Pyric Herbivory | The Ecological Interaction of Fire and Grazing

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This photo, taken at the Oklahoma State University Range Research Station in Stillwater, OK, is a perfect example of a prescribed fire influencing grazing behavior. Pyric herbivory results from the interaction of, and feedbacks between, fire and grazing  that occur on landscapes where fire and herds of grazing animals coexist.

First, a little history. The great plains of the United States are a vast and complex series of ecosystems ranging from tall-grass prairie in the plains of Illinois, Kansas, and Oklahoma, to mixed-grass and short-grass prairie in the northern and higher elevations of Texas, New Mexico, Colorado, Wyoming, Montana, and the Dakotas, extending north into Canada. Historians believe that fire events prior to European settlement were primarily anthropogenic (human caused) and more frequent due to the use of fire by native american tribes for hunting, gathering of food, regeneration of natural resources, and warfare. Native herds of bison were thought to follow fire events to take advantage of new growth generated by the removal of decadent (old) plant material, increased sunlight reaching the bud and seed banks of grasses and increasing photosynthesis, and the recycling of nutrients as nitrogen and carbon were released back into the soil from the combustion of plant material.

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Burned area (front) shows plant regrowth six weeks following fire in central Montana

As European settlers expanded westward across the United States, fire suppression became more common. Crops, livestock, and homesteads dotted the landscape and all fire was suppressed to preserve the livelihood of the settlers. Many areas of the Great Plans, Intermountain West, and Great Basin show encroachment of dense woody shrubs and trees into what was once prairie as a result of suppression of fire.

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Photo (Lepofsky et al. 2003)

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Photo courtesy of Iowa Natural History Foundation

Both fire and grazing can be used separately to manage landscapes, but it is believed there are certain feedbacks that occur when the two are used in combination. Grazing pressure – or how much time animals spend grazing on a given area of land and how much plant material they remove – can influence fire patterns. If grazing pressure is heavier, there is less plant material to be consumed by the fire. If grazing pressure is lighter or non-existent, there is more fuel for the fire to consume and burn patterns are more continuous and often burn hotter. Additionally, research shows that herbivores (we’ll say cows for the sake of simplicity) are attracted to recently burned areas and their attraction to an area will decrease as time since fire increases (Allred et al. 2011). In other words, animals are more likely to be spending their time grazing on areas more recently burned and – as time increases since the area burned, the animals will spend less time grazing there.

Here’s why: fire removes vegetation. It consumes it – that’s just the nature of the beast. There are plants which resprout following fire (most grasses and forbs [flowering plants] and many species of shrubs which can regenerate by resprouting/asexual reproduction as well as seed/sexual reproduction) and plants that don’t (coniferous trees, shrubs including some species of sagebrush just to name a few – these plants regenerate by seed/sexual reproduction). Plants that resprout are stimulated by different environmental factors including increased solar radiation (photosynthesis), removal of plant material (like when you prune your rose bush and it resprouts), or increased nutrient availability from the soil. Fire provides all these things and immediately stimulates resprouting plants to generate new growth. This new growth is very tender, high in protein, and high in nutrient content and it’s like candy to cows. Think about it – if you had the choice between a huge pile of dried out, bleached out bread or a tiny little cupcake, you’d probably be all over that cupcake. Same applies to cows.

This attraction produces feedbacks in the system. In recently burned areas, cows will concentrate their grazing on the recently burned areas and keep the vegetation grazed short. They will not spend as much time consuming the older vegetation so it will grow and create heavier fuel loading. When the next fire burns through, it will burn more of the ungrazed or lightly grazed areas than the more recently burned area that has been heavily grazed, and a new “recently burned” area will appear. As I mentioned before, the cows will focus their grazing on the areas with the most tender, highest quality vegetation, which is now the new recently burned area. The area which was previously burned will be given more of a rest since the cows won’t be focusing their grazing entirely on it, and the vegetation will continue to grow and become more dense. This process repeats itself, creating large and small mosaic patterns across the landscape as areas are grazed and burned, and as overlap occurs with grazing and burning patterns. What results is a heterogeneous landscape with plant vegetation at varying heights and ages, which is important for sustaining multiple populations of wildlife species with different habitat requirements.

This photo was taken one day after a prescribed burn conducted on my research site for my Masters degree on the Charles M. Russell National Wildlife Refuge in central Montana. It is very evident here that there are situations in which only part of the vegetation will burn and that has to do with – among other things – how continuous the vegetation is. This site was dominated by sagebrush with a club moss and sparse grass understory which didn’t carry the fire very well. Although this wasn’t caused by vegetation removal due to grazing, it illustrates the patchy mosaic that can happen when fine fuel (grasses, etc.) isn’t present in quantities high enough to carry the fire through the plant community.

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A juvenile great horned owl utilizing a one year old burned area at the same site in 2011.

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How fantastic is all that! Isn’t nature amazing? There are so many processes and feedbacks going on all around us and under our very feet that we may never fully understand. The interaction of fire and grazing is a fairly new  concept (last 20 or so years) and the exact and specific interactions are not fully understood. My goal is to continue research looking into the complex relationships fire has with other environmental variables (grazing, climate, other unknown factors) so we as land stewards and managers will have a broader understanding of the ecological processes surrounding us. If you have any questions, please ask. I will check back and try to answer them quickly!

Allred, B.W., Fuhlendorf, S.D., Engle, D.M. & Elmore, R.D. (2011) Ungulate preference for burned patches reveals strength of fire–grazing interaction. Ecology and Evolution, 1,1-13.

Lepofsky, D., E. K. Heyerdahl, K. Lertzman, D. Schaepe, and B. Mierendorf. 2003. Historical meadow dynamics in southwest British Columbia: a multidisciplinary analysis. Conservation Ecology 7(3): 5. [online] URL: http://www.consecol.org/vol7/iss3/art5/