Native ruderal species and cheatgrass (Bromus tectorum): Interactions in revegetation

Background/Question/Methods

Cheatgrass (Bromus tectorum) is one of the most widespread invasive weeds in North America, particularly in post-disturbance settings where it often dominates the landscape and prevents the establishment of native species.  Native ruderal species share many characteristics with cheatgrass including high seed production, rapid growth and high resource use and should thus fill a similar niche.  However, native ruderal species are rarely included in restoration seed mixes and seeding rates are often low relative to cheatgrass seed production.  The objective of this study was to determine if a high seeding rate of native ruderal species could be used to suppress cheatgrass establishment in a disturbed site.  We predicted that native ruderal species would suppress cheatgrass establishment and be superior in suppressing cheatgrass establishment to QuickGuard, a sterile cover crop commonly used in revegetation.  We tested this prediction in a field study in northern Colorado where we planted replicated test plots with combinations of cheatgrass, QuickGuard and native ruderal species.  High rates of native ruderal seeds were broadcast and tilled into the soil to mimic a native ruderal seed bank.  QuickGuard and cheatgrass were then broadcast and raked to increase seed: soil contact.  All plots were firmed with a seed roller.

Results/Conclusions

Plots seeded with cheatgrass and native ruderal species showed significant reductions in cheatgrass biomass (p<0.001) and density (p=0.0106).  QuickGuard seeding also resulted in significant reductions in cheatgrass biomass (p<0.0063) and density (p=0.0146), but to a lesser degree than the native ruderal mix.  Cheatgrass did not suppress the biomass of native ruderal species (p=0.2389), but did significantly decrease the biomass of QuickGuard (p=0.0042).  These results suggest that native ruderal plants may be useful in limiting cheatgrass invasion and dominance in restoration settings.  The reproductive potential of native ruderals and lack of suppression by cheatgrass may also provide a longer-term source of cheatgrass suppression in restoration sites.   While QuickGuard does seem to limit the growth of cheatgrass, it seems that cheatgrass also suppresses the establishment of QuickGuard.  QuickGuard does not provide a potential long term buffer against invasion because it is sterile.  If these results continue to be supported in subsequent years, it would suggest that including a high seeding rate of native ruderal species in restoration seed mixes could be a viable approach for reducing cheatgrass dominance in disturbed sites.