Contributions of multi-temporal airborne LiDAR data to mapping carbon stocks and fluxes in tropical forests

Current climate change affects tropical forests functioning in many ways and might jeopardize their role as a global carbon sink. Accurately documenting forest carbon fluxes at a meaningful scale is therefore a pressing challenge. Airborne LIDAR, which can provide a fine-grained description of canopy structure and dynamics has great potential in this respect. This thesis explores the capabilities and limitations of airborne multitemporal LiDAR (ALS) to map patterns of C fluxes (mortality and productivity) over space and time to reduce uncertainty in global models of carbon stocks and fluxes in tropical forests. We relied on a combination of repeated ALS overflights extending over a period of 10 years on the one hand and a large network of plots totaling more than 1.2 km2 of field inventories conducted at the Permanent Research Station of Paracou (French Guiana) on the other hand.
Three main chapters are presented in the form of scientific papers. The first chapter (Q1. Efflux modeling - Mortality) addresses the possibility of developing reliable estimates of biomass, basal area and stem number loss (Efflux) from observed canopy height change with repeated ALS overflights and further evaluates whether these loss patterns are related to local canopy height and local topography. The second chapter (Q2. Allometry and carbon stock) quantifies the error reduction achieved in plot-level AGB estimates by using locally adjusted Height-Diameter allometries. Those allometries are established by merging field inventories and canopy height models derived from ALS and incorporating local canopy height and species identity as predictors. The third chapter (Q3. Influx modeling - Productivity) examines if canopy height gain derived from repeat ALS can be used to map aboveground woody net primary productivity (AWNPP) at a site with different characteristics of structure and dynamics in undisturbed and disturbed plots such as Paracou. A final chapter synthesizes the main conclusions of the first three articles, develops a critical reflection on the work conducted during these three and a half years.

Composition du jury :
M. David Coomes, Professeur des universités, University of Cambridge, Rapporteur/ Examinateur
M. Jérôme Chave, Directeur de recherche, CNRS, Rapporteur/Examinateur
Mme. Sylvie Durrieu, Chargé de recherche, INRAE, Examinatrice
Mme. Adeline Fayolle, Maître de conférences, Gembloux Agro-Bio Tech, Examinatrice
M. Grégoire Vincent, Directeur de recherche, IRD-Montpellier, Directeur de thèse
M. Raphaël Pélissier, Co-directeur de recherche, IRD-Montpellier, Co-directeur de thèse