Extension by Brian Mealor
Wildfire, Weeds and Worry
By Brian Mealor, Extension Weed Specialist
It seems like everywhere you look this summer, something is burning. An open winter followed by a hot, dry spring has apparently led to highly flammable rangelands in Wyoming and all western states.
As of my last unofficial tally, over 260,000 acres have already burned in Wyoming as we move into August. The immediate effects of these fires are relatively easy to see: tragic loss of homes, buildings and other infrastructure, loss of grazing and timber resources and sometimes the loss of lives. The longer-term natural resource impacts may not be as clear-cut and easily observed because vegetation response to fire can depend on multiple factors specific to each situation. Unfortunately, because of this variation in fire response, my answer to many of the fire-related vegetation questions I have received lately has been, “It depends.” But on what does it depend, and are there any generalizations we might make concerning vegetation response to wildland fires?
One consistent generalization regarding fire in our area is that the big sagebrush subspecies do not resprout from rootstock following fire. This may seem like an insignificant observation, but it is important for a number of reasons. If big sagebrush cannot resprout from roots, then reestablishment from seed is necessary. Natural establishment of sagebrush from seed is “episodic,” meaning that the conditions necessary for good seedling establishment only happen every so often. Some estimates for our area indicate that there may be only one in three to 10 years, which are good for sagebrush establishment. Partly because of this inability to resprout and difficulty in establishing from seed, fire-related impacts to sagebrush communities may last as long as 25 to 125 or more years. In areas where sagebrush plays an important role for catching snow, wildlife habitat and for many other reasons, large-scale wildfires can play a detrimental role, especially if the fire burns consistently across large blocks of the landscape. Inconsistent fires that leave some portions of the landscape unburned, resulting in a mosaic of different vegetation, can lead to increased diversity and productivity for both wildlife and livestock in the area.
Hot, slow-burning wildfires may kill existing perennial vegetation rather than just removing aboveground biomass. In such cases, a decrease in plants taking resources from the soil coupled with a flush of nutrients made available from burned plant material may make burned areas highly susceptible to invasion by problematic weeds. Weedy species that are adapted to fast growth in high-nutrient sites can rapidly establish and reproduce under such high-nutrient availability/low competition situations.
Cheatgrass, or downy brome, is probably the most well known of such invasive weeds. It responds quickly to freely available nutrients in the soil and may produce thousands of seeds per square yard. As an annual, it does not waste energy putting down long, well-developed root systems, but instead grows quickly, produces seed, then dries out – sometimes leaving highly flammable fine fuels that can lead to further fires. This attenuation of natural fire regimes in sagebrush grassland systems is one of the worst impacts of large-scale cheatgrass infestations. More frequent fires leads to an even lower probability of sagebrush reestablishment and some areas which were once productive sagebrush grasslands are now annual grasslands dominated by cheatgrass and populated largely by species that can withstand frequent fires.
Do fires in sagebrush grasslands always lead to cheatgrass-dominated rangelands? The answer is no (I know I said earlier the answer is, “it depends,” but since “always” is an absolute, I am more comfortable with a definitive answer, but I digress). Areas with a high population of cheatgrass before the fire have a higher probability of cheatgrass infestation following fire because some of the cheatgrass seed may have survived the fire and are waiting to germinate and grow as soon as conditions are favorable. In some cases, an increase in cheatgrass is not immediately observed after the fire, but several years later if perennial vegetation does not reestablish sufficiently. One hypothesis is that a high portion of cheatgrass seeds may be killed by the fire, and it takes a few years until the population can reach outbreak-type proportions. Many of our native species evolved in systems where fires occurred somewhat often, if not frequently. If the fire is not hot enough to kill perennial vegetation and precipitation comes at a favorable time, then fire may lead to a rejuvenated native grass community, which can serve to limit invasion by undesirable species like cheatgrass.
So, as land managers, should we immediately go out to the hundreds of thousands of acres and begin planting sagebrush and spraying herbicides to control weeds like cheatgrass? I wish I had a definitive answer for that question, but I do not. Immediate concerns should perhaps focus on critical areas where soil stabilization is required to prevent erosion and improve watershed stability. Recent moisture at some locations may lead to an increased potential for recovery of existing plant communities. Sites highly susceptible to cheatgrass dominance (south facing slopes with shallow soils, areas where cheatgrass was problematic prior to fire, etc.) should be monitored for vegetation recovery and cheatgrass increase, and appropriate control actions should be implemented if problems arise. Overall, a strategic approach to post-fire vegetation management including diligent monitoring should improve our chances of success.
Brian A. Mealor is an assistant professor and weed extension specialist at the University of Wyoming and can be reached at firstname.lastname@example.org. More information about ongoing weed science research at the University of Wyoming can be found at weedcontrolfreaks.com.