As we enter a second year of drought in central Nebraska, I’m starting to hear discussions about whether or not it’s a good idea to conduct prescribed fires when conditions are so dry. I have some ideas about this, but am curious to hear other perspectives as well.
Those of you who read this blog regularly will not be surprised that my answer starts with, “It depends upon your objectives.”
Because prescribed burning is inherently dangerous, it shouldn’t be conducted without a clear rationale at any time, but drought can bring some additional risk. For example, low soil moisture can increase fire behavior, potentially resulting in hotter, faster fires. Short sparse vegetation may burn readily, even areas that wouldn’t carry fire under more moderate conditions. In addition, wooded draws or other landscape features that can usually be counted on to stop a prescribed fire might not do so in a dry year. All of these factors can be safely mitigated with good planning – e.g., using wider firebreaks, being more cautious when the relative humidity is toward the low end of the scale, and increasing the rigor of contingency plans. Nevertheless, it’s silly to take risks without good reasons.
Apart from concerns about safety, many people worry about the impact that burning during a drought might have on prairie plants. Burning off the insulating layer of thatch can increase exposure of soil to sunlight, making the soil even drier than it otherwise would be. That can reduce growth and increase stress for many plant species. Prairie plants are generally well adapted to that kind of stress, however, and most will essentially hunker down until better growth conditions return. A few plant species actually thrive when the soil is too dry for their neighbors to grow. Those opportunists include many annual plants, along with some deep-rooted perennials.
Having said all that, the fact that burning can further stress an already drought-weakened plant community shouldn’t be dismissed. This is where objectives come into play. If you’re a rancher, for example, and forage production is a primary objective, a prairie burn that reduces grass growth can be counterproductive. On the other hand, many Nebraska ranchers use fire to control the encroachment of eastern red cedar. Since prescribed fire is most effective at killing trees when they’re still small, it’s important to burn frequently enough to keep trees small enough to control with fire. Nebraska droughts can often last 5-7 years or more, so putting a prescribed fire program on hold during drought could have serious consequences for tree control efforts.
During many Nebraska droughts, cool-season exotic grasses can increase in dominance because what little precipitation we get in drought years tends to come in the spring, and summers are mostly hot and dry. Under those circumstances, grasses such as smooth brome and Kentucky bluegrass grow vigorously during the early part of the year and deplete the soil of its moisture before native warm-season grasses get started. That gives brome and bluegrass a competitive boost. If you’re using prescribed fire to suppress brome and bluegrass, therefore, it can be particularly important to burn during drought years. While burning in the late spring during drought conditions might dry out the soil and suppress warm-season grass growth, it will also prevent cool-season grasses from being able to take advantage of their moisture advantage.
From a wildlife habitat standpoint, diminished plant growth during drought years can make tall dense vegetation a rare commodity. Burning off what little thatchy cover exists can leave some animal and insect species in a lurch. Here in the Platte River Prairies, we’ve reduced the size of some of our planned spring burns to leave a little extra dense vegetation structure standing – not knowing whether there will be much plant growth this coming season. However, we’re still planning to conduct the same number of burns we typically do because that burning (and often subsequent grazing) is an intrinsic component of our plan for habitat management.
There are many other considerations regarding burning in drought, but few clear answers. For example, some pollinators may benefit from an increased abundance of “opportunistic” plants such as annual sunflowers that respond well to both fire and dry weather. On the other hand, drier soil as a result of burning can reduce the flowering of perennial plants that are important to other pollinators. As another example, a spring fire can usually increase seed production in warm-season grasses – a nice trick for those of us harvesting seed for restoration projects. On the other hand, a patch of prairie we burned at the beginning of the drought in 2012 ended up with stunted growth and very little seed production, and we actually harvested a fair amount of seed from a nearby unburned site to compensate. …It’s hard to know what to do.
As with most aspects of prairie management, deciding whether or how to apply prescribed fire during a drought is a complicated process. There are safety concerns that need to be addressed, and impacts that may both support and conflict with management objectives. While it won’t solve all your problems, having a clear idea of what you’re trying to accomplish with prescribed fire can at least help you navigate those complex issues and make sound decisions.
What do you think? If you’re a prairie manager, how do you adjust your prescribed fire plans during drought?