Genetic Differentiation and Phenotypic Plasticity of Forest Herbaceous Species in Iowa, Central United States

The forest herbaceous layer provides important ecosystem services in the central United States. However, human impacts have caused declines of many of these species. Restoration of this layer is uncommon in temperate forests, so best practices are not yet established. There has been widespread concern about negative outcomes (for example, failure due to genetic swamping or outbreeding depression) when plant material is transferred beyond a local scale. Current practice is to use local sources under the assumption that they are optimal genotypes for the site. However, few local sources are available for many species. We examined genetic variability and phenotypic plasticity by comparing performance of local and non-local populations (from sites approximately 250 km apart) of six forest herbaceous species. We used a common garden study to test for genetic differences in plant traits, and a field study to test for phenotypic plasticity. Based on the common garden we found genetic differences between local and non-local populations for each species. Trait differences we observed in greenhouse trials we also detected in the field in the first year. However, these differences diminished in the second year of the field study and we did not detect them in the four species measured in the third year. This provided evidence that phenotypic plasticity was operating, as plant characters responded plastically to local conditions. We found no evidence that local plants consistently outperformed non-local plants. These results suggest less need for strict adherence to locally sourced seeds or transplants.