Plant Species Diversity in Burned Prairies


Phaya Lem, Elisabeth Metzger, Trina Schwed


            We tested the hypothesis that there would be more grasses and forbs than woody plants in recently burned prairies compared to prairies that had been burned earlier or never been burned. We took measurements from eight maintained natural prairies in the Milwaukee area. We discovered that there was a negative correlation between time since burned and percentage of grasses and forbs, with an R-value of 0.7191. This supports our hypothesis, that burning decreases the number of woody plants in prairies.

Keywords:  prairie, grass, forb, woody plants, controlled burning


            We tested our hypothesis on maintained natural prairies. We predicted that ones that were recently burned would have a higher percentage of grasses and forbs compared to woody plants than ones that had gone longer without burning or had never been burned. We believed there would be more woody plants as time went on because of succession. Grasses would shoot up quickly soon after a fire, while woody plants would take longer to get established and grow.

By “maintained natural prairies,” we meant areas that were purposely planted with natural prairie plants but were not laid out or tended like a garden. Areas where grass had simply been allowed to grow wild but was not planted with the purpose of creating a prairie area were not included.

Dormant-season burning is known to increase cover by grasses and forbs. The dormant season occurs during fall and winter (Brockway, et al, 2002). It takes grasslands anywhere between three and 30 months to recover from fire (Ford, Johnson, 2006). Knowing the variety of plants that grow after burning can help those maintaining prairies to decide how often to burn.


Materials and Methods

            To take our measurements, we used string to mark out a meter-by-meter square on the prairie. We then counted the number of grasses, forbs, and woody plants in that square. Grasses were counted in “clumps” rather than individual blades. We took four readings at each site.

 We visited Lake Bluff Elementary School (1600 E Lake Bluff Blvd Shorewood, WI) on October 27, 2010. Lake Bluff has one small prairie area maintained by the school. It is two years old and has never been burned. We visited Havenwoods State Forest (6141 N. Hopkins Street Milwaukee, WI) on November 3, 2010. Havenwoods State Forest had three prairie areas that we visited—one large planted area, one large area that was planted over a landfill, and one smaller area planted on a Nike missile site. The missile site was planted by the Army Corps of Engineers, not the Havenwoods staff. All three areas had been burned six months earlier, in May of 2010. We visited the prairie by the Ben Hunt cabin in Hales Corners (5885 S. 116th Street, Hales Corners, WI), which is maintained by the Hales Corners Historical Society, also on November 3, 2010. The Ben Hunt cabin is surrounded by a small prairie area that was burned seven months earlier, in April of 2010. On the same day, we visited the Stahl-Conrad Homestead prairie (9724 West Forest Home Avenue, Hales Corners, WI), also maintained by the Hales Corners Historical Society. On November 8, 2010, we visited the Urban Ecology Center (1500 E. Park Place Milwaukee, WI). The Urban Ecology Center has two medium-sized prairie areas, neither of which has ever been burned. One is five years old and one is 10.

            Before analyzing our results, we took our total numbers from each site and turned them into percentages. Thus, we had a percentage of grasses, forbs, and woody plants. Since our hypothesis was comparing grasses and forbs to woody plants, we added grasses and forbs together. We then plotted our percent grasses and forbs against months since burned. We analyzed this data using a scatterplot in Microsoft® Excel. We plotted a linear trendline to find the R2 value.



            The R2 value for our data was 0.7191, which suggests a correlation between time since burning and percent grasses and forbs (fig 1). This correlation supports our hypothesis, that there were fewer grasses and forbs the longer it had been since a prairie was burned.









Figure 1—Percentage of grasses and forbs decreases in the months since a prairie has been burned.





            We did find a correlation between time since burned and percentage of grasses and forbs that supported our hypothesis. However, there were many other variables in our test areas.

            It is first possible that the way the prairies were planted affected the species ratios. When we began this project we simply looked for natural prairie areas within the Milwaukee area and did not ask how the prairies were planted until after we began collecting data. There are differences in the way our sample prairies were planted. The Lake Bluff School prairie was planted with a wide variety of prairie plants for educational purposes. The Stahl-Conrad Homestead and Ben Hunt Cabin prairies were also planted with a wider variety of prairie plants. Meanwhile, the Havenwoods prairies and Urban Ecology prairies were just seeded with grass and wildflower mixes. Even within those mixes, there are differences. The grass prairie and landfill sites at Havenwoods were seeded with grass and flower species native to Wisconsin. The Nike site was just seeded with a generic grass mix. Also, all of the areas are seeded after being burned, so their recovery may differ with areas that are burned and then allowed to naturally recover.

The size of the prairies may have also affected our results. There is evidence that smaller prairies have a larger number of species (Simberloff, Gotelli, 1984). Rating the sizes of our prairies on a rough scale from 1-3 and plotting that against percent grasses and forbs does not show a linear correlation. It does show a polynomial correlation of 0.5989, which suggests a slight correlation. Medium-sized prairies seem to have the fewest percentage of grasses and forbs and a larger percentage of woody plants, which could suggest a larger number of species (fig2). Again, the fact that the prairies were planted differently could affect this.








Figure 2—Percentage of grasses and forbs shows a slight polynomial correlation with relative size of prairie



An extension of this project could involve counting number of species of grasses, forbs, and woody plants. It would be best to compare similar prairies, perhaps only extensively sampling from the three Havenwoods prairies. This would show how diversity changes as succession occurs. Another project could involve taking biomass measurements instead of simply counting plants. A small number of woody plants may actually make up a significant amount of biomass.

Works Cited

Simberloff, D., Gotelli, N. 1984. Effects of insularisation on plant species richness in the prairie-forest ecotone. Biological Conservation, 29(1):  27-46. Retrieved 8 November 2010 from ScienceDirect database.

Ford, P.L., Johnson, G.V. October 2006. Effects of dormant- vs. growing-season fire in shortgrass steppe: Biological soil crust and perennial grass responses. Journal of Arid Environments, 67(1):  1-14. Retrieved 8 November 2010 from ScienceDirect database.

Brockway, D.G., Gatewood, R.G., Paris, R.B. June 2002. Restoring fire as an ecological process in shortgrass prairie ecosystems: initial effects of prescribed burning during the dormant and growing seasons. Journal of Environmental Management, 65(2):  135-152. Retrieved 8 November 2010 from ScienceDirect database.