Prairie dropseed is a North American native warm-season bunchgrass. It occupies a large geographic range across the United States, occurring from Massachusetts to Colorado and from New Mexico north into Canada [U.S. Department of Agriculture (USDA), 2022] but is often locally scarce (Ladd, 1995). It is commonly found in well-drained soil and is an indicator of mesic prairies but can be found on drier sites such as hill prairies and rocky prairie slopes (Ladd, 1995). Prairie dropseed is listed as endangered in five eastern states—Connecticut, Kentucky, Maryland, North Carolina, and Pennsylvania—and is listed as threatened in two more, New York and Ohio (USDA, 2022). It is not a protected plant species in Minnesota.
Although prairie dropseed is not protected in Minnesota, it is an ecologically important species. Prairie dropseed is a critical source of food and shelter for at least five species of native Lepidoptera including the federally endangered Poweshiek skipperling, Oarisma poweshiek, and the federally threatened Dakota skipper, Hesperia dacotae (Narem and Meyer, 2017). Prairie dropseed is not only a preferred host plant for Dakota skipper (Dana, 1991), but Nordmeyer et al. (2021) found that Dakota skipper larvae on prairie dropseed plants had the highest survival rates, highest pupation weight, and shortest time to pupation compared with other native grasses. Prairie dropseed is thought to be an optimal forage for native Lepidoptera (Dana, 1991).
Despite its ecological value, prairie dropseed is not commonly included in seed mixes used for prairie restoration (Fedewa and Stewart, 2011). It is sensitive to competition as a seedling and is slow to establish, taking 4 to 5 years to reach maturity (Fedewa and Stewart, 2009). Native plant nursery and restoration specialists confirmed that prairie dropseed is not usually incorporated into restoration projects (B. Carter, personal communication, 26 Aug. 2019; G. Kastey, personal communication, 28 Aug. 2019). Instead, practitioners recommend using plugs of prairie dropseed in restored sites (B. Carter, personal communication, 26 Aug. 2019).
Using plugs in place of direct seeding was determined to be a viable way to establish species that are rare, with limited dispersal or that are difficult to establish from seed (Hedberg and Kotowski, 2010). Plug survival ranges widely, from 23% to 83%, depending on species, planting season, and competition (Page and Bork, 2005). Competition and planting density were found to influence survival, biomass, and flowering of plugs in restorations (Huddleston and Young, 2004). Although plugs are accepted for restoration use, peer-reviewed research has primarily focused on forest restoration with limited research on their application in grasslands or savannas (Palma and Laurance, 2015). This may be due to the increased cost of plugs compared with direct seeding at restoration sites (Palma and Laurance, 2015). Specific research with plugs to establish prairie dropseed could not be located.
Treating plugs with hydrogels immediately before planting may be one method to increase rates of plug survival. Hydrogels are hydrophilic gels composed of crosslinked polymers that can absorb 400 to 1500 times their dry weight (Landis and Haase, 2012). Hydrogels may function like natural polymeric mucilages produced by healthy plant roots and can protect seedlings during planting from sun and wind (Landis and Haase, 2012). For horticultural applications, synthetic hydrogels are most frequently used, but other hydrogels on the market are semisynthetic or starch based (Landis and Haase, 2012). While commonly used for a variety of applications in horticulture, peer-reviewed research on hydrogel use for restoration is limited. Thomas (2008) found that hydrogels reduced mortality and appeared to keep seedlings healthier by increasing leaf retention and accelerating root extension of two tree species commonly used in Australian forestry. Studies with the North American native grass black grama [Bouteloua eriopoda (Torr.) Torr.] found hydrogels did not affect root mass but did increase leaf mass and total leaf area of seedlings planted in a restoration (Lucero et al., 2010). However, both Thomas (2008) and Lucero et al. (2010) studied the effect of hydrogels in an arid environment. We conducted research in Minnesota, which is traditionally semiarid. We could find no research on hydrogels and prairie dropseed survival and growth in restorations.
Similar to using a hydrogel treatment before planting, inoculating plants with beneficial fungi is also a common method of increasing seedling success. Prairie dropseed is a known fungal affiliate and most commonly associates with Glomus species, specifically Glomus fasciculatum (Dhillion, 1992). Spore levels around prairie dropseed range from 7.7% to 48.6% and fluctuate seasonally (Ebbers et al., 1987). Colonization of prairie dropseed roots and spore abundance were found to be inversely related to calcium and magnesium availability (Ebbers et al., 1987).
Restoring the fungal microbiome with vegetation at a site may have major implications for the success of a restoration planting. Koziol et al. (2018) found that plugs treated with native arbuscular mycorrhizal (AM) fungi were 40% more likely to survive the first year and three times more likely to survive over 3 years compared with those left untreated. Plots treated with AM fungi had increased germination and species diversity compared with plots which were not inoculated (Koziol et al., 2018). The fungal species used are critical because the fungal strains found in commercial mixes are usually nonnative strains, and there is a risk in introducing nonnative fungi to a restoration site as their effects on native fungi are unknown (Emam, 2016).
The objective of this research was to determine whether growing plugs in native field soil or treating plugs with hydrogel would increase the survival and growth rates of prairie dropseed planted in restored prairies. We hypothesized that plugs treated with hydrogel as well as plugs grown using native field soil would have higher survival and growth rates than those grown without hydrogel or by using a commercial potting mix, respectively.
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