Klara Bouwen will defend her PhD on July 17, 2025 à 1:30pm titled Interactions between canopy structure and understory microclimate: implications for forest regeneration. The defence will be conducted in English and held at UMR ISPA on the “Grande Ferrade” INRAE campus of Villenave d’Ornon. We will also provide a Zoom link on the same page, so check for updates or contact Klara directly if you want to assist online.
Jury
M. Ogée Jérôme, Directeur de recherche, Université de Bordeaux, Directeur de these
Mme Collet Catherine, Chargée de recherche, INRAE, Rapporteure
M. Ferrio Juan Pedro, Directeur de recherche, EEAD-CSIC, Rapporteur
Mme Rühr Nadine, Professeure, Karlsruhe Institute of Technology, Rapporteure
M. Domec Jean-Christophe, Professeur, Bordeaux Sciences Agro, Co-directeur de these
Mme Porté Annabel, Directrice de recherche, INRAE, Examinatrice
Mme Marie Charru, Maitresse de conférences, Bordeaux Sciences Agro, Co-encadrante de thèse
Abstract
Forest canopy plays a crucial role in buffering understory microclimate and explains understory biodiversity, forest regeneration and forest resilience to future climate change. Forest management practices shape understory microclimate by modifying the structure and composition of the canopy through thinnings, tree species selection, understory control or forest fragmentation. Yet currently, forest managers have no tools to quantify the impact of their practices on microclimate. Using micrometeorological measurements, airborne LiDAR data and model simulations of the physics-based biometeorological model MuSICA, I investigated how variations in canopy structure influence the understory microclimate, with a particular focus on summertime climate extremes and their potential impact on seedling survival.
Performing sensitivity analyses using physics-based models poses a challenge as they require meteorological input data (e.g. air temperature, humidity, wind speed) above the canopy, that are in turn modified by changes in canopy structure. Using the same climate forcing across a range of canopy structures can therefore lead to misleading conclusions. To this end, I codeveloped a new algorithm to account for dynamical feedbacks between changes in canopy structure and the climate conditions just above it. This algorithm relies on existing theories of turbulent flux-gradient similarity relationships within the atmospheric surface boundary layer that I tested against datasets collected in various forest ecosystems across Europe. I found that the influence of canopy roughness clearly affects the shape of the vertical profiles of windspeed and air temperature above the canopy, and that different theoretical framework used to represent these effects performed equally well.
Building on this result, the new algorithm was implemented into MuSICA to investigate how a change in canopy density or vertical complexity shapes understory microclimate during summer extremes. Based on experimental data from a range of forests across Europe, I first identified the existence of a threshold of canopy density below which the microclimate near the ground was becoming warmer than in a nearby open field, meaning that the temperature extremes in summer were amplified, not buffered, in these sparse canopy configurations. This threshold was confirmed by model simulations, and the consequences for the survival of young seedling was also evaluated. Model simulations showed however that the threshold in canopy density was site specific and varied also between years at a given location. Using experimental data and model simulations, I showed that the canopy vertical complexity had limited effect at mitigating this amplification of climate extremes below sparse canopies, unless a dense understory or shrub layer was present.
By linking canopy structure to microclimate and seedling survival, this research advances our understanding of forest–atmosphere interactions and highlights the potential for forest management to either mitigate or amplify summertime climate extremes. The findings lay the groundwork for new decision-support tools that could help forest managers evaluate the microclimatic consequences of silvicultural practices and design strategies that support biodiversity and climate resilience.
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