Abstract :
[en] In Africa, while some plant groups and specifically genera are strictly associated to specific biomes, evidence of widely distributed genera transcending biome or habitat boundaries have also been reported for herbaceous plants and for tree genera. In this thesis, we aim at understanding the evolutionary processes that allowed particular tropical tree lineages to adapt to contrasted environmental conditions and occur in vast areas. While a diversity of methods at different phylogenetic and spatial scales can provide complementary insights, detailed analyses of well-resolved clades are needed to build robust generalizations about niche evolution and biome shifts. Therefore, from species distribution modeling to hard physiological measurements, we attempt to disentangle the processes explaining the vast distribution of Erythrophleum (Fabaceae), a tropical tree genus widely distributed in Africa, Australia and China.
A synthesis is first provided on the Erythrophleum species in Africa (Chapter 2). Erythrophleum encompasses two forest species and two savanna species. The forest species are morphologically similar and sold for timber under the same commercial name, Tali, but genetics revealed contrasted distribution. Erythrophleum ivorense is restricted to the wet coastal forest, while Erythrophleum suaveolens occurs over vast areas from the moist forest up to gallery forests in the savanna.
Secondly, we use species distribution models to determine the bioclimatic constraints on the distribution of the Erythrophleum species and gene pools (Chapter 3). Our results support the substantial role of climate in niche evolution, and speciation, within the genus. Erythrophleum has managed to adapt to extremely contrasted climates, resulting in a distribution encompassing several biomes, from the wet forest to the dry forest and the savanna.
Ecophysiological (Chapter 4) and experimental (Chapter 5) approaches are then used to get a functional understanding of the niche evolution within Erythrophleum. We find that, for the two forest species, the shift into drier environments was associated with a coordinated evolution of the xylem resistance to embolism, controlled by subtle adaptations in wood anatomical traits, and desiccation delay strategies at stem and leaf levels.
Finally, after confronting our results with previous studies on congeneric species, we conclude that, for certain tree lineages, the lability of drought tolerance traits, specifically those related to xylem protection, is an important functional path allowing to transcend habitat and biome boundaries (Chapter 6). We then discuss the generalization of the niche evolution pattern we detected for Erythrophleum to other African genera. Comparing the niche of 1439 woody species at the continental scale, we provide new evidence supporting the niche evolution within many woody genera in tropical Africa (20% of the 532 studied genera).
