Abstract
Comparative ecology has highlighted recurring associations between plant functional traits and their environment. These relationships may vary depending on the level of organization considered – within species, between species, and among groups of species – but this dependency remains poorly studied. A fundamental distinction in life history theories is made between annual species (completing their life cycle in one year) and perennial species (life cycle over more than one year, usually with multiple reproductive events). Annual and perennial herbaceous plants differ in their functioning (growth rate, investment in seed production, allocation to roots, etc.). However, despite their frequent coexistence, few studies have considered potential differences in trait-environment relationships between these two groups. The objective of this thesis is to understand the specific variations in the traits of annual plants depending on resource availability, based on in situ measurements and in a common garden setting.We studied herbaceous communities in the Grands Causses, where annuals and perennials coexist in two contrasting environmental conditions: i) fertilization and high disturbance, and ii) poor soil and less intense disturbance. We show that variations in traits related to growth rate and leaf tissue density are lower in annuals than in perennials. This is explained by (a) a higher species turnover in perennials, and (b) the presence of species with larger differences in trait values between environments in perennials. Intraspecific variations are identical between the two groups of species. Measurements made during this first part were used to complete a trait database under development. On this occasion, I contributed to the structuring of this database through data management work, aiming to propose modalities for sharing functional trait data and associated environmental variables; a synthesis of this work is proposed.Secondly, we analyzed intraspecific variability in annuals from these communities, in order to test its origin (genetic or plastic), to identify the most variable traits in response to fertilization, and to compare this variability between species. Thirty populations were grown in a common garden, with low or high fertilization. The results indicate that i) the observed trait variations in situ are likely of plastic origin; ii) plasticity is low in morphological leaf and root traits but high in biomass allocation and nitrogen content; iii) species preferring nutrient-rich environments are more plastic in their nitrogen content.Finally, a literature review was undertaken to determine which traits are determinant for annual and perennial herbaceous plants, reasoning on demographic components (reproduction, growth, survival), the importance of which differs according to the life cycle. We propose an opinion article aiming to better integrate life cycle and commonly measured morpho-physio-phenological traits.This thesis proposes a study of the relationships between functional traits and the environment at different levels of organization: between life cycles, between species, and within species. It highlights that trait-environment relationships can vary between these levels, fitting into a renewed interest in context dependency in comparative ecology.