Effects of seasonal fire, bison grazing and climatic variation on tallgrass prairie vegetation

Published source details Coppedge B.R., Engle D.M., Toepfer C.S. & Shaw J.H. (1998) Effects of seasonal fire, bison grazing and climatic variation on tallgrass prairie vegetation. Plant Ecology, 139, 235-246.
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Summary

Fire is traditionally used to maintain of North American tallgrass prairies. Without periodic burns, plant litter accumulates leading to decreases in plant productivity and changes in community composition. Historically fires occurred throughout the year but most prescribed burning is now undertaken in spring. As such, relatively few studies exist on the effects of fire seasonality on tallgrass prairie habitats. As part restoration efforts at the Tallgrass Prairie Preserve (TPP) in northern Oklahoma, USA, a population of bison Bison bison were established in 1993. This provided an opportunity to evaluate the influences of bison grazing, season of burn and other environmental factors on aboveground phytomass and abundance of tallgrass prairie plant functional groups.

Study area: The study was undertaken in part of the 15,342 ha Tallgrass Prairie Preserve (TPP) (36º50'N, 96º25'W), northern Oklahoma, USA. The preserve was established in 1989 by The Nature Conservancy. Prior to establishment, the area was grazed seasonally by cattle and burned periodically in spring. Average total annual precipitation was 877 mm; 70% between April and September. Total precipitation during the study was a little above average; 1,058 mm in 1994 and 939 mm in 1995.

Management: The study area was grazed by 3,00 bison (6-7 ha/animal) from October 1993 onwards. Prescribed burns were conducted during late summer (September), autumn (November and December), and spring (March) in 20% of the study area annually.

Vegetation sampling: Twenty 1-ha plots were established in March 1994, located to represent as many different combinations of environmental characteristics as possible. Three additional sites were established on new burned patches in 1995. Sampling took place in early June and mid-August each year.

Bison grazing and other variables: Observations of bison grazing distributions in relation to burn ploys were made 4 to 20 times/month. The cumulative number of grazing bison was divided by the population number to derive an index of bison grazing intensity. Other variables included were: length of time since burning; dry weight of litter (estimated by collecting litter and standing dead material from quadrats); topoedaphic site (defined by soil texture and topographic position; sampling date (June and August); sample year of the study; and burn season.

August phytomass (representing the approximate peak of production) varied between years with a higher average in 1995 (521 g/m²) than in 1994 (435 g/m²). This is best explained by variation in precipitation: in 1994 there was below-average precipitation in spring followed by a wet summer; in 1995 above-average spring precipitation was followed by a slight drought. This difference appeared to strongly influenced peak phytomass. Phytomass was negatively related to bison grazing under all conditions.

Average tallgrass and little bluestem Schizachyrium scoparium (a prairie grass)composition were highest on spring burn plots, whilst those of annual grasses, forbs, and legumes were highest on summer burns. Tallgrasses and sedges and rushes were positively associated with time since burning, while forbs were negatively related. Forbs exhibited a positive relationship with bison grazing intensity.

Topoedaphic site variation was also significant for some species and guilds: e.g. upland sites had the highest relative composition of little bluestem. Annual grasses were most abundant on lowland sites, while sedges and rushes were most prevalent on wetter ‘run-in’ sites.

Conclusions: These results concur with earlier studies that indicate that productivity and vegetation composition in tallgrass prairie is affected by numerous factors such as fire and fire season, grazing, topography, soils and climate. For some groups of plants, a single environmental factor clearly appeared most influential in determining relative abundances (e.g., annual grasses with summer fire, and sedges and rushes with yearly precipitation variation). Other groups are influenced by numerous interactions (e.g. forb abundance with bison grazing, burn season and time since last burn).

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