Reduced Establishment of Canada Thistle (Cirsium arvense) Using Functionally Similar Native Forbs

Canada thistle (Cirsium arvense) is an invasive species which by 1901 was already listed as a noxious weed in 21 states (Hodgson 1968). Currently, Canada thistle is the most frequently listed noxious weed in the U.S. and is listed in 41 states (NRCS 2013). Because Canada thistle is listed as a noxious weed, managers of conservation lands are legally obligated to control thistle which is often costly, requires persistent effort, and can affect non-target species in grasslands (Lym and Duncan 2005). Therefore, preventing the establishment of Canada thistle is needed for better sustainable management. In addition, the presence of Canada thistle in grasslands reduces their value; therefore preventing thistle establishment will increase the value of grassland restorations.

Methods to reduce establishment of Canada thistle are needed at the time of grassland restoration and these methods should reduce success at the seedling establishment phase, particularly recruitment from the existing soil seed bank, because there are proven methods to eliminate adults before restoration (Rowe 2010). Funk and others (2008) have proposed that the reduction of invasive species in restorations can be accomplished by introducing native species with similar resource use traits (functional traits) as the invasive species, along with including a diversity of traits. Using native species with similar functional traits can competitively exclude invasive species because the native species are using the same limited resources as the invasive species. Using several native species in restoration that have similar functional traits to Canada thistle may fill the niches that the thistle could occupy, thus reducing establishment from the soil seed bank or dispersal of seeds into newly restored sites.

Table 1.

Seed mix and amount seeded in the small plots at the Ekre Grassland Preserve and Central Grassland Research Extension Research Center (CGREC), North Dakota.

We investigated a method to reduce recruitment of Canada thistle from existing seed banks based on introducing functionally similar native forbs planted at a high seed density. We selected native species based on a diversity of functional traits in order to increase the probability that competitive exclusion would occur between the functionally similar native forbs and Canada thistle, reducing the establishment of Canada thistle (Biondini 2007). The functionally similar native forbs were added (spiked) to a native seed mix typically used for restoration.

The study was done at two scales: small plot (16 m²) and large plot (>10,000 m²). The experiments at both scales used a paired plot design with one treatment (control) being a native seed mix typically used in the area (non-spike), and the other treatment being the same native seed mix spiked with seeds of 3 or 4 native forbs at a high seed density (spike). The small plot design consisted of six 4m × 4m paired plots at two locations, Ekre Grassland Preserve (Richland County, North Dakota) and the Central Grassland Research Extension Center (CGREC) (Stutsman County, North Dakota), where the seed mixes were hand broadcasted in the fall of 2010. Prior to seeding, the small plots were in an annual crop rotation and were tilled several times right before planting. The native seed mix used in the small plots consisted of 4 or 5 native cool and warm season grasses with 3 native forbs (Table 1). The spiked species with similar functional traits to Canada thistle were common yarrow (Achillea millefolium), black-eyed Susan (Rudbeckia hirta), and prairie coneflower (Ratibida columnifera). Another species, Lewis’ flax (Linum lewisii), with different functional trait characteristics was also used as a spike species. The functional trait characteristics were derived from Biondini (2007) who defined nine quantitative traits related to structure (such as root biomass, root architecture, leaf biomass, shoot biomass, and plant height) and physiology (such as nutrient use efficiency, growth rate, and nutrient uptake rates) for 55 grassland species (Table 2). Only native forbs were used as spike species as native grasses planted at high densities could dominate the community for years, reducing native plant diversity (McCain et al. 2010). The native seed mix was planted at...