Inbreeding depression, functional traits and phenotypic plasticity in an endangered tree species from Congo basin with a mixed mating system

Inbreeding depression, functional traits and phenotypic plasticity in an endangered tree species from Congo basin with a mixed mating system

Abstract

1. Most tree species can suffer from inbreeding depression (ID), which they escape by reproducing predominantly through outcrossing. A remarkable exception is Pericopsis elata, an African timber species naturally producing 54% of self-fertilized seeds in the eastern Congo Basin. This species is highly logged and suffers from a deficit of natural regeneration, so that silviculture is needed for its sustainable management. While selecting good genetic material can increase the value of plantations, we lack fundamental biological knowledge on the effect of inbreeding and competition on growth potential, variability in leaf traits and phenotypic plasticity. We hypothesize that ID in P. elata could result from the expression of deleterious mutations affecting functional traits, or from a reduction of adaptive phenotypic plasticity in inbred genotypes.

2. To test our hypotheses, 540 P. elata seedlings were monitored for 4 years in a Nelder-type device located in the DRC, in which trees were planted along concentric circles to generate a density gradient. Nine leaf morphological traits (including specific leaf area, stomata density and size), eight leaf chemical traits, diameter, and total height were measured regularly, while paternity analyses allowed distinguishing inbred and outbred plants. To explain the observed ID on growth, we tested whether inbreeding affected leaf traits and/or their plasticity expressed across years, across the density gradient or across sunlight exposure.

3. Outbred plants grew faster than inbred ones, demonstrating ID for each level of competition. Despite the significant correlation found between specific leaf area and growth, and the impact of planting density, plant age, and leaf exposure to sunlight on multiple traits, mean leaf trait values did not differ according to inbreeding. However, a few leaf traits (chlorophyl content, maximum stomatal water vapor conductance, and leaf fresh mass) showed significantly higher plasticity in outbred than inbred plants.

4. Synthesis: the observed ID on growth was not explained by a direct effect of inbreeding on the mean values of functional traits but possibly by a reduction of phenotypic plasticity with inbreeding. Additional studies on the interplay between ID, functional traits and plasticity should be conducted at the intra-specific level to identify general patterns.

Key-words : Inbreeding depression, phenotypic plasticity, silviculture, functionals traits, Pericopsis elata, mating system, Nelder device.