ED Sciences et environnements
The first stages of forest regeneration under climate change: Ecological and evolutionary strategies of European Oaks and Firs from germination to seedling in response to rising temperatures
by Marion CARME (BIOGECO - BIOdiversité, Gènes & Communautés)
The defense will take place at 14h00 - Amphithéâtre GABA Allée Goeffroy Saint-Hilaire, Bâtiment B5, 33600 Pessac
in front of the jury composed of
- Marta BENITO GARZÓN - Directrice de recherche - Université de Bordeaux - Directeur de these
- Isabelle CHUINE - Directrice de recherche - CNRS CEFE - Rapporteur
- Marcella VAN LOO - Directrice de recherche - BFW - Rapporteur
- Josep M. SERRA-DIAZ - Maître de conférences - IBB, CISC-CMCNB, AgroParisTech - Examinateur
- Arndt HAMPE - Directeur de recherche - INRAE UMR BIOGECO - Examinateur
Global change is disrupting forests and their important ecosystem services at an unprecedented rate. Ensuring their capacity to maintain and regenerate under the new climatic conditions has become one of the most pressing scientific, political and societal challenges of our time. While the response of adult trees to climate change is relatively well documented, early stages has been much less studied; yet it is a decisive phase for long-term forest establishment, as early tree stages are crucial for adaptation to new environmental conditions. This PhD investigate how the first life stages of trees (germination and seedlings) respond to environmental change (plasticity) depending on their climate of origin (population genetic differentiation). Specifically, it focuses on how future warming temperatures will affect these stages in important European oaks representative of Mediterranean, Submediterranean and Temperate biomes exhibiting recalcitrant seeds: cork oak (Quercus suber), pubescent oak (Q. pubescens), and sessile oak (Q. petraea). To achieve this, we conducted a series of reciprocal transplant experiments under controlled conditions in climatic chambers with acorns collected from range-wide populations for each species, and built hybrid trait-based species distribution models (ΔTraitsSDMs) to characterize and predict germination and seedling responses to temperatures across population gradients. Our results show that warming accelerates germination timing and first leaf emergence phenology across all three species, without consistently affecting germination percentage. In pubescent and sessile oak, warming shifts resource-use strategy toward more conservative (decreasing SLA, height and height growth while increasing stem diameter and photoprotective pigments) and decreases fitness under extreme temperatures, while moderate warming has weak or neutral effects. Acorn mass strongly predicts seedling biomass, growth, and earlier first leaf emergence across species, while germination traits are largely decoupled from both acorn reserves and subsequent seedling performance, suggesting stage-specific developmental controls. Population climatic origin strongly structures responses across a wide range of traits, including germinability, germination timing and synchrony, survival, growth, specific leaf area, photoprotective pigmentation, root architecture, and first leaf emergence time. In cork oak, seeds from warmer provenances germinate more, earlier, and more synchronously than those from colder, more seasonal origins. Warm-dry adapted populations of pubescent oak exhibit the lowest baseline fitness, which is further reduced under warming; by experimentally decoupling temperature from drought, these results suggest that drought adaptation carries physiological costs that become apparent when water is no longer limiting. Cold-adapted continental populations of sessile oak, by contrast, show initially higher fitness but suffer the steepest declines under warming. Populations locally adapted to historical climates may thus face adaptation lags under novel climatic conditions. Spatial projections confirm these trends: pubescent oak faces widespread fitness declines except in northern continental regions, while sessile oak shows stability in western oceanic areas but sharp decline in eastern continental populations. These findings demonstrate that intraspecific genetic differentiation cannot be neglected in climate-resilience strategy, notably in seed sourcing strategies for assisted migration and restoration programs. Besides, modelling early-stage traits across intraspecific gradients captures vulnerability patterns invisible to traditional SDMs, enabling more targeted selection of seed sources across Europe.