Turning the tide on the sagebrush sea: long-term invasive annual grass control and rangeland restoration with indaziflam
Date
2022
Authors
Courkamp, Jacob S., author
Meiman, Paul, advisor
Paschke, Mark, advisor
Dayan, Franck, committee member
Ocheltree, Troy, committee member
Journal Title
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Volume Title
Abstract
The invasive winter annual grass downy brome (Bromus tectorum L.) has invaded vast expanses of sagebrush-grassland in western North America, and the fine fuel associated with invasion increases the frequency of burning such that native plants struggle to persist. Recent research suggests that B. tectorum invasion may expand across an even larger portion of the US Intermountain West in the absence of effective and proactive management. The herbicide imazapic is widely used to manage B. tectorum, but control often declines after one year and reinvasion is typical. Several trials have demonstrated that the newer herbicide, indaziflam, can selectively control annual grasses for three or more years, and past studies indicate that B. tectorum seed banks are relatively short-lived in the field (<5 years). This suggests that consecutive years of control with indaziflam may eliminate B. tectorum seed banks and increase the duration of control, but it is unclear if this will require multiple applications. In addition, existing studies evaluating the effects of treatment on native rangeland plant communities are limited by small plot sizes, and the potential for impacts to native species seed banks is unclear. The studies detailed in Chapter 1 evaluated the effectiveness of imazapic and indaziflam for reducing B. tectorum density and cover over a period of approximately 5 years (57 months after treatment; MAT) at two invaded sagebrush-grassland sites near Pinedale, Wyoming. Treatments included three different indaziflam rates (51, 73, and 102 g ai ha-1) and imazapic (123 g ai ha-1) and were reapplied to half of each plot 45 MAT to evaluate the effects of two sequential applications. Perennial grass cover was also measured because positive perennial grass responses were observed after release from B. tectorum competition in other studies, and perennial grasses may provide resistance to B. tectorum reinvasion. Intermediate and high indaziflam rates (73 and 102 g ai ha-1) resulted in significant reductions in B. tectorum cover and density 45 MAT, and perennial grass cover responded positively to some treatments early in the study. Imazapic reduced B. tectorum initially, but did not have a significant effect on density or cover at either site beyond 21 MAT. Reapplication did not substantially improve B. tectorum control 57 MAT in plots treated with intermediate and high indaziflam rates, suggesting that long-term control with a single indaziflam treatment may be possible in some cases. The studies detailed in Chapter 2 assessed the potential for non-target impacts resulting from indaziflam treatment. Modified-Whittaker multiscale vegetation plots were used to compare diversity (species accumulation) in three treatment (73 g ai ha-1 indaziflam) and three control plots in a sagebrush-grassland plant community near Pinedale, Wyoming that is invaded by B. tectorum. In addition, a seed bank assay assessed the density and richness of shallow (0-1 cm depth) and deep (1-5 cm depth) germinable seed banks in these same treatment and control plots during a 20-week greenhouse study. Vegetation data and seed bank samples were collected during the third growing season after treatment. Species diversity did not differ between treatment and control plots, but this contrasted with the results of the seed bank assay, which showed that the shallow and deep seed banks had significantly fewer germinable seeds and native species richness was significantly lower in the shallow seed bank of treated areas. While significant non-target impacts to native annual seed banks were observed, all impacted species were detected in the aboveground plant community in treatment plots after treatment occurred, suggesting that reduced native annual abundance may be temporary. The results presented herein suggest that long-term B. tectorum control is possible with a single indaziflam application, and that when indaziflam treatment without associated revegetation is a suitable management intervention (i.e., invaded perennial communities), the benefits of protecting and promoting established perennial plants likely far outweigh the potential for non-target impacts to native species and native species seed banks. The ultimate goal of any weed management program is to reduce the impacts of invasive species to the greatest degree possible with the resources available, whether that is through eradication or conciliation and containment. Our results suggest that in the case of B. tectorum invading established sagebrush-grassland plant communities, indaziflam will have a significant role to play in helping managers achieve this objective.
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Subject
downy brome
rangeland restoration
herbicide
cheatgrass