Archives Thèses, HDR

Avis de Soutenance
Daphné ASSE

Soutiendra publiquement ses travaux de thèse intitulés Understand and predict the response of elevation forest ecosystems to climate change with a program of citizen science.

Soutenance prévue le jeudi 15 novembre à 13h30

Lieu : Grande salle de réunion du CEFE


Mountainous regions are particularly exposed to the ongoing climate change. Indeed, in the Western Alps the temperature increased twice faster than in the northern hemisphere during the 20th century. Trees’ annual cycle, as in many other organisms, is largely affected by climate change. Phenology and the fine temporal variations of climate appear key to predict species distribution. The main objective of this PhD thesis work was to understand the response of tree phenology to climate change in the Alps and to develop tools to evaluate this response in future conditions. It has been carried out using the phenological observations (budburst, flowering, leaf senescence) of five tree species (hazel, ash, birch, larch and spruce) of the citizen science program Phenoclim.

Our results show that warmer winters slow down bud dormancy break, and consequently the budburst and flowering dates along the elevation gradient. This effect is stronger at low elevation. The robustness of process-based species distribution models depends strongly on the robust-ness of their process-based phenology sub-model. By comparing different phenology models differing in their level of complexity and we showed that process-based models were the most robust especially when their parameter estimates relied on forward estimation using exper-imental data. Models project a reduction in the phenological cline along the elevation gradient by the end of the 21th century. This is due, on one hand, to an advancement of the budburst dates at high elevation and on the other hand, to a delay of the budburst dates at low elevation. We also tested several hypotheses on the environmental determinism of bud cell growth. However, none of the hypotheses improved signifi-cantly the models’ performance. We then implemented the best phenology models we obtained in the process-based species distribution model PHENOFIT. We carried out for the first time simulations at high spatial resolution. Projections showed that species are expected to move up along the elevation gradient in response to climate change. However, local extinction events may occur in the bottom of the valleys due to late flowering dates that would decrease the reproductive success. Depending on the species, the upper altitudinal limit would be controlled by the risk of flowers’ exposure to late spring frost or to the length of growing season, which determine fruit maturation success.

All of these results, allowed us to provide some answers on the future dynamics of high altitude ecosystems in the face of global climate change. They also allowed us to show that the Phenoclim data were of sufficient quality to be used to address important scientific questions.

Keywords: Phenology, modelling, biological processes, climate change, species distribution, biogeography, Alps

Membres du jury :

Membres du jury :
Dr. Jean-Paul THEURILLAT, Université de Genève                       Rapporteur
Dr. André LACOINTE, CR, INRA Clermont-Ferrand                      Rapporteur
Dr. Stephan HATTENSCHWILER, DR, CEFE - CNRS Montpellier Examinateur
Dr. Philippe ROZENBERG, DR, INRA Orléans                            Examinateur
Dr. Isabelle CHUINE, DR, CEFE - CNRS Montpellier                   Directrice de thèse
Dr. Christophe RANDIN, CR, DEE - Université de Lausanne         Co-directeur de thèse
Dr. Vincent BADEAU, IR, INRA Nancy                                       Co-directeur de thèse
Dr. Anne DELESTRADE, CREA Mont-Blanc                               Membre invité