Public Thesis Defense of Louis de WERGIFOSSE

"Simulating tree growth response to climate change in structurally-complex oak and beech stands across Europe"

In Europe, forests cover approximatively 35% of land area and offer a vast amount of goods and services that could be threatened by global changes. For the last decades, the rise in atmospheric CO2 and the consequent warming have generally had a positive impact on forest productivity but more frequent droughts and heat waves have also locally led to productivity decline and mortality events. Given the future climate projections, it is likely that these trends will be reinforced in the future and have an impact on European forest state but local site conditions could modulate this impact[LdW1] . A promising approach to increase forest resilience is to favor uneven-aged structure and species mixture. Models able to simulate the response of structurally-complex stands to climate change are therefore needed.

During my thesis, I participated to the development of HETEROFOR, an individual-based and spatially explicit tree growth model through the integration of a phenological and a water cycling module, two key processes for understanding how climate conditions affect forest ecosystem functioning. Using this model, I realized simulations according to different climate projections and greenhouse gas emission scenarios in order to predict how climate change will affect oak and beech tree growth in European temperate forests and how this response will be modulated by the local soil, stand and climate conditions.

The simulation results showed that, on average, under constant atmospheric CO2, net primary production (NPP) slightly increased in the temperate continental and mountainous regions and remained unchanged in the oceanic area even if, locally, considerable productivity reduction appeared. The NPP changes were negatively affected by the mean air temperature during the vegetation period and the rainfall decline and depended on the climate model used for the projections. To a lower extent, they were also influenced by soil extractable water reserve and stand characteristics. They were positively affected by longer vegetation periods and negatively affected, for beech, by drought and, for oak, by larger autotrophic respiration costs. For both species, the NPP gain was much larger when considering an increase in atmospheric CO2 concentration mainly due to the CO2 fertilization effect. There is however a large uncertainty regarding the long-term persistence of this effect.

Even if the stand characteristics had a limited influence on the forest response to climate change, they explained a large part of the NPP variability, which gives the forester the possibility to act on the stand productivity and to prepare forests to possible adverse effects of climate change by reinforcing forest resilience.

[LdW1]Peut éventuellement être supprimé pour gagner de l’espace.