Can Mowing Substitute for Fire in Semiarid Grassland?

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Abstract

Accumulating data indicate the importance of fire in rangeland systems. Mowing is a common management technique sometimes considered a surrogate for fire. However, direct comparisons of fire and mowing effects are limited. Our objective was to determine whether mowing can substitute for fire in rangeland by comparing effects on plant biomass, composition, cover, soil nutrients, and forage quality. Three disturbance treatments (nontreated control, spring mowing with clipping removal, and spring fire) were randomly assigned to 21 plots (5 × 5 m) each on silty and claypan ecological sites in a completely randomized design, with seven replications per site. Current-yr biomass was similar among control, mowed, and burned treatments (1 003, 974, 1 022 ± 64 kg ● ha 1). Mowing shifted functional group composition by reducing C3 perennial grass 12% and increasing forbs 8%. Non-native species were a larger component of mowed (12%) than control (6%) or burned plots (4%). Fire increased bare ground 35%, reduced litter 32%, and eliminated previous yrs’ growth the first growing season. Plant-available soil N and S more than doubled with fire, and there was a trend for more P in burned plots. Mowing effects were limited to a trend for less soil Fe. Mowing affected 42% of the forage quality variables with a 2% average improvement across all variables. Fire affected 84% of the variables, with a 12% average improvement. Mowing increased forage P and K, whereas fire increased forage concentrations of N, K, P, S, Mg, Fe, Mn, and Cu. Total digestible nutrients increased 1.1% with mowing and 2.1% with fire. In vitro dry matter disappearance increased 2.2% with mowing and 6.7% with fire. Burned plots had greater in vitro fermentation than controls or mowed plots. Although mowing can be a useful management tool, it is not a substitute for the ecological effects of rangeland fire.

Introduction

Land managers are becoming more aware of the benefits of prescribed fire. However, people are commonly hesitant to begin a burning program because they lack experience applying prescribed fire. Among the mechanisms believed to cause fire effects are nonselective biomass removal and increased light availability near the soil surface. Therefore, it is occasionally suggested that some fire effects can be mimicked with high-intensity, short-term grazing or mowing.

Land managers in rangeland ecosystems recognize the restoration of certain ecological processes (e.g., fire, livestock grazing) as an efficient strategy for improving and maintaining rangeland plant community structure and function (Wright and Bailey, 1982, Milchunas et al., 1988). The effect of fire and grazing on rangelands varies by ecosystem, as some rangeland plant communities are more resilient to fire or grazing (Wright and Bailey, 1982). Prescribed fire and grazing have been used in tandem in tallgrass prairie to enhance forage quality, improve livestock weight gain, reduce woody encroachment, improve wildlife habitat, and increase plant diversity (Hobbs et al., 1991, Collins et al., 1998, Fuhlendorf and Engle, 2004, Joern, 2005, Scasta et al., 2016). Relative to tallgrass prairie, sagebrush steppe rangeland is more susceptible to damage from fire and grazing (Mack and Thompson, 1982, Wright and Bailey, 1982, Chambers et al., 2014). However, prescribed fire and grazing may still be used in the sagebrush steppe ecosystem to maintain productive native plant communities (Davies et al., 2010, Davies et al., 2016). Whether used individually or in combination, fire and grazing are valuable land management tools that can be used to fulfill various ecosystem management objectives (Pöyry et al., 2004, Fuhlendorf et al., 2009, Pyke et al., 2010).

Fire and grazing share certain ecological characteristics; both processes remove plant biomass, convert plant material to biotic and abiotic materials, and have the potential to alter ecosystem properties, including nutrient dynamics, plant production, and diversity (Howe, 1995, Collins et al., 1998, Hart and Ashby, 1998, Wan et al., 2001, Koerner and Collins, 2014). Fire has the potential to produce direct plant mortality in certain species, particularly annuals, and improve overall forage quality of a plant community (Vermeire and Rinella, 2009, Strong et al., 2013, Dufek et al., 2014). Furthermore, fire differs from grazing in that fire does not choose what plants to “consume,” does not have nutrient requirements, removes plant biomass at a relatively uniform utilization level (Bond and Keeley, 2005), and can have direct effects on the animal community (Branson and Vermeire, 2016). In contrast, grazing may cause direct fluctuations in plant community composition and indirectly alter forage quality (Taylor et al., 1993, Vermeire et al., 2008). Hulburt (1988) suggested clipping and fire largely had similar effects in tallgrass prairie in that both increase soil temperature and increase light intensity at the soil surface. However, biomass and reproductive culm density were positively related to nitrogen addition, which is associated with fire. Therefore, while fire and grazing share similar physiognomy, both have the proclivity to set plant communities on distinct ecological trajectories.

Mechanical removal (mowing) of vegetation shares characteristics with fire and grazing (uniform utilization and incomplete vegetation removal) and has been used to test for differences between defoliation mechanisms (Cox, 1988, Moog et al., 2002, Fidelis et al., 2012). Mowing requires less planning than livestock grazing and prescribed fire and may reduce risk compared with fire. Another benefit is the relative ease of mowing applications, which makes it an attractive vegetation management alternative to livestock grazing or prescribed fire. However, as outlined previously, fire and grazing are unique ecological processes and mowing may not reasonably mimic or have the magnitude of effects that the restoration of fire or grazing could have on a rangeland ecosystem (Lodge, 1960, Prober et al., 2008, Kitchen et al., 2009, Pyke et al., 2014).

The objectives of this study were to 1) describe spring mowing and spring prescribed fire effects on plant biomass, composition, cover, soil nutrients, and forage quality; and 2) to contrast the two treatments to each other and nontreated control plots and determine whether mowing is a suitable substitute for fire in rangeland. We hypothesized that spring fire 1) reduces annual grasses and forbs, 2) reduces litter and increases bare ground, 3) increases soil nutrient availability, and 4) increases forage quality to a greater extent than spring mowing.

Section snippets

Study Area

Research was conducted on the Fort Keogh Livestock and Range Research Laboratory (46o23′32′′N, 105o57′09′′W; 809 m above sea level), near Miles City, Montana. The vegetation type is northern mixed prairie. The climate is semiarid, with 341 mm average annual precipitation, 110- to 135-d freeze-free period, and temperatures ranging from − 40oC to 38oC. About 90% of annual net primary production occurs by 1 July (Vermeire et al., 2009) and is most affected by April and May precipitation (Vermeire

Biomass and Cover

Current-yr biomass was similar among control, mowed, and burned treatments (1 003, 974, 1 022 ± 64 kg ● ha 1, respectively; P = 0.7538). Despite the similarity in production, mowing shifted functional group composition relative to control and burned plots by reducing C3 perennial grass (P = 0.0029) and increasing forbs (P < 0.0001; Fig. 1). No differences were detected among treatments for C4 perennial grass (P = 0.7802) or annual grass (P = 0.3725). Across functional groups, non-native species

Discussion

Early spring mowing caused changes in functional group composition and diversity and improved some measures of forage quality, whereas spring fire did not alter functional group composition or diversity. However, fire increased bare ground through litter reduction, increased plant-available soil nutrients, and improved forage quality of current-yr growth. The hypothesis that spring fire reduces annual grasses and forbs was not supported by the data, but results supported the hypotheses that

Implications

Many rangelands evolved with recurrent fire, grazing, and variable weather. Two of the three, fire and defoliation, can be manipulated through management and both, operating in tandem, are likely required to maintain rangeland integrity. Defoliation can mimic fire in reducing standing biomass and has been successfully employed for species-specific management. However, fire effects are more complex than simply reducing standing biomass. Mowing-induced changes in functional group composition and

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    This work was partially supported by the US Forest Service (13-IA-11011800-012). Mention of any trade name or proprietary product does not constitute a guarantee or warranty by the authors or US Dept of Agriculture  Agriculture Research Service (USDA-ARS), nor does it imply the approval of these products to the exclusion of others. The USDA-ARS, Plains Area, is an equal opportunity/affirmative action employer, and all agency services are available without discrimination.

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