Fall Fire for Effective Management: Is There a Seasonal Affective Disorder in Prescribed Fire?

Over the years, I have noticed several intriguing responses in natural communities to seasonal variations in prescribed fire. I've discussed this issue with many folks, have initiated several interesting, though largely unsatisfying, email strings on the subject and wrote an essay on the subject in a recent Missouri Natural Areas Newsletter. Through this process, I have found that the increasing utilization of spring burning, as opposed to the more historically relevant application of autumnal anthropogenic burning, is primarily driven by convenience rather than ecological soundness. And, that the assumptions underlying the prescription of spring fire (that either spring fire is harmless or that it is at least better than no fire at all) are potentially more wishful than accurate. Having observed significant losses in plant diversity and subsequent increases in weedy shrubs like sumac and blackberries in areas where spring fire has reigned supreme, my concern has only grown. Steve Buback (Natural History Biologist for the Missouri Department of Conservation), sharing my concerns, suggested we investigate the matter via a summary of the pertinent literature on the subject and present the summary at the Missouri Natural Resource Conference. So, that is what we are doing. On Friday, February 5, 2016 Steve Buback and I will be presenting a talk on the effects of spring and summer prescribed fire compared to fall and winter prescribed fire.

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. 

In summary
  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. 
The End

Well, that's it. I'd be interested in anyone's thoughts or questions. Thanks!

Literature Cited

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


  1. In Towne and Craine 2014, 3rd point for forbs:
    -autumn and fall saw increase in 6 species (average C-value=6.2)
    The difference between fall and autumn is rather confusing!!!

    1. Thanks for catching that. It should be "fall and winter".

    2. Do you think that consistance with the usage of either word is necessary? Already the dates bookending the seasons aren't too clear and certainly change every year...

    3. I tried to only use "fall", but didn't make that decision until a few autumns slipped in. I prefer "fall" because it is more descriptive, though "autumn" is certainly more poetic.

    4. I agree with your decision: a bird head like me needs to be able to grasp concepts with ease...

  2. Did you find any studies that directly contradict your "spring fire bad, fall/winter fire good/better" assertion?

    1. The studies don't offer anything as judgmental as "spring fire bad, fall/winter fire good/better", but rather a "spring, winter, fall, summer fire different" stance. In fact, none of the papers pushed or really even addressed the biodiversity loss. Because it was there and blatant, and because it is the question at hand, I have put a spotlight on it. I've always thought summer burning was a bad idea, but have learned through this process that if you had a C4 grass dominated restoration (as there are many such) a summer fire could be very beneficial.

      That said, despite a thorough search and tons of asking around there do not appear to be any studies to the contrary; just nearly a century of research demonstrating the same phenomena.

  3. In Towne and Kemp 2003, also found that...
    -Cool season grasses increased 22% in autumn and winter; decreased 5% spring
    I assume this is regarding productivity!

  4. Yes. Unfortunately, none of the studies looked at population dynamics or reported much in the way of effects on individual species. So we are forced to assume that the decrease in productivity corresponds to decreases in diversity (which it kind of has to) and richness (which it may not, but probably does). Some one really needs to directly test the hypothesis that spring fire decreases C3 graminoid and forb diversity and richness.

    It is my understanding that the authors of the Konza based research in my post are governed or heavily influenced by the agriculture school as KSU and that they are careful not to shout too loudly about these types of things. A good friend of mine knows Gene Towne really well and has said as much. In fact, because of the Towne and Craine paper, Towne was "let go" from Konza after something like 30 years of employment there for suggesting that spring fire was potentially harmful. He had been trying to say it for decades (thus his earlier work) but he was more or less silenced by the system. I'm glad he finally got the word out, but sad he lost his job over it and that Konza lost a really good manager. It is all rather scandalous.

  5. Towne and Craine 2014
    -Carex spp. increased 8 to 14 percent in autumn and winter
    -Carex spp. decreased from 6 to 1 percent in spring
    Remove the word FROM: it is confusing!

  6. I look forward to the presentation later this week. Based on the studies reviewed and your conclusions, I am wondering why you focus specifically on fall burning rather that using the term dormant season more generally. The studies reviewed and your conclusions generally seem more consistent with the term 'dormant season.' Late September and early October are fall of course, but many herbaceous species are still non-dormant at these times.

    Also, there has been some literature on burn season in long-leaf pine savanna communities. I realize it's not Ozark woodlands, but I figure you'd be interested if you aren't already familiar with this literature. This recent issue of the Southern Fire Exchange newsletter discusses and includes links to a few recent fire seasonality studies that do address fire seasonality effects on under story vegetation. http://www.southernfireexchange.org/newsletters/v5-5.pdf

  7. Thanks for your comments, Quinn. Indeed, "dormant season" is a more apt and direct phrasing. Because they used "fall" in most of the papers, I stuck with it. But, the knowledge that winter burning appears to be as, if not nearly as, beneficial as "fall", "dormant season" is a nice summary term. I'll use dormant season from now on given the clarity it does provide to the topic.

    I've read several papers involving fire in Long Leaf Pine communities. I've gleaned that seasonality is a very different thing in the deep south than it is for the Midwest; with very unique communities defined/initiated sometimes by growing season fires. I think people often confused the kinds of results reported from the south as being possible in the Midwest; proof to me that regional systems should dictate the application of fire. I'm looking forward to seeing your talk on invasives Tan-Tar-A.

  8. Enjoyed reading this summary! Since I had the time right now, thought I would provide you with some of my thoughts and comments as I was reading the post. This is just a perspective as to what someone in the audience could be thinking as you present the material. By no means do I have a strong background in prescribed burning, nor the mid-western flora anymore. I had to break up my overall response into many replies since it was too big to publish it at one time.So here goes:

    Comments and questions on your first section before summary of papers:
    “Through this process, I have found that the increasing utilization of spring burning, as opposed to the more historically relevant application of autumnal anthropogenic burning”
    • So how long ago did this ‘transition period’ from fall to spring burn occur?
    “Having observed significant losses in plant diversity and subsequent increases in weedy shrubs like sumac and blackberries in areas where spring fire has reigned supreme, my concern has only grown.”
    • Yes, its obvious but the negative results of spring fires seem to be LOSS of plant diversity and INCREASE in weedy shrubs (sumac and blackberries)
    “I'll start the talk with a rundown of why people used to burn and why we burn now.”
    • I know this is a summary of papers and presentation, but I am thinking what about ‘natural’ wildfires. When do they usually occur or what season? What is the observed result of a natural wildfire during a specific season on plant diversity or on weedy woody species? I would guess the same but do I really know?

  9. Here is the next part:
    Comments and questions on summary of papers and data:
    Towne and Kemp 2003
    o “woody species showed no change (low density to begin with)”
     What species? Are they ‘weedy shrubs’?

    Towne and Craine 2014
    o “grass biomass remained unchanged from autumn, winter and spring burns”
     Are these C3 or C4 or assumed just a mixture of each?
    o “spring burns reduced biomass by 50%”
     Did the forbs increase in autumn and winter then”
    o “spring burns had increase in 5 species (average C-value = 4.7)”
     Just curious as to what species
    o “winter and fall saw increase in 6 species (average C-value=6.2)”
     Just curious as to what species

    Howe 1994
    o “spring fire hurts early perennials and encourages late perennials/summer fire hurts late perennials and encourages early perennials”
    o Do we consider any of these perennials as ‘weedy species’
    o Do early perennials or late perennials have more of a significant benefit when it comes to plant diversity in this specific area? I guess what I am saying is if you had to choose over late or early flowering perennials what one would you chose and why?

  10. Comments and questions on your summary and ‘put simply’:
    “Others show that late spring and growing season fires increase woody plants like sumac, blackberries and dogwood.”
    o Sumac was highlighted in the paper summaries though not blackberry and dogwood?
    o Also what species of blackberry and dogwood are we talking about?
    “after forbs and C3 grasses break dormancy (late February to early March) their rates of mortality increase with the lateness of burning season”
    o What about perennial forbs and Carex spp. according to T&C, 2014, and early flowering perennials according to Howe
    “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)”
    o The only data presented in the paper comments relating to ‘shrubby species’ was H,H,W,2011on sumac and T&K, 2003 that stated there was ‘no change’ even though there was low density to begin with.

    “Fall fire increases richness, diversity and productivity of C3 grasses, C4 grasses, and
    forbs and doesn’t encourage shrubby species”
    o Seems T&C,2014 stated “Grass biomass remained unchanged from autumn, winter, and spring burns” not sure what that means.

    There you go, hope it added a little to help you prepare!

  11. Hi I'm a forest ecologist by training and I have some questions. Why have people been so determined to burn in the spring? Is it logistics of burning in that season or is it an attempt to kill woody species when they are breaking dormancy? I know that there has been a switch to growing season burns in upland forest ecosystems in an attempt to kill red maple and others in the understory.

    How are dormant season burns historically relevant? Do you mean that they were the practice in the past or is there literature somehowing constructing historical fire regimes? How did often and when did native Americans burn?

    Thanks for the read.

  12. Thanks for your questions. I've questioned many of the fire management folks (in Missouri, at least) and they all say that fire behavior is logistically more convenient in the spring. And, so folks say the fall/winter fires disrupt deer hunting season. Neither reason has anything to do with ecological dynamics.

    There are many historical eyewitness accounts of native American fire practices from explorers, settlers and scientists. With very few exceptions, they documented these fires in journals with fall/winter dates and/or make comments like "their practice of burning after the first frost/after leaves drop".

    A colleague of mine knows an Environmental History professor that has been poring over old medical doctor's journals from east-central Kansas; early to mid 1800's. These doctors documented daily weather and included native American fires. They are almost exclusively Oct-Nov.

    Hope that helps.


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