Colonization processes and the maintenance of genetic diversity: insights from a pioneer rainforest tree, Aucoumea klaineana

Abstract

Despite recurrent episodes of range expansion and contraction, forest trees often harbour high genetic diversity. Studies of temperate forest trees suggest that prolonged juvenile phase and high pollen flow are the main factors limiting founder effects. Here, we studied the local colonization process of a pioneer rainforest tree in central Africa, Aucoumea klaineana. We identified 87% of parents among trees up to 20-25 years old and could thus compare direct parentage structure data with classical population genetics estimators. In this species, genetic diversity was maintained during colonization. The absence of founder effects was explained by (i) local random mating and (ii) local recruitment, as we showed that 75% of the trees in the close neighbourhood participated in the recruitment of new saplings. Long-distance pollen flow contributed little to genetic diversity: pollen and seed dispersal was mainly within stand (128 and 118 m, respectively). Spatial genetic structure was explained by aggregated seed dispersal rather than by mother-offspring proximity as assumed in classical isolation-by-distance models. Hence, A. klaineana presents a genetic diversity pattern typical of forest trees but does not follow the classical rules by which this diversity is generally achieved. We suggest that while high local genetic variability is of general importance to forest tree survival, the proximate mechanisms by which it is achieved may follow very different scenarios.

Figure 1

Location of study site and samples. (a) Location of the Lopé National Park in Gabon; (b) location of the study area in the savannah–forest mosaic landscape and (c) distribution of reproductive trees in the forest and of collected saplings in the colonization front. DBH, diameter at breast height in cm (black circles, 5.00–14.58; grey circles, 14.59–80.60; white circles, 80.61–160.00); AD1, old adult tree cohort; AD2, young adult tree cohort; SAP, sapling cohort.

Figure 2

Average kinship coefficient plotted against geographical distance between individuals for (a) the AD cohort, (b) the SAP cohort and (c) among individuals from different cohorts (AD and SAP). Standard errors were assessed by jackknifing data over each locus.