Print PagePrevalence of sexual and asexual reproduction by plants influences genetic and spatial structure of populations. We investigated reproduction in five populations of Dirca palustris that span the distribution of the species from Florida to North Dakota, and identified the contributions of sexual and asexual reproduction to the clustering of individuals characteristic of the species. In each population, maximum density of plants was documented by assessing the 10 clusters that appeared most dense. We also surveyed these clusters for evidence of recent recruitment, sowed locally collected fruits to estimate potential for germination, and estimated the prevalence of clonality by using ISSR genetic markers. Moreover, we used ISSR markers with broader population-wide sampling to assess genetic structure among the five populations.
Results/Conclusions
Maximum density of plants varied considerably among populations, and denser clusters also had evidence of greater recruitment over several years. Germination of seeds one year after locally collected fruits were sown varied from 7 to 56 % among populations. Our analysis of ISSR data, and our observation of cotelydonary scars on young plants, indicate restricted seed dispersal rather than clonal reproduction explains the occurrence of D. palustris in clumps. Plant density and recruitment were greatest in North Dakota, which raises questions about what governs the distribution of D. palustris. Are environmental constraints on survival or reproduction, or simply constraints on the capacity of D. palustris to disperse into new habitats during northward postglacial colonization, responsible for the present northern and western distributional limit? The population in Florida, where the species is listed as endangered at the southern limit of its range, had the least evidence for recent recruitment, the least germination, and the greatest extent of granivory. In our genetic analysis, 54% percent of the total genetic variation was among populations, and a Mantel test of genetic data showed evidence for historically limited gene flow (isolation-by-distance). Moreover, the population in Florida was the most genetically distinct because of its numerous loci not found in other populations. D. palustris is especially rare in the South, and our data suggest populations in that region are genetically unique and characterized by limited recruitment. Therefore, we conclude that populations near the southern limit of the species should be prioritized by conservationists.
