Deciphering the growth form variation of the Mediterranean chamaephyte Thymus vulgaris L. using architectural traits and their relations with different habitats

International audience; There is a growing interest for methods in plant biology that summarize different plant growth forms and which can identify patterns of growth form variability at ecological scale in a global context. The use of detailed architectural descriptions can bring valuable information for understanding what developmental features underlie the transitions from one growth form to another. In this paper, we investigate the developmental attributes responsible for growth form variation in Thymus vulgaris and whether they are linked to habitat preference. We analysed architectural development in T. vulgaris from 12 populations in southern France, which includes two contrasting habitats of the species. Architectural development in T. vulgaris is based on three hierarchical levels of organization, which include the module, the branched complex and the whole plant. T. vulgaris is found as three different growth forms that can vary from basally branched to more apically branched growth forms. Differences in growth form result from variations in the sequential repetition of branched complexes and are also linked to the type of habitat. Individuals growing in hot, dry, rocky habitats are dominated by plants that display mostly a basally branched (basitonic) architecture – a potentially adaptive architectural trait for exploiting space in open woodland. Individuals growing in colder, more humid habitats that are dominated by tall grasses display mostly apically branched (mesotonic and acrotonic) architectures resulting in more elevated growth forms. The architectural method provides additional insight into how variations in form are developed in small-bodied woody plants such as Thymus. We found that reiteration processes are important for forming different organisational levels and variations in growth form. Architectural traits provide additional markers for ecological studies that wish to understand how living space is exploited by the whole plant. The study represents a promising approach for identifying whole plant traits at the community level.