Native Grasses Collected from Invasions Demonstrate Invasion Resistance

Native grasses can persist in areas dominated by invasive species, yet resistance to invasion by the selected remaining natives is largely unknown. We examine native grass lineages differing in history of survival within invasions, when transplanted into association with 2 invasive perennials. Invaded plants were collected from Russian knapweed (Rhaponticum repens) invasions, and non-invaded plants were collected outside invasions. We assess growth and reproduction of alkali sacaton (Sporobolus airoides) in the presence of Russian knapweed (the original invader) and Canada thistle (Cirsium arvense), an invasive species to which the grasses lacked exposure. On 2 field sites (Crowheart, WY and Cheyenne, WY) and in an environmentally controlled lab at the University of Wyoming, we monitored plant growth (longest leaf, basal circumference, and tiller production) of grass lineages from the 2 histories with and without the presence of the invaders. We monitored invasive species recruitment, grass seedling germination, and grass seedling establishment to determine grass fecundity and resistance to invasive encroachment. At Crowheart, when competing with Russian knapweed, invaded plants were larger than non-invaded plants, irrespective of sampling date. At Cheyenne, invaded plants competing with Canada thistle grew larger than non-invaded grasses irrespective of sampling date. In the controlled growth chamber, growth of the 2 histories differed on few dates, yet non-invaded lineages had greater leaf growth, and seedling germination was greater for invaded grasses. Lines of native grasses collected from within historic invasions display competitive growth traits in field transplants that may be useful for reclamation.