I was able to help with a couple prescribed fires at our Niobrara Valley Preserve this spring. The two burns occurred under very different weather conditions – especially with respect to relative humidity. Each fire burned well, consuming the majority of dried Sandhills prairie vegetation within its boundaries, but we are waiting to see whether the two were equally successful at meeting our primary objective – killing eastern red cedar trees.
The first burn took place on a bright sunny day when relative humidity was near the low end of our weather prescription (the range of weather conditions we felt were acceptable for both safety and to meet objectives). We were trying to burn about 1,000 acres of Sandhills prairie that spanned the border between The Nature Conservancy and our neighbor to the south. Chad Bladow, our burn boss, had some difficult decisions to make. We had a large crew – numbering about 30 people or more – that included staff from several organizations and nearby ranches. However, by the time we were ready to start the burn, the relative humidity had already dropped to about 30%, considerably lower than the forecast said it would be by late morning.
Chad decided we would do a small section of the larger burn unit – about 3 acres – and see how things went, before starting on the larger burn. By the time we finished that three acres, the temperature was in the mid 70’s and relative humidity was in the mid-20’s, and still dropping. Despite fairly light winds, Chad made the smart call to hold off on burning more until later in the evening when temperatures started doing down and relative humidity starting going up. I had to leave before that second ignition started, but the crew ended up burning about 500 acres that evening, and finished the remaining 500 acres a few days later, under similar dry conditions.
The second burn unit, consisting of about 750 acres, was burned under very different conditions than that first 1,000 acres. Skies were overcast and our Platte River Prairies crew drove through several hours of rain as we made our way north to the Niobrara River to help with the fire. When we arrived, Chad said it had very lightly sprinkled, but a nearby rainstorm had mostly missed the area. However, the temperature was cool (50’s) and humidity was pretty high (60’s, but predicted to drop into the 50’s). We ended up burning with a crew of about a dozen people and burned about 750 acres of Sandhills prairie in about 4 hours. Relative humidity readings during the fire started in the high 50’s and were at 64% when we lit the final head fire. The temperature dropped throughout the burn and ended up at about 45 degrees F.
Fire behavior and risk of escape was very different between those two burns. During the hot and dry burn, the grass burned quickly and easily, and we had to keep a sharp eye out for the fire creeping or jumping through or out of our mowed fire breaks. It took a lot of people, equipment and water to make the burn go off safely – which it did. Even so, we stopped burning during the hottest and driest part of the afternoon, choosing instead to delay until evening when relative humidity started to rise again.
During the second burn, the fire moved very slowly across the unit and we had to do a lot of interior ignition to ensure that most of the grass across the site burned. Containing the fire was much easier, and we felt pretty comfortable using a much smaller crew than we did on the first fire, despite having to patrol several miles of burn breaks around the perimeter. A narrow wet line (band of water sprayed along the edge of the mowed break) was sufficient to keep the fire from leaving the unit, and we needed to use very little additional water from the multiple vehicles patrolling the boundary.
If the objective of our burning at the Niobrara Valley Preserve was simply to turn the ground black within the fire breaks, both burns were equally successful. The second fire, though, needed far fewer people and less equipment, and also presented lower risk of the fire escaping containment. Maybe it would be smart for us to plan all our burns for days with high humidity and a chance of rain!
However, our actual objective for those fires was to kill eastern red cedar trees. Both burn units had been cleared of most large trees prior to the burn, so the fires were intended to kill the smaller trees that were missed by clearing equipment or that had sprung up after the clearing effort ended. Having only small trees to deal with gave us a lot more leeway in terms of weather conditions.
Killing large trees with fire in the Sandhills can be difficult because of the relative sparseness of the vegetation. It can take warm, dry, windy conditions to create the heat intensity and flame heights necessary to take out big trees in sparse grasslands – and even then, there are limits to the size of tree we can kill. With smaller trees, a less intense fire can still be lethal, though there are many other factors involved, some of which we’re still learning about – including live fuel moisture of the trees, time of year, speed and intensity of the flaming front when it hits the trees, and more. The amount of dry vegetation present (fuel loading) also comes into play. With higher fuel loading (more vegetation), fires can carry well with tree-killing intensity under cooler and more humid conditions than when there is less fuel to burn.
We are pretty confident that we killed cedar trees with both fires, but we’ll wait a few more weeks before venturing out to assess how many are actually dead at each site. By measuring the percent and size of trees killed with fires set under various weather conditions – including other fires at various locations around the Sandhills – we hope to continue learning about how best to safely and successfully prevent cedar encroachment with fire. Even if our low intensity fire on the cool wet day didn’t end up killing very many cedar trees, we still got some tree piles burned up, which was an additional objective since we weren’t able to get to them over the winter. More importantly, though, we will have an opportunity to document and learn about cedar control under higher relative humidity conditions. If both the humid and dry day fires killed similar numbers of trees, that’s good information too – especially for ranchers and others who have limited experience, crew, or equipment capacity, and want to burn on days with the lowest possible risk of a fire escaping.
I’ve written before about the importance of having clear objectives for prescribed fire and matching the weather and fuel (dry vegetation) conditions to those objectives. Prescribed fire is a useful tool for prairie management, but it also comes with risk, even for experienced crews with plenty of equipment and water. Taking on even minimal risk when there’s no chance of achieving an objective is silly and dangerous. Sometimes, however, it can be hard to know whether or not burning under a particular set of conditions will be successful. On those days, if the burn is carried out anyway, it seems particularly important to come back and assess (and share with others) the effects of the fire. We will try to fulfill that responsibility this summer by measuring the relative impacts of the two fires I mentioned above, as well as some others. Hopefully, what we learn will help guide the way we and others conduct fires in the future, allowing us to better judge the risk/reward trade-offs under various scenarios.
Be safe out there.