Northern Prairie Wildlife Research Center
|Figure 3. Provisional model for managing smooth brome with prescribed burns where Y means "yes" and N means "no."|
We suggest that the most important question managers need to ask is whether or not native warm-season tall grasses are present (Fig. 3, Question 1). The importance of this was made clear by the results of fire treatments at Mead and Pipestone. At Mead, burning smooth brome during tiller elongation reduced secondary tillers by a maximum of 58 percent to a density significantly different from the control, while at Pipestone, the maximum reduction was about 37 percent, which resulted in a density not significantly different from the control (Fig. 4). Although we timed the Pipestone burn treatment properly to kill smooth brome tillers and minimize secondary tiller production, secondary tiller were not subjected to competitive stress from warm-season tall grasses. At Mead, we did not determine the minimum amount of big bluestem needed to suppress smooth brome, but, in another experiment, Dill and his colleagues (1986) found that a single spring application of atrazine restored a pasture dominated by smooth brome when big bluestem and Indiangrass comprised only 20 percent of the pre-treatment vegetation. From the results at Mead where big bluestem was estimated at 50 percent cover, and from those of Dill and his colleagues, it appears that warm-season tall grasses must comprise more than 20 percent of the species mix of a degraded site before a prescribed burn will effectively reduce smooth brome. If native warm-season tall grasses are absent, or nearly so, as in the situation at Pipestone, we believe that fire alone will not restore a degraded tallgrass site that is dominated by smooth brome.
|Figure 4. Mean smooth brome tiller density (number of tillers per 0.1 m²) in control plots and in treatment plots that were burned at the time of tiller elongation. The experiments took place in 1989-1990 at Mead, Nebraska (MEAD), and in 1990-1991 at Pipestone National Monument (PIPE) in Minnesota. Means with the same letter within years are not significantly different (P>0.05, LSD).|
At severely degraded sites where there are not enough tall, native grasses, managers should consider other methods to control smooth brome, such as herbicide applications followed by sod-seeding warm-season tall grasses. In the Pipestone study, atrazine was the only treatment that we found effective in reducing smooth brome (Willson and Stubbendieck, 1996). However, like all herbicides, atrazine should be used carefully because it may negatively affect other vegetation, such as native forbs (Gillen and others, 1987; Bragg and Sutherland, 1989).
When warm-season tall grasses are present in sufficient numbers, then we suggest that managers consider prescribed burns that are timed to affect the smooth brome when it is most vulnerable (Fig. 3, Question 2). At the Mead site, we learned that burning during tiller elongation was most effective in reducing smooth brome tiller density and biomass, while simultaneously providing the greatest stimulus to big bluestem (Willson and Stubbendieck, 1997). Managers can determine when smooth brome tillers are elongating by examining a tiller for an aboveground node or, alternatively, by counting the number of green leaves per tiller (Willson 1991). Generally, burning should be done when most elongating tillers have five or more green leaves.
If burning can be repeated on an annual basis, managers may have an additional option to control smooth brome (Fig. 3, Question 3). At Mead, we learned that burning during tiller elongation in consecutive years kept tiller density and biomass at reduced levels, whereas not burning consecutively allowed tiller density and biomass to fully or partially recover. We also found that consecutive annual burns before tiller elongation at Mead resulted in a decline in smooth brome biomass in 1988-90, but not in 1989-91. We believe these results suggest that annual burns conducted too early to kill tillers may have some negative effect on smooth brome tiller biomass and its ability to persist with native tall grasses. Similarly, Becker (1989) found that estimates of smooth brome foliar cover decreased 10 percent after five consecutive burns that were conducted before smooth brome tillers became elongated. In addition, after ten years of annual late-April burns, Bragg and Stubbendieck (pers. comm.) found that plots originally co-dominated by big bluestem and smooth brome became nearly pure stands of big bluestem. However, managers should be aware that annual spring burning can reduce species diversity by eliminating or preventing the establishment of cool-season forbs (Collins and Gibson, 1990; Knapp and others, 1998). They should also recognize that a single burn before tiller elongation that removes most of the litter may increase the growth of smooth brome as a result of increased light reaching growing leaves. For example, in 1991, we observed that the annual growth of smooth brome more than doubled the year after a tiller emergence burn under normal precipitation at the Mead study site. In addition, results from the Mead study indicate that early spring burning of smooth brome-dominated sites when precipitation is below normal can lead to less soil moisture and lower soil temperaturesfactors that can limit the expansion of big bluestem (Willson and Stubbendieck, 1995).
Finally, if tiller inflorescences are visible (Fig. 3, Question 4), we recommend that managers delay burning smooth brome to a subsequent year. The study at Mead showed significant reductions in the density and biomass of smooth brome secondary tillers following burns conducted during the heading and flowering stages. However the reduction of biomass was not as great as those after burns conducted at tiller elongation. In addition, the study at the Mead site showed that smooth brome tiller density and biomass did not recover to preburn levels the year after a burn during tiller elongation, but did the year after a burn during heading.