Thesis by LEGROS Thibaud “The litter of Mediterranean forests as a source of Biogenic Volatile Organic Compounds (BVOC) emissions in a more arid climate: what are the atmospheric consequences on a regional scale?”
17 December 2025 @ 2pm - 16h00
Mr. LEGROS Thibaud, doctoral student at IMBE, team Functional ecology: from socio-ecological systems to molecules, will publicly defend his thesis on Wednesday 17 December 2025, at 2pm in the PONTE Amphitheatre, Campus Saint Jérôme.
In front of a jury made up of :
- Dr Alexandre ARMENGAUD, Guest member, AtmoSud
- Dr Karine SARTELET, rapporteur, CEREA, Ecole nationale des ponts et chaussées
- Dr Joan LLUSIÀ, Rapporteur, CREAF, Spain
- Dr Raluca CIURARU, examinatrice, Université Paris-Saclay, INRAE
- Professor Philippe Cuny, Chairman of the jury, AMU-MIO
- Professor Catherine Fernandez, thesis co-director, IMBE
- Dr Elena Ormeno-Fuentes, PhD supervisor, IMBE
Summary of work:
Emissions of Biogenic Volatile Organic Compounds (BVOCs) from the canopy have been characterised since the 1980s and their contribution to the formation of O3 emissions have been quantified from the regional scale (South PACA region) to the global scale. This contrasts with VOCB emissions from soils and their litter, which are rarely characterised, even in the Mediterranean region, which is heavily affected by O3 and that it represents a hot spot for climate change, which could have a rapid impact on VOCB flows. To fill this knowledge gap, this thesis has two parts. On the one hand, the experimental part aims to characterise the fluxes of VOCBs in soils covered with litter in two typical forest formations in the Mediterranean region: the deciduous forest of Quercus pubescens within the experimental platform in natura of the O3HP - where VOCBs were measured in real time by PTR-ToF-MS - and the mixed forest of Pinus halepensis and Quercus ilex within the site in natura Fontblanche, where VOCBs were studied via off-line GC-MS analyses. These fluxes were measured in late spring or summer under conditions of natural water stress (SN) and long-term amplified water stress (SA) respectively, via precipitation restriction measures implemented over the last 10 or 9 years respectively. Secondly, the modelling component was used to assess the current contribution of soils (covered in litter) from forests (deciduous and coniferous) in the Sud-PACA region, using the Tingey algorithm. These emissions were then compared both with the VOCB emissions in a drier future scenario and with the VOCBs emitted by the modelled canopy. via MEGAN. Finally, the formation of O3 has been estimated via the CHIMERE model. The experimental component illustrates that the soil (litter cover) of the mixed forest is characterised by a high emission factor (emissions at 30°C) of VOCBs (184 µgC.m-2.h-1 on average) made up of monoterpenes and sesquiterpenes, mainly due to the participation of P. halepensis emissions. Deciduous forest, on the other hand, has a lower emission factor (22 µgC.m-2.h-1 on average) of highly volatile compounds (acetone and acetaldehyde). In both cases, the total flux of VOCBs in soils decreases very sharply under AS conditions (84% at Fontblanche and 43% at O3HP). In terms of modelling, VOCB flows from forest soils represent, on the scale of the Sud-PACA region and during the summer of 2022, 26 tC.day.-1 under SN and 7 tC.day-1 under AS, leading to an increase of 4% and 2% (respectively) in O3 on a regional scale.