixture effect on forest process-es, wood production and litter decomposition. Finally, simulations were conducted to identify management scenarios promoting mixtures and allowing the maintenance of ecosystem services in the context of climate change.

Keywords : productivity, decomposition, diversity, stability, climate change, Alps

Membres du jury :

Stephan HÄTTENSCHWILER, Directeur de recherche, CNRS                       Directrice de thèse                          
Xavier MORIN, Chargé de recherche, CNRS                                                 Co-Encadrant de thèse
Michael SCHERER-LORENZEN, Professeur, Université de Freiburg               Rapporteur        
Bart MUYS, Professeur, Université de Leuven                                               Rapporteur
Céline MEREDIEU, Chargée de recherche, INRA                                          Examinatrice
Claude PLASSARD, Directrice de recherche, INRA                                       Examinatrice

Miriam BUITRAGO, Animatrice du secteur forêt et changement climatique, ADEME          Membre invitée