Inbreeding depression affects the growth of seedlings of an African timber species with a mixed mating reproductive system, Pericopsis elata (Harms) Meeuwen

Selfing or mating between related individuals can lead to inbreeding depression (ID), which can influence the survival, growth and evolution of populations of tree species. As selective logging involves a decrease in the density of congeneric partners, it could lead to increasing biparental inbreeding or self-fertilization, exposing the population to higher ID. We assessed the influence of inbreeding on the growth of a commercial timber species, Pericopsis elata (Fabaceae), which produced about 54% of self-fertilized seedlings in a natural population of the Congo basin. We followed the survival and growth of 540 plants raised in a plantation along a gradient of plant density (0.07 to 15.9 plants per m2). Parentage analysis allowed us distinguishing selfed and outcrossed seedlings. The annual growth was higher for outcrossed than selfed plants, on average by 10.8% for diameter and 12.9% for height growth. Based on the difference in above ground biomass between selfed and outcrossed seedlings after 41 months, we estimated the level of ID at d = 0.33, while a lifetime estimate of ID based on the proportions of selfed plants at seedling and adult stages led to d = 0.7. The level of ID on growth rate did not change significantly with age but tended to vanish under high competition. Pericopsis elata is a particularly interesting model because inbreeding depression is partial, with about 26% of reproducing adults resulting from selfing, contrary to most tropical tree species where selfed individuals usually die before reaching adulthood. Hence, the risks of ID must be considered in the management and conservation of the species.

Methods
Seeds were harvested between 17 September and 12 December 2016 in the 400 ha plot of Biaro forest, where all 196 Pericopsis elata trees (diameter at breast height ≥ 10 cm) had been inventoried. Seed trees were retained only if they produced mature seeds and were sufficiently isolated to avoid any confusion of seed provenance (19 seed trees eventually contributed to the seed pool used). One week after harvesting, seeds were sowed in polyethylene bags containing soil with natural manure, and an ID label indicating the seed tree. The nursery was located next to the botanical garden of the Science Faculty campus of the University of Kisangani (DR Congo), under mild shading. After germination, we randomized the positions of the seedlings in the nursery and followed their growth for one year before transplantation.

We installed three plantation trials in October 2017 according to the design Ia proposed by Nelder (1962), whereby plants are located along a set of concentric circles ensuring a gradient of sapling density. Each unit (Nelder 1, Nelder 2 and Nelder 3) is referred to as a replicate in the following sections. The experiment was carried out over an area of 135 x 45 = 6,075 m2 (including areas around each Nelder design) and each replicate covers an area of 908 m2. This area was cleaned from former vegetation
(fallow land), allowing to prevent the destruction of the designs by uncontrolled bush fires.

The replicates were placed next to each other to maximize the similarity of environmental conditions (edaphic, topographic and sun exposure). Each Nelder design consisted of 12 concentric circles, each one with 18 equally spaced saplings. The saplings located on the first and last circles formed the buffer zones, placed respectively at a distance of 0.6 and 15.6 m from the centre of the circles, so that a total of 180 saplings out of 216 were monitored for each replicate. The local spacing between neighbouring saplings ranged from 0.2 x 0.2 m to 3.8 x 3.8 m.

Saplings from the nursery were transplanted on October 2017, when they had reached heights ranging from 40 cm to 160 cm (median around 65 cm). As available sample sizes varied widely among maternal families, we could not conceive a well-balanced design. In Nelder 1 and Nelder 2, where 19 and 10 families were represented, we ensured that saplings from the same mother tree were never contiguous. This was not feasible in Nelder 3, represented by seven families but with two of them constituting 88% of the saplings. Within each Nelder design, we ensured that represented families were well spread among the different circles to avoid any confounding effect between maternal tree and local density. Initial sapling height did not differ significantly among replicates. Dead saplings were replaced immediately when found.

The total height and diameter at 10 cm from the collar were measured after two months and then at three months intervals on all saplings between December 2017 and March 2021, using a ruler and a caliper graduated in cm and mm, respectively. The total height was measured from the collar to the terminal bud of the longest branch. Weeding was carried out before each measurement period. We computed a competition index (C) for each sapling as a function of the distance and diameter of neighbouring saplings situated within a circle of radius K, according to Hegyi, 1974 and Vanclay et al., 2013.