Fire and Mowing Have Effects on the Density of Asteraceae and Fabaceae

Tillers 2000, 2, 39-45

Fire and mowing have effects on the density of Asteraceae and Fabaceae

DAN CARTER, DAN LESH, and ANN MOGUSH Biology Department, Grinnell College, PO Box 09-04, Grinnell, IA 50112

Abstract

Burning and mowing are two common prairie restoration/reconstruction tools. Both are important in determining the prairie’s composition of forbs. We investigated effects on Asteraceae and Fabaceae, the two most common forb families on the tallgrass prairie. We measured the density of both families and species belonging to these families on plots where mowing and burning were treatments. Burning led to a significant increase in the density of Fabaceae but not Asteraceae. Mowing had a significant effect on Amorpha canescens and Solidago sp. The effects of these treatments indicate that they influence the presence of the two largest forb families on tallgrass prairie in Central Iowa.

favoring Fabaceae species capable of fixing Introduction their own nitrogen. Asteraceae may be hindered by fire. Fire removes deep litter,

Forbs comprise the majority of species in and dominant C4 grasses have been shown the tallgrass community, despite being to increase after deep litter removal infrequent in comparison to dominant C4 (Hulbert 1969). Competition with grasses (Howe 1999). Asteraceae and grasses may be to the detriment of Fabaceae are two of the most common Asteraceae, especially since many forb families in the tallgrass prairie region. Asteraceae species bloom concurrently Species belonging to Asteraceae tend to with the main thrust of grass growth. have daisy-like flowers and bloom from Therefore, fire is capable of altering the summer into fall. Many species belonging balance between forbs and grasses on the to Fabaceae can fix nitrogen and have prairie. compound leaves. Most bloom from Grazing is another factor that can spring into summer. To manage the affect the prairie community. Bison and prairie ecosystem, it is necessary to other large grazers roamed the prairie prior examine the factors that influence the to European settlement. Large generalist presence of major forb families and their grazers tend to increase diversity by representative species. Fire and grazing reducing dominant vegetation (Howe are management tools and forms of 1999). Mowing is an appropriate disturbance that effect the composition of substitution because it is the ultimate forbs on prairie. generalist, cropping a given area equally. Fire is an environmental factor Therefore, grazing (mowing) is likely to that has a profound effect on the prairie favor forb species because it levels the ecosystem (Whelan 1995). Before playing field between the forbs and the European settlement, fires ignited by dominant grasses. lightning and set by Native Americans Kucera and Koelling (1964) periodically swept across the prairie. Fire showed that annual spring burn may favor Fabaceae on the prairie. significantly reduced Asteraceae at a Frequent burning may result in chronic tallgrass prairie site in Missouri. Because nitrogen deficiency (Collins 1992), of this and what we know about fire’s

© 2000 Grinnell College 40 D. CARTER et al. effects on nitrogen budgets, we expected all combinations of treatments in the that annual fire treatment alone would Mow/Burn experiment. lead to a decrease in Asteraceae density and no change or increase in Fabaceae density. Because grazing (in our Results experiment, mowing) reduces C4 dominance, we expected grazing to lead to Effects of fire on Asteraceae: a general increase in the density of forb We found that burning did not species. significantly influence the density of the family Asteraceae in the prairie (t=-0.14, df=18, p=0.89). Mean Asteraceae density Methods was 4% lower on burned than unburned plots, but is not significant (Fig. 1). To investigate the effects of annual Burned plots had a higher mean burning and mowing on the density of density of Solidago sp. than unburned Fabaceae and Asteraceae species, we plots, but the difference was not collected data from Conard significant (t=.52, df=16, p=0.61, Fig 2). Environmental Research Center on the Unburned plots had a higher following dates: October 4, 9, 11, 23, 25, mean density of Ratibida pinnata than and 30 2000. We collected data from burned plots, and burning did twenty plots in the burn experiment, ten significantly influence its density (t=-2.38, each on burn and no burn plots. The df=18, p=0.02, Fig 3). burn experiment had been burned every spring since 1997. We also collected data Effects of fire on Fabaceae: from twenty plots on the mow/burn The family Fabaceae was more experiment, five replicates each of burn/no abundant on burned than unburned plots mow, burn/mow, no burn/mow, and no (t=2.73, df=17, p=0.01, Fig. 4). burn/no mow (see Appendix A). Each Mean Lespedeza capitata density plot measures ten meters by ten meters, was significantly higher on burned than and ten-meter strips of uncontrolled unburned plots(t=2.52, df=18, p=0.02, prairie separate each plot from the next. Fig 5). To increase our sample size in the Mean Baptisia leucantha density mow/burn component of our experiment, was significantly higher on burned than we included data from the ten unburned unburned plots (t=2.63, df=16, p=0.02, plots and ten burned plots in the burn Fig. 6). experiment. In each replicate we randomly laid Combined effects of Mow and Burn on a ten meter transect and counted the total Asteraceae and Fabaceae number of stems of each forb family and Mowing, burning, and the species within half a meter of the transect. combined effects of mowing and burning We calculated the density of, total had no significant effect on the density of Asteraceae, individual Asteraceae species, Asteraceae or Fabaceae in the two-factor Fabaceae, and individual Fabaceae species experiment (Table 1). However, mean in each sample. We ran t-tests on Solidago sp. and Amorpha canescens density Microsoft Excel to compare density were significantly reduced by mowing and between burned and unburned samples in not burning or the combination of the burn experiment. We ran an ANOVA burning and mowing (Table 1). on Minitab to compare density between

Figure 1. Mean Density(stems/ Figure 2.Density Denisty(stems/m2) Asteraceae on Burned vs. Unbu Goldenrod sp. on Burned vs. Unb Plots at CERA (+/- S.E.) Plots at CERA (+/- S.E.) 10 5 8 4 6 3

Mean 4

Mean 2 Asteraceae 2 1 Goldenrod sp.

Density(stems/m2) of 0 0 Density(stems/m2) of Burn No Burn Burn No Burn Treatment Treatment

Figure 3. Density(stems/m2) of pinnata on Burned vs. Unburned at CERA (+/- S.E.)

3 2.5 2 1.5

Mean 1 0.5 Ratibida pinnata 0 Density(stems/m2) of Burn No Burn Treatment

Figure 4. Density (stems/m2) Fabaceae on Burned vs. Unburned at CERA (+/- S.E.)

4 2 Fabaceae (stems/m2) of Mean Density 0 Burn No Burn Treatment

Figure 5. Density (stems/ LespedezaLespedeza capitata capitata on Burne Unburned Plots at CERA (+/-

5 Mean Density (stems/m2) of 0 Lespedeza capitata burn no burn Treatmen

Figure 6. Density (stems/m BaptisiaBaptisia leucantha leucantha on Burne Unburned Plots at CERA (+/- 0.8 0.6 0.4 0.2 Baptisia leucantha 0 Mean Density (stems/m2) of Burn No Burn Treatment

Table 1. Analysis of Variance, using Adjusted SS for Tests Burn Mow Burn and Mow F-value P-value F-value P-value F-value P-value Asteraceae 0.32 0.575 1.31 0.26 0.05 0.832 Solidago sp. 0.07 0.797 4.56 0.04 0 0.992 Fabaceae 2.4 0.13 1.35 0.253 1.32 0.258 Amorpha canescens 0.17 0.681 4.62 0.038 0.79 0.381 F-value is the ratio of variation among groups to the variation within groups.

Discussion Since our study site was burned during the spring before the growing season, The Burn Experiment Hartnett’s effect may have been offset for Our data indicate that the yearly spring Asteraceae and Solidago sp. burning regime at CERA prairie has a The ability of Fabaceae to fix significant impact on Fabaceae but not nitrogen and their time of growth and Asteraceae. Our results showed that reproduction gives them an advantage Fabaceae was more abundant in burned after a fire. Our data for Asteraceae, the than unburned plots, so were Baptisia largest family on the prairie, shows that, in leucantha and Lespedeza capitata, two general, spring burning does not have a Fabaceae species. Kucera and Koelling significant effect on the prairie. We (1964) found that Fabaceae abundance encourage further study of summer and was not affected by fire but they cited the fall burn treatments and study of less findings of Whelan (1954) that fire has frequent burns, because plants would be been shown to increase Fabaceae. Fire impacted at different points in their decreases the soil nutrient content growth cycle, and effects on soil nutrients (Hartnett, 1991). Since most Fabaceae with less frequent burns may be different. species host nitrogen-fixing bacteria, it makes sense that Fabaceae would have an The Mow/Burn Experiment advantage after fire. Other factors are The only significant difference in likely important in determining Fabaceae Asteraceae density in any treatment was a density. At our site, the prairie is burned decrease in goldenrod sp. in response to every spring before the growing season. the single factor of mowing, and the only This should benefit Fabaceae plants difference in Fabaceae density was because they generally grow and flower Amorpha canescens in response to mowing. earlier than C4 grasses and avoid grass Our study of Asteraceae and Fabaceae competition (Christianson and Muller, density showed no significant effects from 1999). Heat has been shown to increase burning or the combination of mowing germination of some Fabaceae species and burning in this experiment. The (PBC). This also explains increased decline of these two species because of numbers on burned plots. mowing and the absence of significant Mean abundance for Asteraceae change in density for any other species are species was greater on unburned plots unexpected. Reduction of dominant than on burned plots, but our data were grasses is thought to favor the less not significant between burned and dominant forbs (Howe 1999). On plots unburned plots. However, we also where both burn and mowing treatments analyzed the individual species Solidago sp. were applied, it is possible that these two and Ratibida pinnata and found that fire factors worked against each other to yield had no effect on Solidago sp. and the insignificant result. decreased the abundance of Ratibida Mowing may not be truly pinnata. Hartnett (1991) also found that comparable to grazing. Large grazers tend frequent burning reduces Ratibida pinnata to graze in large patches (Briggs et. al, density. Hartnett suggested that fire 1993). It is possible that within these increases C4 grass vigor and density, patches the grazing is more intense, thus resulting in decreased soil moisture, compromising the dominant species decreased nutrient content, and decreased further. Mowing may have different light penetration through the canopy. effects because it is even more general than Howe (1993) found that March fires ungulate grazers are. Reduction of favored late species such as Asteraceae. Amorpha canescens, for example, could

© 2000 Grinnell College 44 D. CARTER et al. occur because mowing is too general. such as reproduction, size, and density Amorpha canescens is a shrubby perennial would also help to determine where and if (Christianson and Muller, 1999). these two factors have opposite effects on Mowing arbitrarily cuts Amorpha canescens forb density. to a particular height regardless of the coarseness of its stems, but a grazer might avoid eating the woodier parts of the Acknowledgements plant. Bison, for example, prefer a diet of grasses (Oakland Zoo). Thus, mowing We would like to thank Professor Caruso may unduly reduce the density of for guidance in formulating our project Amorpha canescens by not allowing it to idea and Professor Brown for help with become a small, woody shrub. statistical analysis of our data. We also Solidago sp. reduction in response thank Judy in the Writing Lab for to the mowing factor is difficult to reviewing our initial draft. explain. Within the plots included in our study, most Solidago sp. had a tendency References to be concentrated into rather dense patches. For example, in plot five, ten American Bison (Oakland Zoo) [online]. plants were recorded, and 127 were http://www.oaklandzoo.org/atoz/azbison. recorded in plot seventeen. These plots html both received the same treatment. Since our sample size is small for this Briggs, John M., Elmer Finck, David C. experiment, missing a patch could account Hartnett, and Mary Ann Vinton. 1993. for Solidago sp. reduction. Interactive effects of fire, bison (Bison Mowing may have favored forbs to bison) grazing and plant community some degree and burning might have composition in tallgrass prairie. American favored dominant species to on plots that Midland Naturalist 129:10-18. received both treatments. Our insignificant results on these plots could Christianson, Paul, and Mark Muller. very well be a result of both factors 1999. Iowa Prairie Plants. University of canceling each other. Burning may have Iowa Press, Iowa City. reduced litter, which increased grass dominance, and mowing may have Collins, Scott L. 1992. Fire frequency reduced grass dominance by opening up and community heterogeneity in tallgrass its canopy. Since it is likely that many of prairie vegetation. Ecology 73(6): 2002- the less dominant Fabaceae and Asteraceae 2006 family members are at the mercy of dominant C4 grasses, their net Davidson, Coleen and Kelly Kindscher. opportunity or loss might have been near 1999. Fire, grazing, and mowing on zero once both of these factors were taken tallgrass prairies. Ecological Restoration into account. 17(3): 136-141. Amorpha canescens may illustrate the inability of mowing replicate the Hartnett, David C. 1991. Effects of fire in influence of grazing. Further scientific tallgrass prairie on the growth and study must provide a superior replication reproduction of prairie coneflower or a direct application of grazing if this (Ratibida columnifera: Asteraceae). function is to be clearly defined. Further American Journal of Botany 78(3): 429- study of the separate effects of fire and 435. mowing on specific plant characteristics

Howe, Henry F. 1999. Dominance, central Missouri prairie. The American diversity, and grazing in tallgrass Midland Naturalist 72(1): 142-147. restoration. Ecological Restoration 17(1&2): 59-66. Prairie Bush Clover (PBC) [online]. http://econ.usfs.msu.edu/gla/tesweb/ESAs Hulbert, Lloyd C. 1969 Fire and Litter /esalele.htm . Effects in Undisturbed Bluestem Prairie in Kansas. Ecology 50:874-7. Whelan, R.J. 1995. The Ecology of fire. Cambridge University Press, New York. Kucera, C.L. and Melvin Koelling. 1964. The influence of fire on composition of a

Appendix A