Population density as a predictor of genetic variation for woody plant species

As the focus of conservation biology shifts toward multispecies and ecosystem conservation and management a principal question becomes how we manage species to conserve their long-term evolutionary potential. Few criteria exist for prioritizing which populations within a species should be protected to conserve maximal genetic variation. We designed this study to explore the genetic consequences of using population density as a criterion for selecting populations of woody plant species for conservation. Population density may be an effective gauge of genetic variation for two reasons. First density often reflects ecological population size, particularly for continuously distributed species, and density is much easier to measure in the field than population size. Second, from an individual species' perspective, population density may be an indicator of habitat quality. We evaluated the relationship between standard genetic diversity indices and densities of seedlings, small trees, and large trees, and we investigated the association between genotypic composition and density measures with canonical correlation analysis for three common tree species (Carya tomentosa, Sassafras albidum, and Quercus alba) from the Missouri Ozarks. We found that population density was not correlated with genetic diversity in large populations of plant species, brit density was associated with genotypic composition of populations. That is, populations with small densities had different genotypes than those with large densities To sample a maximal amount of regional genotypic variation, we recommend choosing plant populations representing a range of densities. Findings from our study should be generally applicable to Plant populations that have occupied habitats long enough for natural selection to affect focal genotypic composition. Used in conjunction with other established criteria, population density may be a useful rule of thumb for conservation practitioners concerned with the maintenance of adaptive genetic variation in plant species.