How do gap dynamics and colonization of a human disturbed area affect genetic diversity and structure of a pioneer tropical tree species?

• We studied pioneer tree populations subject to human and/or natural disturbance.
• Primary and early successional forests harbored similar levels of genetic diversity.
• Allelic richness and genetic structure differed between forest types.
• Gap dynamics, colonization, and gene flow were determinants of genetic structure.
• Genetic diversity in both forest types is important for conservation of this tree.

Pioneer tree species exhibit life-cycle characteristics and population structures that are mainly affected by natural or human disturbances. In primary forests, demographic dynamics of pioneer species may resemble those defined for a metapopulation. In early successional forests, the patterns of establishment, survival and reproduction are mainly determined by microclimate and biota modifications of the site after the human disturbance. The aim of this study was to investigate how the ecological processes and the population genetic factors associated with disturbances can affect the genetic diversity and structure of populations of a pioneer tree species: Croton floribundus Spreng. (Euphorbiaceae). Nuclear and chloroplast microsatellite markers were examined in plants of two size classes sampled in four gaps of primary forest and four sub-areas of early successional forest. Despite presenting similar genetic diversity levels, the genetic diversity was distributed differently between forests. The combined effects of seed dispersal and colonizations (and extinctions) were determinants of the fine-scale genetic structure of C. floribundus. The main finding was that human disturbances seem to boost the influence of founder effects in populations of a species with limited seed dispersal. Results suggested that gene flow by pollen was responsible for maintaining the genetic diversity within populations of C. floribundus in both forests, but in the early successional forest, gene flow by seeds was equally important. We conclude that gap dynamics, colonization, and pollen and seed dispersal affect the genetic diversity and structure of the pioneer species depending mainly on the number of colonizers, the number of source populations, the gene flow rates, and the level of human disturbance.