Seasonal fire effects on the diversity patterns, spatial distribution and community structure of forbs in the Northern Mixed Prairie, USA

  • Published source details Biondini M.E., Steuter A.A. & Grygiel C.E. (1989) Seasonal fire effects on the diversity patterns, spatial distribution and community structure of forbs in the Northern Mixed Prairie, USA. Vegetatio (now Plant Ecology), 21-31.


Prairie plants are variously adapted to fires but their responses are dependent upon factors such as fire intensity, fire frequency, season of fire, and weather conditions. Forb (herb) composition within recently burned patches generally changes with time from a community dominated by annual/biennial species to one of, often rhizomatous, perennials. Perennial grasses gradually become the dominant plants, whilst forbs become less frequent. In the study summarised here, the effects of fire season on forbs were determined for a northern Mixed Prairie site in South Dakota, USA.

Study site: The study was undertaken within the Samuel H. Ordway, Jr. Memorial Prairie (a 3,160 ha native mixed prairie preserve managed by The Nature Conservancy) in South Dakota, north-central USA. The vegetation was characterized by typical mid-prairie and high-prairie plant communities.

Burn treatments: In 1982, 12, 30 m by 30 m plots were located within a 97 ha enclosure with 4 treatments and 3 replications randomly assigned. The vegetation had been lightly to moderately grazed by bison Bison bison prior to commencement of the study and unburned for at least 10 years. The treatments applied in 1982-84 were:

annual spring (dormant season) burns applied between April 19-26;

alternate year summer burns applied between August 3-5;

annual autumn (dormant season) burns applied between October 5-17;

unburned controls.

Monitoring: Plant density data were collected from each of the study plots during mid-July through early August with randomly located quadrats

Alpha diversity (plant diversity within a particular area i.e. the different treatment plots, measured by counting the number of taxa within the ecosystem eg. at family, genus or species level) and beta diversity (species diversity between ecosystems, or in this case, treatments; comparing the number of taxa unique to each) were investigated.

The different burn treatments and unburned controls were compared over a 3-year period characterized by both above- and below-average precipitation which appeared to affect forb diversity and community patterns.

Forb diversity: Significant differences among treatments in species richness occurred only in 1983 (following a year of above normal summer rainfall); unburned and summer burns had a higher number of species than spring and autumn burns (an average of 51 vs. 38 species). The average number of species across treatments dropped from 45 in 1983 to 33 in 1984-1985 (rainfall in these two years was 26% below average). Alpha and beta diversity showed similar responses: autumn and the spring burn plots had lower alpha and beta diversity than unburned controls or summer burn plots in 1984 and 1985.

Forb community patterns: Forb density was significantly higher on autumn and spring burns compared with unburned control and summer burn plots in 1984 and 1985 (dry years). In 1983 there was no difference in forb density among the burn treatments but they had higher forb densities than the unburned control plots. The control and the summer burn plots showed large declines (54% and 60% respectively) in forb densities from 1983 to 1985 whilst that in the autumn and spring burn plots exhibited far more moderate declines (9% and 19% respectively).

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