On the surface this appears to be a dense or debatable topic. However, after an exhaustive review of the literature, I was surprised to find that it really isn't. It is quite simple really. Given this, Steve and I agreed to post the literature summaries in advance. We are hoping a few people will help catalyze any questions/discussion via this forum and thereby help us to be better prepared for Q&A after the presentation. We are also hopeful that this little bit of publicity will get more folks in attendance, since the talk is on Friday (last day of the conference) and the title isn't horribly provocative.
So, I'll start the talk with a rundown of why people used to burn and why we burn now. Basically modern burning (in North America, mind you) falls into two camps; 1) managing rangeland for cattle production with a heavy bias for grasses and 2) to promote and/or support biodiversity. There may also be a tad of fire use to promote forestry here and there. Because most prescribed fire in Missouri is done through public and private conservation agencies, most of it is directed toward promoting and/or supporting biodiversity; a rich assemblage of plants, animals and all creatures great and small.
Unfortunately, I could find zero published results on the effects of seasonality of fire in woodlands and forests. All of the literature involves prairie habitats and even then it is sparse. The phenomenon shouldn't be much different, and if it is different, one would expect to see more exaggerated results in woodlands and forests since there is a higher ratio of C3 graminoids and forbs to C4 grasses in woodland and forests: the effect we are concerned about. Here are the results of the studies directly testing hypotheses of seasonality on plant community structure and composition followed by a summary/synthesis:
Towne and Kemp 2003 – 8 year study using annual fire (previous 3-4 year interval)
-Spring burning (late April) compared to winter (February) and fall (late November)
Spring Burning resulted in…
-decreased richness of C3 grasses and perennial forbs
-decreased diversity of C3 grasses and perennial forbs
-decreased productivity of C3 grasses and perennial forbs
-complete loss of five annual species
-no increase in C4 grass productivity
Also found that…
-Big Bluestem increased regardless of season
-Indian Grass proliferated with spring burning
-Carex spp. increased 14% with autumn and winter; decreased to 3.5% in spring
-Cool season grasses increased 22% in autumn and winter; decreased 5% spring
-most legumes tolerated but did not benefit from spring burning-woody species showed no change (low density to begin with)
Towne and Craine 2014 – continuation of 8 year to a 20 year study
-grass biomass remained unchanged from autumn, winter and spring burns
-June Grass increased from 3 to 11 percent in autumn and winter
-June Grass was eliminated from spring plots
-Indian Grass stayed constant in autumn and winter
-Indian Grass doubled in spring (increase indicative of lost diversity in glades)
-spring burns reduced biomass by 50%
-spring burns had increase in 5 species (average C-value = 4.7)
-winter and fall saw increase in 6 species (average C-value=6.2)
-Carex spp. increased 8 to 14 percent in autumn and winter
-Carex spp. decreased from 6 to 1 percent in spring
-woody plants stayed constant across treatments
Owensby and Anderson (1967) and Aldous (1934) – early studies from Konza
Compared early (Mar 20), mid (Apr 10), and late spring (May 1)
to fall dormant season burns (not in Owensby and Anderson 1970).
-C4 grass productivity increased from fall (lowest) to late spring (highest)
-weeds decrease by a ~ ¼ after early spring and ~ ½ after late spring
Weeds as defined by Aldous (1934): “There are several species of forbs, or broad-leafed plants. In pastures most of these are considered as weeds and will be designated as such. These forbs fluctuate in abundance from year to year. The most abundant ones include:
-perennial ragweed (Ambrosia psilostachya) [C=3]
-many-flowered aster (Aster ericoides) [C=5]
-pasture sage (Artemisea ludoviciana) [C=3] [
-whorled milkweed (Asclepias verticillata) [C=2]
-prairie cat’s-foot (Antennaria neglecta) [C=4]
-Missouri goldenrod (Solidago missouriensis) [C=6]
-stiff goldenrod (Solidago rigida) [C=5]
-blue-eyed grass (Sisyrinchium campestre) [C=5]
-wild flax (Linum texanum) [C=5]
and about a half dozen other species of minor importance.”
Howe 1994 – early flowering vs. late flowering in experimental prairies in WI
-compared spring (March 31) and mid-summer (July 15) burns
-early flowering perennials (flowering before mid-July) planted prairie
-cover was 17% in controls, 6% in spring burn and 46% in summer
-late flowering perennials (flowering after mid-July) in remnants
-cover was 80% in controls, 92% in spring burn and 47% in summer
-spring fire hurts early perennials and encourages late perennials
-summer fire hurts late perennials and encourages early perennials
Hajny, Hartnett and Wilson 2011 – 2 and 10 year study of fire effects on Smooth Sumac
Fall (November), Winter (January), Spring (April), Summer (July); backfires and head fires
Part 1: Effects of seasonality
-all treatments had dramatic stem mortality (80 to 99%)
-summer burns had fewer resprouts post treatment
-but lower mortality due to fuels (no net loss)
-spring had the most resprouts
-fall had no significant change in resprouts
-fire increased plant size and fruit set compared to unburned
-thus no reproductive cost to sumac (resprouts or fruit)
-summer and fall had the lowest seed set and lowest resprout
-winter and spring had highest (>double fall or summer) 2 year study.
-suggests that summer burns could be the best management for sumac
-10 year data demonstrates dramatic increase from summer burns
-long term, summer burning is the worst scenario for stem density
-In terms of resprouts, new sprouts and seed production in long term study
-spring burns showed most dramatic increases overall (worst option)
-fall burns showed most decrease (best option)
-winter was second best option
-summer was third best option
Part 2. Fire Intensity (backfires vs. headfires)(no summer component)
-spring backfires resulted in higher density of stems than
fall or winter backfires or head fires
-backfires and headfires in all seasons but winter and fall
resulted in increased sumac
-fall backfires resulted in zero population growth
-winter backfires resulted in the only decrease
Given the long-term results, the lower number of stems and seeds produced, the stable population growth and suitability to herbaceous grass and forb diversity and productivity (from other studies), low intensity fall backfires provide for the greatest biodiversity and ecological function.
Minor disagreement – whether late spring increases productivity of C4 grasses
-Contemporary research demonstrates no difference between seasons
-Older research demonstrated that late spring was best but for production of quality for
forage, not biodiversity(weeds)
Minor disagreement – seasonal effects of fire on woody plants
-Most show nothing deters established woody plants in the long-term
-Others show that late spring and growing season fires increase woody plants like sumac, blackberries and dogwood.
There is unanimous agreement (Abrams and Hulbert 1987; Heisler et al. 2003; Briggs et al. 2005; Lett and Knapp 2005; Bidwell and Engle 1992; Engle and Bidwell 2001; Towne and Kemp 2008; Dacy and Fulbright 2009; Owensby and Anderson 1967 and Anderson et al. 1970; plus above) that…
-burning increases productivity compared to not burning (but maybe not vs. haying)
-after forbs and C3 grasses break dormancy (late February to early March) their rates of mortality increase with the lateness of burning season
-any herbaceous plant that is actively growing will be negatively impacted by fire to some degree
-the degree depends on how advanced it is into growth cycle and fire intensity
-fire does not deter established shrubby plants (maybe winter backfires)
If an herbaceous plant is actively growing, it will be negatively impacted by fire.
Spring fire decreases C3 grasses and forbs, favors C4 grasses and increases shrubby
species recruitment and densities (carries high potential of net loss to biodiversity).
Fall fire increases richness, diversity and productivity of C3 grasses, C4 grasses, and
forbs and doesn’t encourage shrubby species (if shrubs aren’t an issue, winter is next best).
We don’t know enough about summer fire to apply it except under special circumstances
where the collateral damage is understood and acceptable.
Well, that's it. I'd be interested in anyone's thoughts or questions. Thanks!
Abrams MD, Hulbert LC (1987) Effect of topographic position and fire on species composition in tallgrass prairie in north-east Kansas. American Midland Naturalist, 117, 442-445.
Aldous AE (1934) Effect of burning on Kansas bluestem pastures. Kansas Agricultural Experiment Station Bulletin 38.
Anderson KL, Smith EF, Owensby CE (1970) Burning bluestem range. Journal of Range Management 23, 81-92.
Bidwell TG, Engle DM (1992) Relationship of fire behavior to tallgrass prairie herbage production. Journal of Range Management 45, 579-584.
Briggs JM, Knapp AK, Blair JM, Heisler JL, Hoch GA, Lett MS, McCarron JK (2005) An ecosystem in transition: causes and consequences of the conversion of mesic grassland to shrubland. Bioscience 55, 243-254.
Dacy EC, Fulbright TE (2009) Survival of sprouting shrubs following summer fire: effects of morphological and spatial characteristics. Rangeland Ecology and Management 62, 179-185.
Engle DM, Bidwell TG (2001) Viewpoint: the response of central North American Prairies to seasonal fire. Journal of Range Management 54, 2-10.
Hajny KM, Hartnett DC, Wilson WT (2011) Rhus glabra response to season and intensity of fire in tallgrass prairie. International Journal of Wildland Fire 20 709-720
Heisler JL, Briggs JM, Knapp AK (2003) Long-term patterns of shrub expansion in a C4-dominated grassland: fire frequency and the dynamics of shrub cover and abundance. American Journal of Botany 90, 423-428.
Howe HF (1994) Response of Early- and Late-Flowering Plants to Fire Season in Experimental Prairies Ecological Applications, Vol. 4, No. 1 (Feb., 1994), pp. 121-133
Lett MS, Knapp AK (2005) Woody plant encroachment and removal in mesic grassland: production and composition responses of herbaceous vegetation. American Midland Naturalist 153, 217-231.
Owensby CE, Anderson KL (1967) Yield responses to time of burning in the Kansas Flint Hills. Journal of Range Management 20, 12-16.
Town EG, Kemp KE (2003) Vegetation dynamics from annually burning tallgrass prairie in different seasons. Journal of Range Management 56, 185 192.
Towne EG, Kemp KE (2008) Long-term response patterns of tallgrass prairie to frequent summer burning. Rangeland Ecology and Management 61, 509-520.
Towne EG, Craine JM (2014) Ecological Consequences of Shifting the Timing of Burning Tallgrass Prairie. PLoS ONE 9(7): e103423