We have heard a lot of buzz in the tallgrass prairie fire community recently surrounding the practice of fall burning. The recent release of a research paper by Towne and Craine (2014) describes a 20-year dataset, where the authors looked at effects of burning at different times during the year on the plant community. They conducted the study on ungrazed sites at the Konza Prairie in Kansas, where they burned in late-November, mid-February, and mid-April. They found similar grass productivity across burn season and upland/lowland sites. However, culm production was greater with spring burns. However, forb productivity decreased for spring burns in upland sites. Woody species, often a target for prescribed burners, increased across all treatments in the upland and lowland sites. Grass productivity, an important consideration for livestock producers, seemed to be more sensitive to precipitation in the fall and winter than spring. Subtle shifts in species composition encouraged the authors to contend that “autumn and winter burns promote a broader phenological diversity of species than spring burns.”
Land managers in the Great Plains have conducted prescribed fires most often in the spring. The spring window of opportunity was thought to last longer than other times of year, providing more opportunities to complete burns, however Roberts et al. (1999) found that fewer burn days were actually available in April than in other months (see also Weir 2011). Spring moisture can also make fire more manageable than during droughty periods in summer. The Towne and Craine findings are important to the Flint Hills range community, where the Intensive Early Stocking (IES) system, most commonly used there, has recommended early April burning (Launchbaugh and Owensby 1978, Towne and Owensby 1984). Because of the wide adoption of IES grazing system, many ranchers burn simultaneously, resulting in regional smoke management challenges (see http://ksfire.org/ for smoke management plans and background). A flexible burn period, if adopted, has potential to reduce smoke management concerns in the spring.
Unfortunately, Towne and Craine’s research was not conducted in grazed pastures, so the direct implications for grazed lands are unclear. However, a meta-analysis on burn-season research in grazed grassland by Engle and Bidwell (2001) demonstrated the resilience of tallgrass prairie. Consistent with Towne and Craine, they found that productivity varied little across sites burned at different times, including grazed prairie, but other compositional shifts varied by topography, precipitation, soil, fire history, and other factors. Other grazing-fire research found that sedges and other cool season grasses increase with fall or summer burning consistent with the Towne and Craine results. Sedges are a key winter and spring forage for bison in particular (Coppedge and Shaw 1998, Coppedge, et al. 1998)
In spite of variable results from burn-season studies, some very good reasons encourage us to consider vary time of burning in fire management plans. (See Leis 2008, chapter 1.1 for a thorough discussion of fire history in the Heartland, including frequency and season). We know that fires occurred year-round historically, but anthropogenic burning occurred most frequently in spring and fall periods, while lightening ignitions dominated in summer (Howe 1994, Bragg 1995). Burning in fall protected winter settlements and created grazing incentives. Research clearly tells us that most grassland grazers, especially those of the tallgrass prairie, prefer recently burned patches. (See annotated bibliography for a sampling of relevant citations ) Therefore, having fresh plant growth that would persist for most of the winter season attracting game would encourage hunters to burn nearby grasslands. So, varying burn season would be consistent with historical fire occurrence.
Diversity in plant phenology is another reason to vary burn season (and probably one of the best reasons). Plants reach different lifecycle stages at different times through the year. Altering burn season provides opportunities for organisms to thrive in various years as burn timing changes. For example, spring burns typically reduce the spring forb guild and native cool season plants, while fall or late summer burns may give these species a temporary boost (Engle et al. 1998, Towne and Crain 2014). Similarly, wildlife species that are vulnerable in one period may benefit from varying burn season in some years. For example, herpetofauna come out of hibernation sluggishly in the spring, making them susceptible to mortality from fire, but a dormant season burn might find them sheltered from harm (see Gaetani et al. 2010 for more discussion of fire effects on wildlife).
Fire intensity affects plant responses to fire. Unfortunately, little research has effectively controlled fire severity to help us sort out how to reach our goals (See GPE 2013-23 for a discussion of this). Recent work has quantified fuel moisture levels needed to effectively burn eastern redcedar (Weir and Scasta 2014). Choosing timing when live fuel moistures would aid in attaining target fire intensities might more successfully achieve objectives than would locking into a single burn season when ideal conditions might not exist (Weir). Varying burn season would allow managers to select for the right combination of fuel load, winds, fuel moistures, and phenological stage to best achieve objectives.
Lastly, varying burn season might provide logistical benefits. If you burn at a time when there is little demand for fire crews and when funding is available, you might have greater success in conducting the burn, than at times when there is high demand for crews and funding cycles are restrictive. The best place to start in determining the ideal time to burn should always begin with evaluating objectives for each fire you plan.
Bragg, T. B. 1995. The physical environment of Great Plains grasslands. Pages 49-81 in A. Joern, and K. H. Keeler, editors. The changing prairie. Oxford University Press, New York.
Coppedge, B. R., and J. H. Shaw. 1998. Bison grazing patterns on seasonally burned tallgrass prairie. Journal of Range Management. 51:258-264.
Coppedge, B. R., D. M. J. Leslie, and J. H. Shaw. 1998. Botanical composition of bison diets on tallgrass prairie in Oklahoma. Journal of Range Management. 51:379-382.
Engle, D. M. and T. G. Bidwell. 2001. Viewpoint: The Response of Central North American Prairies to Seasonal Fire. Journal of Range Management 54:2-10.
Gaetani, M. S., K. Cook, and S. A. Leis. 2010. Fire effects on wildlife in tallgrass prairie. Natural Resource Report NPS/HTLN/NRR—2010/193. National Park Service, Fort Collins, Colorado.
Howe, H. F. 1994. Managing species diversity in tallgrass prairie: assumptions and implications. Conservation biology 8:691-704.
Launchbaugh, J. L. and C. E. Owensby. 1978. Kansas Rangelands: Their management based on a half century of research. Kansas Agricultural Experiment Station Bulletin 622.
Leis, S. A. 2008. Prescribed fire resource kit for grassland parks in the Central Grasslands, U.S.: information resources for prescribed fire planning. Natural Resource Report NPS/HTLN/NRR—2008/027. National Park Service, Fort Collins, Colorado.
Roberts, K. W., D. M. Engle, and J. R. Weir. 1999. Weather constraints to scheduling prescribed burns. Rangelands 21:6-7.
Towne E. G. and J. M. Craine J. M. 2014. Ecological consequences of shifting the timing of burning tallgrass prairie. PLoS ONE 9(7): e103423. doi:10.1371/journal.pone.0103423
Towne, G. and C. Owensby. 1984. Long-term effects of annual burning at different dates in ungrazed Kansas tallgrass prairie. J. Range Manage. 37:392-39.
Voleti, R., S. Winter, and S. Leis. 2014. Patch burn-grazing: an annotated bibliography. Great Plains Fire Science Exchange GPE:2014-16.