ARS | Publication request: Returning succession to downy brome dominated rangelands: roadblocks to perennial grass establishment

Submitted to: Western Society of Weed Science
Publication Type: Abstract Only
Publication Acceptance Date: November 9, 2011
Publication Date: March 15, 2012
Citation: Harmon, D.N., Clements, C.D., Young, J.A. 2012. Returning succession to downy brome dominated rangelands: roadblocks to perennial grass establishment [abstract]. Western Society of Weed Science. 64:44.

Technical Abstract: The most common cause of successional retrogression in the Great Basin is wildfires fueled by downy brome (Bromus tectorum). Downy brome invasion has reduced fire intervals from an estimated 60-100 years down to 5-10 years. Our previous research found that establishment of long-lived perennial grasses is the best known method to suppress downy brome and reduce wildfire disturbance in order to assist succession. Shrubs provide downy brome understory safe sites and therefore perennial grass establishment and downy brome suppression must occur first. At the seedling stage, perennial grasses only compete to survive. Perennial grass seedlings do not out compete downy brome, established mature perennial grass compete for resources an adequate amount to suppress downy brome. Reports of successful perennial grass establishment fall below 20 percent. We hypothesize that multiple factors are affecting the high failure rate such as downy brome resource competition and the species of perennial grass seeded. In a plot level seeding experiment we tested three treatments, 1) downy brome removal with Glysophate (Roundup®) at 5% rate), 2) seeded species (native vs. introduced mix) and 3) seeding depth (2cm vs. 6cm). We measured the number of seedlings/m2 as the response variable. Seedlings were counted in May and July 2011. Our results found a significant effect (P < 0.05) of downy brome removal on seedling establishment (removal=19.5 seedlings/m2, no-removal=0 seedlings/m2). Complete die-off occurred by July if downy brome was not controlled. Most alarmingly, even with complete downy brome removal, the native grass seed mix established very poorly. Seedling establishment of introduced species, 34.6/m2, was significantly (P < 0.05) greater than native seed mix, 4.3/m2. Contrary to most seeding depth recommendations, we experienced higher seedlings survival with increased seeding depth, 34.6/m2 versus 4.3/m2, respectfully. Our results find that effective downy brome control is paramount to establishing perennial grasses in an effort to decrease downy brome densities and fuel loads. Appropriate species selection and proper seed placement are also critical needs to increase success.