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Single burns of smooth brome at the tiller emergence stage in 1989 and 1990 did not change tiller density or biomass as compared to the unburned controls (Figure 1). Results of this study agree with those of Garrett (1992), who found that single, late winter (February) burns in Kansas just before smooth brome tiller emergence had no effect on herbage yield when compared to no burning. In addition, these results are similar to those of Probasco and Bjugstad (1977), who determined that burning tall fescue (Festuca arundinacea Schreb.), another cool-season perennial grass, just before active growth in late February did not significantly affect herbage yield compared to no burning.

Figure 1. Mean smooth brome tiller density (number m^-2) and biomass (g m^-2) in the fall following single burns at different growth stages (CO = unburned control, TE = tiller emergence, TL = tiller elongation, HE = tiller heading, and FL = tiller flowering) in 1989 (area #30) and 1990 (area #13) at Mead, Nebraska. Means are adjusted by the 1988 or 1989 pretreatment covariates. Means with the same letter within years are not significantly different (P > 0.05, LSD).

Other studies (Becker 1989, Rosburg and Glenn-Lewin 1992) noted that a reduction in litter by burning in March and April increased the cover of quackgrass (Agropyron repens Beauv.) and biomass of tall fescue and Canada bluegrass (Poa compressa L.). Furthermore, prairie managers (R. Baynes, superintendent, Homestead National Monument of America, pers. com.; V. Halvorson, superintendent, Pipestone National Monument, pers. com.) have observed more vigorous growth of smooth brome following late March and early April removal of heavy litter. Although litter reduction was not measured in this study, visual inspection of the burned plots indicated that most of the litter was consumed by the fire. Lack of an increase in smooth brome tiller density and/or biomass may be due to lower than normal precipitation in the first 6 months of 1989, with March and April precipitation being particularly low (15 mm and 25 mm, respectively). Similarly, although the first 6 months precipitation in 1990 was near the 30-year average, only 85 mm of precipitation were recorded in April. In this study, burning smooth brome at tiller emergence (late March/early April) may have further reduced soil moisture. This assumption is supported by Anderson (1965), who found differences in soil moisture due to time of burning: early spring (March 20) burned areas were significantly drier than unburned areas. In central Wisconsin, the association between soil moisture and plant response was investigated by Zedler and Loucks (1969), who observed that soil moisture differences between depressions and ridges in a sandy prairie resulted in differences in postfire biomass production of Kentucky bluegrass (Poa pratensis L.) and little bluestem. These results suggest that, in a dry spring, the negative effects of reduced soil moisture may counterbalance the positive effects of litter reduction on smooth brome tiller development.

Tiller-Elongation Burns

Single burns of smooth brome at tiller elongation significantly reduced tiller density and biomass in the fall of 1989 and 1990 (Figure 1). These responses may have been due to removal of the apical meristem and loss of secondary tiller biomass similar to that reported in clipping and grazing studies (Teel 1956, Eastin et al. 1964, Paulsen and Smith 1969, Krause and Moser 1977). Burning smooth brome at tiller elongation may have reduced secondary tillering because of low carbohydrate reserves, low basal bud activity, and differential levels of growth regulating hormones at the time of tiller removal (Winch et al. 1970, Kunelius et al. 1974). Furthermore, smooth brome and other cool-season grasses burned at this time endure not only physiological stress from forced regrowth but also competitive stress in mixed stands from enhanced growth of warm-season grasses (Rosburg and Glenn-Lewin 1992, Blankespoor and Larson 1994). In a companion study at Mead, big bluestem flower culm density significantly increased following the burn at the smooth brome tiller elongation stage in 1989 and 1990 (Willson 1994). These results are consistent with other studies of big bluestem, which show the greatest enhancement in flowering occurs after late spring fire (Henderson et al. 1983, Benning and Bragg 1993). In big bluestem, flowering response is associated with increased biomass production and may be an indicator of competitive vigor (Henderson et al. 1983).

Tiller Heading and Tiller-Flowering Burns

Single burns of smooth brome at more developed growth stages negatively affected both tiller density and biomass. In 1989 and 1990, a burn treatment at tiller heading significantly decreased smooth brome tiller density and biomass compared to control plots. In 1990, burns at tiller flowering also produced significant decreases (Figure 1).

In 1989, burning at tiller heading was less damaging to smooth brome than burning at tiller elongation: biomass following the tiller-elongation burn was signficantly lower than biomass following the tiller-heading burn (Figure 1.). Kunelius et al. (1974) found similar results following cutting of smooth brome at eight developmental stages. In their study, dry matter yields from smooth brome regrowth after first cutting were higher at advanced stages of development (heading or later) than at less mature stages (elongation to early heading). The effect of burning smooth brome at the tiller-heading stage on secondary tillering should be less than that at tiller elongation because of higher carbohydrate reserves at heading. Why a similar response was not found in 1990 is not known. Conversely, both tiller-elongation and tiller-flowering burns produced similar reduced secondary tiller responses (Figure 1) because both of these smooth brome growth stages are low in carbohydrates.

Single and Repeated Burns

The 1989-90 and 1990-91 tiller-emergence burn/unburned treatments produced no significant differences in smooth brome tiller density among years (Figure 2). The effect of the 1989-90 tiller-emergence burn/unburned treatment on smooth brome biomass production was similar to that on tiller density, but biomass production more than doubled in 1991 under the 1990-91 burn/unburned treatment (Figure 3.). Reduced litter, adequate precipitation, and the absence of burning may have allowed this increase in biomass production that year.

Figure 2. Mean smooth brome tiller density (number m^-2) prior to burning (fall 1988 and 1989) and following single and repeated burns at different growth stages (CO = unburned control, TEBU = tiller emergence burn/unburned, TEBB = tiller emergence burn/burn, TLBU = tiller elongation burn/unburned, TLBB = tiller elongation burn/burn, HEBU = tiller heading burn/unburned, HEBB = tiller heading burn/burn, FLBU = tiller flowering burn/unburned, and FLBB = tiller flowering burn/burn) in areas #30 and #13 at Mead Nebraska. Means with the same letter within treatments are not significantly different (P > 0.05, LSD).

Figure 3. Mean smooth brome tiller biomass (number m^-2) prior to burning (fall 1988 and 1989) and following single and repeated burns at different growth stages (CO = unburned control, TEBU = tiller emergence burn/unburned, TEBB = tiller emergence burn/burn, TLBU = tiller elongation burn/unburned, TLBB = tiller elongation burn/burn, HEBU = tiller heading burn/unburned, HEBB = tiller heading burn/burn, FLBU = tiller flowering burn/unburned, FLBB = tiller flowering burn/burn) in areas #30 and #13 at Mead, Nebraska. Means with the same letter within treatments are not significantly different (P > 0.05, LSD).

Repeated burns of smooth brome at tiller emergence produced significantly lower biomass in the 1989-90 burn/burn treatment but not in the 1990-91 burn/burn treatment (Figure 3). Although single burns at this morphological stage do not adversely affect smooth brome (Figure 1 ), repeated burns, even when conducted too early to kill tillers, still may have some negative effect on smooth brome. For example, Becker (1989) found reductions in smooth brome shoot height and flowering and seed production after 5 years of annual burns, conducted before smooth brome tiller elongation, from middle to late April. Similarly, in central Alberta, Anderson and Bailey (1980) found 24 years of annual early spring (April) burns decreased both frequency and canopy cover of smooth brome. In addition, T. Bragg (pers. com.) found that plots at Mead, Nebraska, that were originally codominated by big bluestem and smooth brome were nearly pure stands of big bluestem after 10 years of annual late-April burns. The reasons for this are not known but may involve a combination of severe defoliation of smooth brome tillers (the tiller is still alive but substantial leaf area is removed) (Henderson et al. 1983) and an enhancement of growing conditions for competing big bluestem (Willson 1994) and other warm-season grasses (Henderson et al 1983).

In the 1989-90 tiller-elongation burn/unburned treatment, smooth brome tiller density and biomass declined and remained below preburn levels, although only tiller density remained significantly lower (Figures 2 and 3). This persistent effect is consistent with the report by Gates et al. (1982), which showed that there was a suppression of smooth brome yield 1 year after an April 29 burn. In addition, Blankespoor (1987) found that cool-season biomass in smooth brome-dominated plots was below the preburn level 1 year after a May 14 burn.

Repeated burns of smooth brome at tiller elongation in the 1989-90 and 1990-91 burn/burn treatments maintained tiller density and biomass significantly below preburn levels (Figures 2 and 3). These results show that in mixed stands with big bluestem, repeated burns of smooth brome at tiller elongation can maintain smooth brome tiller density and biomass below preburn levels.

Except for biomass in 1990-91, the responses following the 1989-90 and 1990-91 tiller heading burn/unburned treatments showed both smooth brome tiller density and biomass significantly declining and returning to preburn levels (Figures 2 and 3). In the 1990-91 burn/unburned treatment, tiller biomass did not change from preburn levels (Figure 3). These results may reflect seasonal trends in smooth brome carbohydrate reserves, which increase during tiller elongation until heading (Teel 1956). They also may reflect seasonal differences in precipitation and litter reduction following burn treatments. In May 1990, high carbohydrate reserves at heading, reduced litter, and adequate moisture may have enhanced secondary tiller growth of smooth brome, thus mitigating year-of-burn tiller losses. Furthermore, high carbohydrate reserves at heading may enhance tillering and tiller growth in the year following a burn. These results are in contrast to repeated burns at tiller heading, which produced 2 years of reduced smooth brome tiller density and biomass that differed significantly from preburn levels (Figures 2 and 3).

The significant decline and recovery of smooth brome tiller density following the tiller flowering burn/unburned treatment (Figure 2) were similar to the response following the tiller elongation burn/unburned treatment, which corresponds to an early season carbohydrate low. This result is, again, consistent with the expected response based on previous studies on carbohydrate reserves in smooth brome. Teel (1956), for example, showed carbohydrates decreased during flowering to a second seasonal low at anthesis. In contrast, the absence of significant changes in tiller biomass following the tiller-flowering burn/unburned treatment closely paralleled the response following the tiller-heading burn/unburned treatment (Figure 3). Near normal precipitation in May and June 1990 and reduced litter after the burn may have promoted robust growth of secondary tillers. Repeated burns at tiller flowering maintained both smooth brome tiller density and biomass at significantly lower levels than occurred before treatment (Figures 2 and 3). This response was similar to that found after the tiller-elongation and tiller-heading burn/burn treatments.