At the Platte River Prairies, we conduct prescribed burns for various purposes. Some fires are intended to kill eastern red cedar trees or to suppress cool-season invasive grasses. Other fires are aimed at removing thatch and old vegetative growth – creating lush regrowth that concentrates cattle grazing in one portion of a prairie. For each objective, we prescribe a certain set of outcomes that need to be met in order for a burn to be successful, and a parallel set of conditions (especially timing and weather conditions) that will get us to those outcomes. If we’re just trying to remove most of the old dead growth from a prairie, we don’t need the same kind of fire intensity as when we’re trying to kill cedar trees. If we’re targeting cool-season grasses (and won’t be following up with grazing), we try to burn about the time those grasses are starting to flower.
On the last day of March this year, we assembled a crew that combined our staff with employees from the Central Platte Natural Resources District and got ready to burn some hilly sand prairie. Our objective was to remove at least 75% of the thatch and old growth from the burned area so subsequent cattle grazing would be focused in that burned patch while the remainder of the prairie went largely ungrazed. The forecast had predicted pretty high relative humidity readings, but we thought we’d be ok as long as we didn’t have overnight fog or mist. Unfortunately, on the morning of the fire, the grass litter along the ground was more damp than we’d hoped and since the sun was hidden behind clouds it didn’t seem likely that litter would dry much. After considerable discussion and delay, we finally decided to conduct a test fire in the downwind corner of the burn unit to see what kind of burn results we’d get before deciding whether or not to burn the entire 70 acre unit. We also figured it was an opportunity to learn more about how fire behaves under humid conditions. At 1 pm, it was 46 degrees F, 71% relative humidity, and we had winds at about 10 mph.
The test fire was definitely instructive. The upper portions of grass plants were sufficiently dry that they burned fairly well, but the dampness along the ground made the fire creep along very slowly, even when pushed by the wind. Following along behind the fire, I was kicking away ash to find that quite a bit of litter was still unburned and covering the soil. After we burned an area about 40 x 40 feet, we extinguished the fire and had a group discussion. The grass was burning well enough that we could probably burn the unit, and much of the area inside the firebreaks would ignite and turn black. On the crests of the hills where vegetation was dominated by bunchgrasses and there was considerable bare ground beneath plants, we’d probably get a pretty complete burn. However, in lower areas where there was more dense vegetation, including some cool-season invasive grasses, we didn’t feel like it would burn very completely at all. In total, we didn’t think we’d reach our goal of removing litter from 75% of the area. Importantly, the areas that wouldn’t burn well (and thus wouldn’t attract grazing) were the ones we most wanted cattle to graze (to suppress invasive grass growth). After talking through our options with the whole crew, we decided to postpone the burn until we had a day with better conditions.
It’s never an easy decision to call off a burn when you’ve got crew and equipment on site. As a burn boss, I’ve had to do that multiple times, but usually when we’re worried about safety because the weather conditions are too far on the hot, dry and/or windy side. In this case, there were no extraordinary safety concerns, but every fire comes with risks to people and property. It never makes sense to burn and not achieve the desired result. We needed near complete consumption of the dead vegetation to attract cattle grazing and carry out our management plan for that season. Since we weren’t going to achieve that, we didn’t burn.
As it turned out, we only had to wait four days for another opportunity to burn that unit. On April 4, most of the same crew members assembled and we set up to try it again. Our weather conditions at 11 am weren’t all that different from our previous attempt (46 degrees F, 65% RH, and 12-15 mph winds) but the grass litter was much drier, and while the sky was cloudy, the clouds were more patchy and the sun was even popping through once in a while.
Our downwind firebreaks were two gravel roads, so it didn’t take long to get those lines lit and blacked out. At that point, however, I walked out into the black to see how much litter consumption we were getting. While it was much better than the previous week, there were still some unburned patches. Since we had solid firebreaks, we paused ignition to wait for everything to warm up and dry out just a little more. About a half hour later, relative humidity had dropped nearly 10% and the temperature had risen about 5%. We restarted ignition and pretty quickly finished up the rest of the fire.
Here are a couple timelapse videos of the lighting of the “flanking head fires” toward the end of the burn. They are a little jumpy (sorry) because I was just hand-holding my phone and taking repeated photos, but it shows how different the fire behavior was from the slow creeping fire of our first attempt 5 days earlier. If you can’t see the videos, click on the title of this post to open it in a web browser or click on these links: Video 1, Video 2.
I’m glad we waited for more favorable conditions to burn this unit. We wouldn’t have accomplished what we needed to on the first day, and though it was hard to turn down a potential burn opportunity and assembled crew, I think we made the right call. As it happened, we didn’t have to wait long for a better day, and we got what we wanted out of that fire. At the same time, I’m also glad we decided to try a test fire on the first day. It turned into a good learning experience and fodder for fruitful discussion among the crew. The whole situation was a good reminder that while we can achieve many important objectives through prescribed burning, it isn’t a toy we play with for fun. Instead, we want to burn only when we can do so safely, and when we can achieve clear and specific objectives.
If you want to learn more about how we combine prescribed fire and grazing to manage for habitat and species diversity, you can read more here.
Just did our burn (4 year) two weeks ago. The cool weather grasses are now showing. Using a grass specific herbicide to get rid of them and Garlon to kill volunteer trees that are now much easier to see.
Thanks for the timely blog post, Chris! I’m looking at doing two small burns tomorrow morning before the wind cranks up, but RH is supposed to be at 62% at ignition time. I’ve never burned with that high of RH so I wasn’t sure if things would even burn (brome/crested wheatgrass pasture. 1 year deferment). Based on this post, I think we’ll give it a try.
Thank you for the post. It is a good reminder to me to be clear about my purpose as I prepare for spring burns on my natural areas. Nicely written.
Great post and excellent illustrative videos Chris!! Saying “not today” is always a hard thing to do, but it is also one of the most important things to do when it is necessary! A good crew will never be overly disappointed when they are sent back to wait until another day – when conditions are better for safety and/or meeting burn objectives.
I hope you have some fun burning. When burning you are a virtual god. You take the power of nature stored over a number of seasons and release it deliberately to shape your environment. Your work determines if prairie or trees will win. You determine where herds of very large animals will graze in the coming season. This shapes your environment right down to the composition of wildflowers. You’re a burn boss, is there any better job.
Do you take into account the effect fire has on the insect (and any vulnerable fauna) population?
Yes, absolutely. It’s one of the big reasons we only burn a relatively small percentage of each prairie. While they’re far from the only vulnerable animal, we’ve been tracking regal fritillary populations and how they react to our management, and their populations seem to do well under our (modified) patch-burn grazing regimes. If you’re interested, you can read a report of our results here: https://outdoornebraska.gov/wp-content/uploads/2015/12/NLP_Project_EnhancingDiversityAndAtRiskSpeciesHabitatInDegradedPrairieLandscapes_2010.pdf
Thanks for posting a link to the report. Long, but interesting. Hope you don’t have to write reports like that too often! Do you still routinely set aside areas within patch-burn sections where cattle are excluded to promote flowering and seed production by some of the forbs that cannot do so under even a light grazing regime, like the milkweeds you mentioned?
Yes, we still do that, and no, I don’t have to write long reports like that very often. I’m glad I wrote that one, though – it’s a good one to refer people to when I don’t want to go through a lengthy explanation of what patch-burn grazing is all about!
Hot dirty work
Chris, this is a perfect follow up to what we saw in person the day of the first burn attempt. In fact you can see our car in one of the shots; which we would have moved as soon as it was clear the burn was proceeding. Not knowing the management objective the day of had me wondering, and a bit frustrated, as to why you didn’t proceed but now all is clear. A fine example of having an objective and based on current conditions or new data, adjusting your work. Although, I must admit the highlight of our trip would’ve been sitting at the house and watching the flames roll over the sublime sand hills we’d walked the day before.
I’ve been involved with prescribed burning for 26 years at my current job, and I’m glad you pointed out the fact that we still can learn from fires set under previously untried conditions. Indeed, I continue to learn from fires under what seem, from years and fires past, to be familiar conditions. Excellent post and responses to comment/questions, Chris.