Aim Edaphic heterogeneity may be an important driver of population differentiation in the Amazon but remains to be investigated in trees. We compared the phylogeographic structure across the geographic distribution of two Protium (Burseraceae) species with different degrees of edaphic specialization: Protium alvarezianum, an edaphic specialist of white-sand habitat islands; and Protium subserratum, an edaphic generalist found in white sand as well as in more widespread soil types. We predicted that in the edaphic specialist, geographic distance would structure populations more strongly than in the edaphic generalist, and that soil type would not structure populations in the edaphic generalist unless habitat acts as a barrier promoting population differentiation.
Location Tropical rain forests of the Peruvian and Brazilian Amazon, Guyana and French Guiana.
Methods We sequenced 1209–1211 bp of non-coding nuclear ribosomal DNA (internal transcribed spacer and external transcribed spacer) and a neutral low-copy nuclear gene (phytochrome C) from P. subserratum (n = 65, 10 populations) and P. alvarezianum (n = 19, three populations). We conducted a Bayesian phylogenetic analysis, constructed maximum parsimony haplotype networks and assessed population differentiation among groups (soil type or geographic locality) using analysis of molecular variance and spatial analysis of molecular variance.
Results The edaphic specialist exhibited considerable genetic differentiation among geographically distant populations. The edaphic generalist showed significant genetic differentiation between the Guianan and Amazon Basin populations. Within Peru, soil type and not geographic distance explained most of the variation among populations. Non-white-sand populations in Peru exhibited lower haplotype/nucleotide diversity than white-sand populations, were each other’s close relatives, and formed an unresolved clade derived from within the white-sand populations.
Main conclusions Geographic distance is a stronger driver of population differentiation in the edaphic specialist than in the generalist. However, this difference did not appear to be related to edaphic generalism per se as adjacent populations from both soil types in the edaphic generalist did not share many haplotypes. Populations of the edaphic generalist in white-sand habitats exhibited high haplotype diversity and shared haplotypes with distant white-sand habitat islands, indicating that they have either efficient long-distance dispersal and/or larger ancestral effective population sizes and thus retain ancestral polymorphisms. These results highlight the importance of edaphic heterogeneity in promoting population differentiation in tropical trees.