Fire and Grazing

Site: 
Bison herd grazing on a watershed burned every 4 years at Konza Prairie. Bison are allowed to graze across a 961-ha are that includes watersheds with different fire frequencies (annual, 2-, 4- and 20-year fire return intervals) to study the interactive effects of fire and grazing on ecological processes. 6/2007
Matt Whiles

Understanding the ecological effects of fire and grazing in grasslands is an important aspect of research at the Konza Prairie (KNZ) LTER site. The tallgrass prairies of North America were shaped by natural disturbances that included periodic fires and the activities of ungulate grazers, and these remain important processes in tallgrass prairie and other grasslands around the world. In order to study the ecological role of fire and grazing, KNZ scientists pioneered the use of long-term, large-scale fire and grazing experiments. The KNZ LTER program features a unique site-based experiment, established in 1977, that includes replicate watersheds subject to different fire and grazing treatments (Fig. 1), which has led to numerous important discoveries about how fire and grazing interact to affect the ecological dynamics of grassland communities at scales ranging from patches to landscapes, with relevance to grasslands globally.

Fire is integral to the ecology of mesic grasslands worldwide. Tallgrass prairie occurs in the transition zone between wetter forests to the east and more arid shortgrass prairies and desert grasslands to the south and west, and includes some of the most productive grasslands in North America. The high productivity of the grasses allows for the rapid accumulation of fine, combustible fuel in the form of dead grass (detritus). As a result, fires were widespread and common throughout the history of these grasslands. Today, fire is used as tool for the management and conservation of tallgrass prairies. Fire removes accumulated plant detritus and allows for the vigorous regrowth of grasses, enhancing the productivity of the prairie. Fire also limits the spread of woody plants that would otherwise displace the grasses and forbs characteristic of tallgrass prairie. KNZ research has revealed how fire alters the structure and function of grasslands at multiple spatial and temporal scales. It changes the light and soil environment of emerging plants, altering their physiology, growth and population dynamics. Fire enhances growth of the dominant grasses, while also reducing soil N availability and impacting plant competition for light and N. These changes contribute to reduced abundance and richness of many subordinate plants, reduced frequency of exotic species, and a decline in overall plant biodiversity. Changes in vegetation structure, composition and tissue quality in turn elicit changes in consumer population dynamics, species composition and habitat use.

Grazing by bison, like fire, was historically important in tallgrass prairie, and grazing by cattle is now the dominant land use. In fact, changes associated with the management of grazers are among the most significant in the recent history of mesic grasslands worldwide. To address the role of native grazers and important fire - grazing interactions, bison were reintroduced in 1987 to a 1000-ha area of Konza that includes replicate watersheds burned at 1-, 2-, 4- and 20-year intervals and a range of topography and vegetation types. Comparative studies of native (bison) vs. introduced (cattle) ungulates are also being done to understand the impacts of changing land use in North American grasslands.

The Konza Prairie LTER site features watershed-level fire and grazing treatments that are part of the KNZ core LTER program. Watersheds with native ungulate (bison) grazing (‘N’) are green, and cattle-grazed watersheds (‘C’) are red. Cattle-grazed treatments include both annual spring burning/season-long stocking and a 3-yr rotational burning patch-grazing treatment initiated in 2010. All other watersheds are ungrazed. Numbers designate fire return intervals for spring-burned watersheds. Watersheds subject to different seasons of fire (W=winter, F= fall, Sp= spring, SU=summer) are brown, and Fire Treatment Reversal (‘R’) watersheds are yellow. King’s Creek watershed (USGS Benchmark Stream) is blue. Many plot-level experiments are located at the headquarters areas (HQ) in the northwest portion of the site.
For further reading: 
Blair, J.M. 1997. Fire, N availability and plant response in grasslands: A test of the transient maxima hypothesis. Ecology 78:2539-2368.
Collins, S.L., A.K. Knapp, J.M. Briggs, J.M. Blair, and E.M. Steinauer. 1998. Modulation of diversity by grazing and mowing in native tallgrass prairie. Science 280:745-747.
Collins, S.L. and M.D. Smith. 2006. Scale-dependent interaction of fire and grazing on community heterogeneity in tallgrass prairie. Ecology 87:2058-2067.
Joern, A. 2005. Long-term disturbance from fire and bison grazing modulates grasshopper species assemblages (Orthoptera) in tallgrass prairie. Ecology 86:861-873.
Knapp, A.K., J.M. Blair, J.M. Briggs, S.L. Collins, D.C. Hartnett, L.C. Johnson, and E.G. Towne. 1999. The keystone role of bison in North American tallgrass prairie. BioScience 49:39-50.
For further information: 
Dr. John Blair
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