Testing the divergent adaptation of two congeneric tree species on a rainfall gradient using eco‐physio‐morphological traits

Abstract

In tropical Africa, evidence of widely distributed genera transcending biomes or habitat boundaries has been reported. The evolutionary processes that allowed these lineages to disperse and adapt into new environments are far from being resolved. To better understand these processes, we propose an integrated approach, based on the eco-physio-morphological traits of two sister species with adjacent distributions along a rainfall gradient. We used wood anatomical traits, plant hydraulics (vulnerability to cavitation, wood volumetric water content, and hydraulic capacitance), and growth data from the natural habitat, in a common garden, to compare species with known phylogeny, very similar morphologically, but occupying contrasting habitats:Erythrophleum ivorense (wet forest) and Erythrophleum suaveolens (moist forest and forest gallery). We identified some slight differences in wood anatomical traits between the two species associated with strong differences in hydraulics, growth, and overall species distribution. The moist forest species, E. suaveolens, had narrower vessels and intervessel pits, and higher vessel cell-wall reinforcement than E. ivorense. These traits allow a high resistance to cavitation and a continuous internal water supply of the xylem during water shortage, allowing a higher fitness during drought periods, but limiting growth. Our results confirm a trade-off between drought tolerance and growth, controlled by subtle adaptations in wood traits, as a key mechanism leading to the niche partitioning between the two Erythrophleum species. The generality of this trade-off and its importance in the diversification of the African tree flora remains to be tested. Our integrated eco-physio-morpho approach could be the way forward.Abstract in French is available with online material.

Tree growth and mortality of 42 timber species in central Africa

“…The differentiation of two species of Erythrophleum was difficult: they were assigned following the maps of Gorel et al (2019) or to the Erythrophleum sp. taxon in one site (Djoum) where the two species coexisted (Gorel et al, 2019).…”

Climatic niche lability but growth form conservatism in the African woody flora

“…On the African continent and elsewhere, despite the prevalence of niche conservatism in plants (Crisp et al, 2009; Linder et al, 2012; Segovia et al, 2020), many clades have transcended major ecological barriers, such as the forest‐savanna boundary (see White, 1978 for Diospyros ; Gorel et al, 2019 for Erythrophleum and references therein), but the direction, timing, functional trait association and distribution of such shifts over plant phylogeny have not been characterised at the scale of a whole flora. Here, we determined the climatic niche lability of the African tropical woody flora.…”

Two Co-occurring Liana Species Strongly Differ in Their Hydraulic Traits in a Water-Limited Neotropical Forest

“…Whenever possible, the branches were cut at a length of at least 2 times their maximum vessel length (see below), after which a second piece was cut under water to remove embolisms induced by the cutting. As in Gorel et al (2019), the cut end of the branches was immediately put in a vial filled with water, and the branches were enclosed in a humidified black plastic bag for transport to the lab.…”