Germination Characteristics of a Native Non-Indigenous Prairie Forb in Prairie Plantings

A native, non-indigenous species is a species existing as an introduced population outside of its historical range, often within the same ecosystem where it is native (Simberloff et al. 2013). A native non-indigenous species present in recent tallgrass prairie plantings is Ratibida pinnata (pinnate prairie coneflower). Whether through intentional or unintentional human action this species has become established in native species plantings in North Dakota, far north of its traditional range. Historical records show no R. pinnata recorded in North Dakota with the northern edge of the traditional range going from far, eastern South Dakota, east through southern Minnesota and Wisconsin, south to Michigan and Ohio, then west to eastern Kansas (USDA, NRCS 2017).

When an introduced population is included in restoration activities, there are several ways that the native, non-indigenous species can influence the restoration (Clewell and Aronson 2013). The long-term influence of a native non-indigenous species on a restoration depends on the species’ ability to sustain itself outside of its normal range. One aspect to sustainability is the ability to reproduce and expand its population. If a species cannot reproduce even if it was able to establish itself during the restoration process, then its long-term effect is limited. As found by Gibson et al. (2016), information on reproduction and establishment when moving species outside of its normal range is lacking for most native species. Currently there is no information if R. pinnata can reproduce or expend as an introduced population far north of its traditional range, so we chose to fill this knowledge gap. In addition, R. pinnata is an aggressive species in its native range if it has adequate resources (Knee and Thomas 2002). This aggressive nature could be important in how it interacts in tallgrass prairie plantings outside of its normal range. Our objectives were to: 1) perform a greenhouse study aimed towards learning the germination rate of R. pinnata grown outside of its traditional range, establishing if R. pinnata is able to produce viable seed and at what level; and 2) perform a field study where R. pinnata seed from an existing prairie plantings in North Dakota is seeded into the same prairie planting where no R. pinnata was present to establish if the species can expand.

We collected R. pinnata seeds from three restoration sites in North Dakota: North Dakota State University (NDSU) (46°54′15.48″ N, 96°48′33.48″ W); Woodworth Waterfowl Production Area (WPA) (47°7′37.2″ N, 99°14′24″ W); and Strong Waterfowl Production Area (47°5′2.4″ N, 99°17′38.4″ W). The NDSU site is located the farthest east, near the Minnesota border while Woodworth WPA and Strong WPA are located closer together just northwest of Cleveland, ND. The prairie plantings at the NDSU site were seeded in 2008 with seed additions in 2010 and 2013. The seed additions consisted of 5–15 native forbs intended to increase forbs in the plantings. Ratibida pinnata was unintentionally part of the 2010 seed addition which involved only a portion of the site leaving the rest of the site without the species. The prairie plantings at the Woodworth and Strong WPA were seeded in 2008 with R. pinnata being part of the original seed mix. Seed heads of R. pinnata from all three sites were collected in September of 2015. Seed heads from the NDSU site were used both in the greenhouse and the field study; seed heads collected from Strong WPA and Woodworth WPA were only used in the greenhouse study.

The seed heads collected from the sites were air dried in a box with heat (32°C) and forced air. After drying, we processed and cleaned the seed heads so only seed was present. There was no effort to separate out non-viable seed or estimate the percentage of non-viable seed. The clean seed was added to a stratifying mix of seed and moist sand and placed into a cooler for 30 days (3°C). The number of seeds/gram in the stratifying mix...