Multi-scale genetic diversity in populations of an introduced, weedy tree species (Albizia julibrissin)
Pardini, Eleanor Ashley
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This dissertation was motivated by a desire to understand the evolutionary history of a weedy tree species in its introduced range. Albizia julibrissin Durazz. (Fabaceae) was introduced as an ornamental from Asia to the United States approximately 220 years ago and has since become naturalized and weedy throughout the southeast. Previous analyses have documented high levels of allozyme diversity and pollen gene flow, but little is known about patterns of seed movement or quantitative trait variation in populations of this species in its introduced range. We used a hierarchical paternity analysis to estimate the effective number of pollen donors and document correlated mating within tree crowns. Individual trees receive pollen from many donors but mating patterns within plants are not random, particularly within inflorescences. Once we established genetic structuring of seed pools within trees, we examined patterns of spatial genetic structure within the next generation by analyzing fine-scale genetic structure in six actively colonizing populations. Spatial genetic structure was pronounced and persistent in populations. Structure decreased with increasing distance, indicating leptokurtic seed dispersal and overlapping of maternal seed shadows. Structure was nearly absent in populations with a single colonist, suggesting most recruits are half- and full-sibling progeny of the founder. Levels of relatedness were impacted by the number and relatedness of founding individuals and adjacent seed sources. Patterns of spatial genetic structure were used to infer recruitment history in these populations, and indicated the importance of colonization for structuring genetic diversity within populations in this species. We established a common garden experiment in a northern and a southern location in which we planted individuals from 14 seed sources encompassing most of the latitudinal span of A. julibrissin’s naturalized range. Populations were differentiated for early growth traits (height, diameter) but exhibited a clustered pattern rather than a smooth, clinal relationship with latitude. Furthermore, populations exhibited differentiation for vegetative phenology traits. These results suggest that populations of A. julibrissin appear to be in the process of evolving adaptive trait variation in response to local climatic conditions within the naturalized range. Frequent colonization plays a major role in structuring diversity in this species and likely contributes to observed patterns of geographic differentiation.