Annual Burns
There are complexities and misconceptions surrounding “annual burning.” Some of this comes from the different meanings of “annual.” Pioneer accounts often lack clarity, not explaining if the reported fire was burning in the same area or if fires were simply observed somewhere on the landscape each year. Historical accounts may have been misinterpreted or misreported.
One cannot transpose past “annual” burning with present practices because of several issues. Historically, fire moved across landscapes in a patchy and less predictable manner than prescribed burns practiced today in our fragmented landscape. Additionally, early studies on fire regimes focused on tall C4-dominated tallgrass prairies and grazed pasturelands, where grasses were more desired than forbs.
Annual and frequent fires that reduce biodiversity. This is well researched. See this list of studies on loss of biodiversity. Burning the same area each year favors a near monoculture of tall C4 grasses, while reducing forb diversity.
So, why do people burn the same area every year? Some of the more common reasons are that it makes the landscape look nice, like spring cleaning or it’s what was done in the past. When questioned, few understand the full ramifications of annual burns and when information is provided, few want to believe it. Human perspective is a funny thing.
Ecosystem and Region Specific
Tailoring management practices to specific ecosystem conditions seems essential for long-term restoration success. Extrapolating annual burning from one region to another isn’t practical, especially when comparing systems like the tallgrass prairies of Kansas or the longleaf pines of Florida to more diverse and moisture-rich systems like those in the Driftless Area. The differences in climate and weather make a one-size-fits-all approach incompatible.
Annual burns tend to favor the tall C4 grass species (sometimes referred to as the bully grasses) at the expense of forbs. It’s also important to note the significance of landscape diversity and woodlands and savanna composition in influencing fire’s ecological impact. This supports the idea that smaller, fragmented ecosystems—like many found in the eastern U.S.—require a more nuanced fire management strategy.
“The effects of fire on vegetation, especially natural grassland, is indeed a controversial subject. This is not surprising when one considers the complexity of prairie communities and the versatility of the component plants. Advantages or disadvantages of burning depend on a myriad of factors peculiar to a given location, time, and desired objective” (Ehrenreich 1959).
6 Effects of Annual Burns
Annual burns may provide short-term benefits for managing vegetation (Ehrenreich 1959), their long-term cumulative effects may harm soil microbes, reduce biodiversity, alter nutrient availability, compromise soil health, and destabilize the ecosystem. The concerns raised by the various researchers noted in this paper align with the growing evidence that repeatedly removing the decomposing plant materials is not optimal for ecosystem functioning and can lead to negative consequences for the ecosystem.
The nuanced response of ecosystems to fire demonstrates that annual burns can have long-lasting, detrimental effects on soil health, plant diversity, and ecosystem function. It suggests that a tailored approach, with variable fire frequencies, is necessary to avoid the negative cumulative effects while still utilizing fire as a management tool.
Let’s explore further some of these consequences.
Harms Soil Microbes
Long-term annual fires are devastating to soil microbes (Alcaniz et al. 2017, Garcia and Rice 1994, Knicker 2007, Shaw et al. 2015, Ojima et al. 1994). Soil microorganisms depend on the decomposition of organic matter for food and nutrients. Long term annual burn studies discovered the microbes in the soil were starved of fresh inputs reducing mineralization by 1.5 times, leading to diminished soil health and fertility (Soong et al. 2015, Ojima et al. 1994, Killham 1994). This additional stress can make allelopathic chemicals in the environment more potent (Pedrol et al. 2006).
Source: Vecteezy
Reduces Biodiversity
Annual and frequent burns negatively affect biodiversity. The dominance of tall C4 grasses after frequent and annual burns suppress forbs that many vertebrates and invertebrates depend on for food, cover, rearing their young, and habitat (Collins et al. 1998). For reptiles and amphibians, a small decline such as 1% each year, can have a cumulative effect that can “push some populations beyond recovery” (Briggler 2014).
Destabilizes the Community
Annual fire destabilizes a community (DeBano et al. 1998). Our ecosystems work continuously to maintain balance. Extreme fluctuations, like annual fire, that don’t give the ecosystem time to adapt, threatening its functional stability (Christensen et al. 1996, Odum 1969, Landi et al. 2018).
Forbs Decrease
Annual burns have direct negative effects on forbs in several ways (Briggs and Knapp 2001). This is mostly through the increased domination of the sod-forming warm season grasses but there are several other reasons. The reduction of forb diversity affects pollinators and herbivores that rely on them (Brown et al. 2016). Native seedbanks are reduced, and seedling recruitment is not favored in annual fires (Simpson et al. 2016, Hartnett and Fay 1998). Forbs that are active later and flower mid- to late summer show reductions in growth, flowering, and vegetative reproduction (Hartnett and Fay 1998) and clonal forbs such as Canada goldenrod love disturbances and can increase (Benson et al. 2006, Becker 1989, Kucera and Koelling 1964).
Indian grass (tall C4 grass) monopolizing the forbs
Changes Nutrient Availability
Changes nutrient availability in the short term and the long term (DeBano et al. 1998). All extractable soil nutrients except copper were decreased; P, Na, Fe, Mn decreased significantly (Brye 2006, Alcaniz et al. 2017). Annual burning causes significant reduction in soil organic C, lower microbial biomass, and higher C:N ratios in SOM, and lower soil nutrient levels (Ojima et al. 1994, Collins and Steinauer 1998, Brye 2006, Kitchen 2009, Dai et al. 2006, Alcaniz et al. 2018, Knoepp et al. 2005). Annually burned soils have higher soil respiration, causing losses of sequestered carbon to the atmosphere (Soong et al. 2015, Kitchen et al. 2009, DeBano et al. 1998). Over time, this increases the atmospheric carbon, which is concerning in the context of climate change.
Compromises Soil Health
Annual burning can compromise soil health, negatively affecting soil properties and structure (Alcaniz et al. 2017). It can lower and/or remove the soil organic matter (SOM) and disrupt the natural decomposition process of the litter layer, leading to a cascade of effects (Rice et al. 1998, Ehrenreich and Aikman 1963, Neary et al. 1999, Soong et al 2015a, 2016, DeBano et al. 1998, Ojima et al. 1994). It increases desiccation possibilities which reduces electrical conductivity. The soil can no longer sponge up moisture and be a reservoir during times of drought (Thorp 1948, DeBano et al. 1998, McAllister et al. 1998, Collins and Steinauer 1998, Fynn 2004, Daubenmire 1968). This soil drying and the decrease of essential plant nutrients can increase salinity and decrease soil infiltration, which can have a detrimental effect on plants (Alcaniz et al. 2017, Brye 2006, DeBano et al. 1998, Ehrenreich and Aikman 1963, Daubenmire 1968).
Do Annual Burns Decrease Invasives?
The indirect effect of fire on invasives could be the reason some feel it’s effective. Frequent and annual burns favor sod-forming C4 grasses at the expense of native forbs. As native forb diversity is lost to these outcompeting grasses, then indirectly, invasive forms could also be outcompeted. In my research on various non-grass invasive species, it is seldom found that fire had a direct positive effect.
The literature consistently shows how fire alone, even with high frequency, doesn’t effectively control resprouting woody plants and it is less effective at eliminating extant woody plants than it is in preventing establishment, even when burned annually (Heisler et al. 2004). After years of annual burns some trees and most of the resprouters persisted in the tallgrass prairie (Hartnett and Fay 1998, Briggs and Gibson 1992). Much of the early research on this was done in C4-dominated grasslands with trees that aren’t as prolific resprouters (e.g. elm, cherry) as our modern-day invasive, woody shrubs (e.g. honeysuckle, multiflora rose). Control is most effective when fire is coupled with other techniques such as cut and treat.
It is unlikely that any single management tool will be completely effective in restoring native plant communities because the invasive species use many different mechanisms to succeed .
Conclusion
The sources cited span decades of research, from foundational to more recent studies. Hard to separate the various aspects of restoration as one effects the other. It’s necessary to study each one then work toward management strategies using this information. If only one aspect is considered, the restoration effort will be lopsided. For example, using annual and frequent burns changes the plant composition by increasing C4 grasses and decreasing forbs. This changes the insects/pollinators/parasitoids and changes the quality of the soil which changes the soil ecosystems/inverts/microbes.
The goal of restoration is to produce resilient ecosystems that are resistant to invasion, capture and use resources efficiently, they must contain biological complexity to function effectively (Heneghan et al. 2008). This can only be done if we stop using studies of tall C4 grasses to compare to high-diversity remnants. The resiliency fallacy stems from thinking the tallgrass biome with its ability to handle annual fire and harsh environmental conditions can be extrapolated to all midwestern habitats (Ladd 2011).
Homogenous management leads to homogenous prairies. A diversity of tools and management strategies is imperative (Helzer 2010). Fear of, or belief that, changes to the fire regime will change an ecosystem to something else and habitats and species will be lost can be put aside (Myers 2006). While we don’t know what the historical fire regime was, we know that it no longer applies. Our ecosystems are not the same as they have been sectioned up and reduced in size. Additionally, much has happened in 100+ years that the fire regime was lost long ago. It is unlikely sufficient ecological information exists to be certain of the ecological effects of any prescribed fire. “Putting all eggs in one basket, by imposing the same, regular fire regime across a landscape, will be an unwise strategy” (Whelan 1995). When using fire in your ecosystem, it’s important to maintain a balance between fire use and soil conservation and biodiversity.
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