Last week, we conducted two small prescribed fires at our Platte River Prairies (25 acres and 12 acres). Well, when I say WE, I mean the crew of Nature Conservancy and partner staff, who actually did the work. My job was to just photograph the event for educational purposes, including this blog post, which is intended to explain what summer prescribed fires look like and why they are used.
Fire photographer is definitely my favorite role on a prescribed fire. The stress level is very low compared to my usual role of burn boss. Instead of managing a complex operation consisting of numerous crew members and lots of potential risks, I pointed a camera at those people and pushed a button many times. I also flew a drone around a little. Low stress.
So – why would we burn a prairie in August? Also, do prairies even burn when they’re that green? Both are good questions. The answer to the second is yes, as long as there is enough dry vegetation from previous years’ growth to fuel the fire. The above video shows what one of the prairie burn units looked like about an hour before we lit it on fire. The image below shows a cross section of that prairie’s vegetation – see the layer of dead brown grass at the base of those green plants? That’s what carried the fire.
The ‘Why?’ question is more complicated. We had a couple objectives for last week’s burns. One objective was to experiment with growing season fires as a way to control woody encroachment by eastern redcedar, dogwood, and other shrubs and trees. We, like many others, are struggling to understand and control the flood of woody plants that can swamp out prairies.
In some places, summer fires apparently do a decent job of setting back re-sprouting deciduous species like dogwood and sumac and killing non-resprouting species like eastern redcedar. My limited experience with summer burns has not given me a lot of confidence that mid-to-late summer fires do much long-term damage to deciduous shrubs in this part of the state, but I need more data points. Plus, I don’t really have a lot of better options to suggest.
A second big objective was to use a combination of summer fire and cattle grazing to diversify both the habitat (vegetation structure) and the plant composition of the two prairies. Last week’s burns should have a pretty strong negative impact on the vigor of warm-season grasses like big bluestem, Indiangrass, and switchgrass, which were just beginning their flowering period. It will also have similar impacts on wildflowers that haven’t yet wrapped up their growth cycle for the year. The temporary suppression of those species will open up some space for other plants who don’t normally compete well with those big plants.
In addition, both prairies we burned have cattle in them, and as the new burned patches (which make up small portions of each site) start to green up, cattle will quickly shift their grazing to that lush green growth and start grazing it hard – especially the grasses. In this case, we expect much of that grass to consist of fall-growing (cool-season) species, including lots of smooth brome and Kentucky bluegrass. Without grazing, summer fires around here tend to shift the balance of power to those invasive grasses, which can then crowd out the wildflowers we’re trying to encourage. Cattle grazing can negate that shift of power by keeping those invasive grasses short, giving wildflowers a break from competing with both warm-season and cool-season grasses, and allowing them to really thrive for a year or two.
Both these prairies are being managed with patch-burn grazing, and both have patches that were burned back in the spring of this year. Cattle have been grazing those burned patches intensively, but should now shift at least some of their attention to these summer burn patches for the rest of this year and until more patches are burned sometime next year (probably early or late spring). The combination of spring-burned/grazed patches, summer-burned/grazed patches, patches recovering from previous burns/grazing, and fully recovered patches creates a broad array of habitat structure for prairie animals – vertebrates and invertebrates. It also creates a wide variety of growing conditions for plants, each of which favors a different subset of the plant community. The result is the kind of shifting mosaic of habitat and biological diversity we’re shooting for.
Like most prescribed fires, our summer burn operations began by lighting a backing fire on the downwind edges of the burn unit. A mowed firebreak helped to keep the fire from being able run downwind like it wanted to, and crew members sprayed water on the mowed grass and the edge of the fire to further control it. Since they couldn’t move downwind, the flames just backed slowly into the wind, creating an ever-widening zone of black ash. Because all the grass within those black zones burned, the zones created effective barriers to the head fire, which was ignited later and extinguished itself when it ran into the black.
Once there was a solid line of black along the entire edge of the burn unit, crews started lighting into the wind, still following the edges of the mowed fire breaks. A flanking fire has a little more energy to it than a backing fire because the wind can push it a little more, but because the flames aren’t being pushed directly into the break, they are even easier to control than they are on a backing fire.
Once both the backing fire and flanking fires were lit and nice wide black zones were created, the crews finally lit the upwind edges of the fire and let the wind push the flaming front (head fire) through the unit until it hit the black zones and put itself out. The speed of that head fire is much slower on a summer fire than on a spring fire, but the fire still moved steadily.
The head fire is what consumes the majority of the prairie inside the burn unit. It’s also the hottest and most showy part of the fire.
Most photos you see of prescribed fires come from the head fire. That’s because the flames are most spectacular at that stage, but also because crew members have time to pull out their cameras to photograph the fire. The hard work to create the black zones downwind is all done, so all that’s left is to watch the head fire as it makes its way to the black and dies from a lack of fuel. Head fires during a summer burn tend to be less spectacular than those seen during the dormant season when all the vegetation is dry, but we still saw some pretty intense flames last week.
Importantly, there are some obvious negative impacts that can come from summer fires. You may have read the post I wrote a couple weeks ago about the tradeoffs involved in the use of any prescribed fire. Because you can go back and revisit that post, I’ll avoid repeating everything I said back then. However, it’s worth re-stating that summer fires – as with any other prescribed burns – need to be planned thoughtfully to avoid wiping out entire populations of potentially vulnerable species of plants or animals. Leaving lots of unburned refuges around the burned area – and, ideally, within it too – is critically important. A summer burn should always cover a relatively small proportion of the total prairie habitat within a management unit unless there are clear and well-considered objectives that outweigh the ecological risk of larger fires.
Crew safety is also an important factor when burning in the summer. When air temperatures are hot and the fire is even hotter than that, crew members can easily get overheated. Heat exhaustion and dehydration are constant threats that need to be monitored and mitigated for. That’s another good reason to stick to smaller burns that don’t take all day. In addition to the heat, smoke from summer fires is also intense, so making sure nobody spends long periods of time in that smoke is really important.
That said, burning in the summer is a way to extend the window for getting prescribed fire done throughout the year, which can be really helpful when fire is a major tool for managing prairies. Our spring fire seasons are often windy and wet (except when they’re windy and dry), and we rarely feel like we get enough burning done. Burning in the summer and fall can help make up for that – recognizing, obviously, that those burns will also have some different impacts than spring fires.
Most importantly, we need to keep learning about the effects of summer fires on woody encroachment, plant composition, and habitat diversity. There are too many challenges in prairie conservation for us to rely only on a few old strategies. We’ll be watching the results of last week’s burns closely and learning from whatever we see. Then we’ll adjust as needed and keep moving forward. Just like always.