Regional Genetic Differences in Forest Herbaceous Species

The appropriate collection zone for seeds and transplants of native shrubs, grasses, and forbs is a key concern for plant restoration ecology, and has been the subject of a number of extensive reviews (Hufford and Mazer 2003, McKay et al. 2005). This issue was also the topic of a 2014 workshop that included academics, practioners, and nursery growers (Herman et al. 2014). “How local is local?” is a succinct way to summarize the issue of uncertainly about the size of collection zones (McKay et al. 2005).

If natural selection produces ecotypic variants that are better suited to the local environment compared to nonlocal genotypes, this is often termed home-site advantage. As a result, there are multiple concerns about the potential failure of non-local genotypes in restoration, including outbreeding depression, founder effects, and introduction of maladapted genotypes (Handel et al. 1994, Hufford and Mazer 2003, McKay et al. 2005). At the opposite end of the spectrum, it is also possible that highly successful non-local genotypes could swamp local populations numerically or in fitness advantage, resulting in species-wide loss of genetic diversity (Hufford and Mazer 2003).

In contrast, a high degree of gene flow, which connects populations genetically through the movement of pollen or seeds, can hinder among population genetic differentiation (Loveless and Hamrick 1984). Breeding system, floral morphology, pollinator system, seed dispersal, and life cycle are related to degree of gene flow and therefore to among population genetic structure (reviewed by Loveless and Hamrick 1984).

Despite the questions outlined above, we lack information about among population genetic variation for most native shrub, grass and forb species that would be candidates for restoration (McKay et al. 2005, Rice and Emery 2003, Johnson et al. 2004). This is especially the case for forest herbaceous species, which historically have been less widely used in restoration by conservation professionals, and less known about and supplied by nursery professionals (Altrichter et al. 2017).

Table 1.

Families and 10 species included in a study of regional genetic differentiation in Midwest, USA, forest herbaceous species.

My goals in this study were to: 1) determine whether there was genetically based regional variation in traits for forest herbaceous species that would be candidates for forest ground layer restoration, and 2) assess whether any among population genetic variation observed could be related to whether species were primarily self pollinated or outcrossed. My goal was not to delineate collection zones per se, but to evaluate a very practical issue related to restoring woodland groundlayer species: most species of interest are not available for purchase locally, and practioners must look to regional nurseries to supply plant material (Altrichter et al. 2017). Thus, knowledge of regional genetic differences are needed to help guide practical decisions about where to purchase plants.

To test for among population genetic variation at the regional level, I compared populations of forest herbaceous species between central Iowa and northeast Iowa/southeast Minnesota, approximately 320–400 kilometers apart. I chose to investigate at the regional level because this is the appropriate scale for common, widespread species (Montalvo and Ellstrand 2001), and because in this region there are few nurseries that supply shade tolerant species, making it necessary to obtain plant material regionally rather than locally (Altrichter et al. 2017).

Specifically, I chose 10 species across six families that are offered by native plant nurseries in the Midwest region, USA (Table 1). These species, while not rare, are likely to be included in woodland and savanna restoration due to their beauty, resilience, and nutrient uptake capacity (Mottl et al. 2006, Mabry et al. 2008, Gerken Golay et al. 2013). Species were also selected to represent a range mating systems (Table 1), which may influence gene flow, and thus influence the potential for among population genetic differences (Loveless and Hamrick 1984). Nomenclature follows USDA Plants Database (USDA, NRCS 2016).

I obtained non-local seeds of each species from Prairie Moon Nursery, Winona, MN, with seed source originally from Filmore, Winona, Brown, Pine, and Houston Counties in Minnesota; Clayton and Allamakee Counties in Iowa, and Rock and Pierce Counties...