The Interaction of Wildfire with Post-Fire Herbivory on Arid and Semi-Arid U.S. Rangelands: A Review

Abstract

In the United States, rangelands comprise 30% of the total land cover and serve as a valuable resource for livestock, wildlife, water, and recreation. Rangelands vary in climate and are often subject to disturbances like drought and wildfire. Historic wildfire trends have indicated an increase in wildfire size and frequency, raising societal and ecological concerns about the management of these lands, both pre- and post-wildfire. While there has been investigation into the effects of grazing prior to a wildfire on fire severity and plant mortality, there is limited research related to grazing post-wildfire even though current management paradigms suggest deferring grazing rangeland for two years after a wildfire to avoid additional stress on native plant species. Based on the diversity found across rangeland ecotypes and history with wildfire, the two-year deferment recommendation may need to be reconsidered for some ecosystems. Species found in perennial bunchgrass rangelands like Pseudoroegneria spicata (bluebunch wheatgrass) and Festuca idahoensis (Idaho fescue) may be less susceptible to post-fire grazing than initially thought, necessitating the need for research into different rangeland ecosystems.

1. Introduction

Rangelands are typically characterized by native vegetation and rainfall that is too low or erratic to support crops [1]. This definition encompasses the grasslands, shrublands, savanna woodlands, and deserts that make up almost 70% of the earth’s land cover [2]. In the United States, 30% of the total land cover is considered rangeland, approximately 311 million hectares [3]. Though a common land cover, rangelands are primarily found in the western half of the United States [2]. Erratic precipitation patterns and vegetation make livestock grazing one of the key economic enterprises for the land. The passage of the Taylor Grazing Act (1934) led to the public domain being divided into districts to regulate and prevent overgrazing. Since then, government agencies like the National Forest Service (NFS) and Bureau of Land Management (BLM) have become the primary overseers for public rangelands. In 2020, these two agencies supported 24,250 livestock grazing permittees with 15.7 million animal unit months (AUMs) on public rangelands [4,5]. Though permits are authorized for various livestock species, 92% of public grazing land is used by beef cattle [6]. As a major agricultural industry, beef cattle production in the United States generates approximately USD 34.7 billion each year [6]. Maher and coworkers [7] estimate the value of forage production in western rangelands to be approximately USD 3.8 billion annually.

Livestock production is not the only economic value for rangelands; wildlife, recreation, water, and other ecosystem services bring the total economic value of rangelands to USD 24.5 billion annually [7]. In addition, western rangelands are home to over 3000 wildlife species ranging from large herbivores and small vertebrates to migratory songbirds and pollinators [8]. The high prevalence and value of wildlife on these rangelands puts additional pressure on rangeland managers to balance the optimization of livestock production and maintenance of wildlife habitats [8,9]. In addition to terrestrial species, rangeland conditions have a direct impact on aquatic species habitats [10,11]. Though little research has delved into the impacts of fire on aquatic habitats, it is known that wildfires can mobilize nutrients and increase erosion [11]. Sediment from burned areas makes its way through watersheds, potentially altering habitats (Figure 1) [11].

Rangelands also provide water resources for agricultural, city, and industry use [12]. Precipitation received in these watersheds is often drained into waterways or through the soil into underground springs [12]. The condition of the soil and vegetation communities in these rangelands is crucial when evaluating the quality and quantity of water for human use [2,12]. Strong plant communities and healthy soils reduce the incidence of flooding, erosion and subsequent silting of rivers, and coliform bacteria counts in reservoirs [2].

Rangelands are critical for a variety of resources, making disturbances like wildfires a significant management concern. Because of their importance for livestock and recreation industries alike, determining the most sustainable management for these diverse rangelands is crucial. This review aims to understand how post-fire grazing management influences soil health, vegetation regrowth, biodiversity, and overall ecosystem stability following wildfire events. By evaluating the effects reported in various studies, this review seeks to provide insight into the best management practices that can support sustainable land use and livestock production.

2. Knowledge Gap

To date, most of the literature regarding the ecological concern of herbivory post-wildfire has centered on the arid and semi-arid rangelands of the United States, Canada, and to a lesser extent, Australia. These drier rangelands are more ecologically susceptible to vegetation and/or soil changes in response to disturbances such as wildfire and herbivory. Due to the social and political structure of land management for these areas, the management of herbivory post-wildfire has been a contentious issue for livestock managers and private/public land managers. A search for “ecology” and keywords “beef cattle”, “grazing”, “rangelands”, “post-wildfire”, and “wildfire” was conducted on 28 May 2024 using the Web of Science search engine. This search yielded 164 results. Most research was conducted in the United States and Canada. Therefore, this review focuses on post-fire grazing on arid and semi-arid rangeland vegetation and soil in the United States.

3. History of Fire and Rangelands

Though fires can have detrimental environmental impacts, it has been widely reported that aboriginal burning was practiced in North America for thousands of years [13,14]. In fact, Pyne [13] argues that our current idea of a pristine wilderness is “a fundamental part of a national creation myth” and that the American wilderness has been cultivated and modified by human-caused fire for thousands of years. After all, fire was the most powerful tool humans possessed at the time [14]. The American wilderness has not been occupied by Native Americans who left no trace; rather, its landscape has been defined by their use of fire [13]. Thus, the idea of removing human influence contrasts with the historical evidence of human-caused fires that were prevalent across North America, long before Europeans arrived.

Early Americans were not careless in their application of fire, however; their application varied among ecosystems and was greatly dependent on the resource they were managing [15]. There are various reasons for the early use of fire, with the most notable being to modify ecosystems and “improve game grazing/browsing potential” [14,15]. Early Americans noticed that fire stimulated the next year’s productivity and used this knowledge to alter the grazing behavior of large herbivores [13,14,16]. By burning an area, they could increase the amount of new growth in an area, attracting wildlife to these areas.

Historically, wildfires incited an increasing concern for the sustainability of natural resources and public safety. In 1910, a wildfire nicknamed the “Big Burn” consumed 3 million acres of timberland in northern Idaho and Montana [17]. In response to the Big Burn’s devastation, the U.S. Forest Service implemented an aggressive wildfire suppression policy: the 10 AM Policy. This policy required that following discovery, all fires had to be suppressed by 10 AM the following day. This rule was followed throughout the twentieth century [18]. This strategy succeeded in nearly eradicating recurring fire from these ecosystems that had evolved to exist with fire. As a result, fine fuels did not burn regularly, leading to an accumulation of these fuels and an increase in fire severity [19]. This surplus of fuels, paired with changing climate, has contributed to the increase in both wildfire size and frequency. Dennison and coworkers [20] reported that wildfires have increased by 35,500 km² per year and 7 fires per year. For perspective, wildfires burned an average of 3.1 million hectares per year over the past ten years, a substantial increase from the approximate 445,154 hectares burned in 1984 [21].

4. Climate and Fire

Since 1880, average global temperatures have risen by an average of 0.08 °C each decade, resulting in a 1.12 °C total increase [22]. This excess heat has been credited with shifts in snow cover, rainfall, and streamflows. Westerling and coworkers [23] have identified a positive relationship between temperature, drought prevalence, and wildfires. In warmer years, spring snowmelt tends to occur earlier, lowering fuel moisture and lengthening wildfire season [12]. Further, timely precipitation in the spring and early summer leads to more plant production [13]. When partnered with summer drought and increased temperatures, this abundant plant matter is available as low-moisture fuel, is easy to ignite, and allows for rapid fire spread [14].

From 1970 to 2003, 56% of wildfires occurred in years with early snowmelt, with 73% of all early springs occurring after 1986 [23]. It is difficult to predict future wildfire behavior, but current models for 2050 show a 1.5 °C increase in global temperature [22]. Historical trends imply that this increase in mean temperature may lead to earlier snowmelt, prolonged drought, and greater incidence of wildfire [22,24,25,26]. Reflecting the historical increase in wildfires, the U.S. federal budget for wildfire suppression and prevention has grown from USD 728 million to over USD 2.5 billion [27]. The cost of fire-caused damages is more difficult to calculate, but the National Oceanic and Atmospheric Agency (NOAA) estimated USD 11.6 billion for damages in the 2021 wildfire season [28]. In 2018, one of the most devastating years for wildfires in the western U.S., damage costs reached USD 29 billion [28]. It is important to note that none of these estimates can accurately account for societal impacts like supply chain disturbances, health hazards, ecological alterations, and livestock displacement. Thus, all economic estimates should be considered conservative [7].

5. Current Recommendations

Though the economic consequences of wildfires are significant, the ecological impacts are equally important. Since rangelands serve as the primary forage source for various livestock and wildlife species, proper management of both pre- and post-wildfire landscapes is critical. Most research has focused on land management prior to burning, either by prescribed or wildfire, but there has been little research into management strategies after a wildfire specifically related to wildlife and/or livestock grazing.

For most public rangelands, the post-fire management strategy follows the recommendations outlined in Blaisdell’s Managing Intermountain Rangelands [29]. Blaisdell [29] recommends deferring grazing for two years after a fire, a suggestion that both the NFS and BLM have adopted into their management models. However, there is little research to support or discount this recommendation. Blaisdell [29] argues that “only a small amount of forage is produced the first year, and grazing may cause serious damage to soil and desirable perennials”. However, the literature indicates that plants have variable responses to fire. Some works have reported that rangelands experience an initial depression in plant cover immediately after a fire, but recovery is rapid, often within the first three years [20,30,31,32,33]. Li and coworkers [20] evaluated the impacts of different burn severities on vegetation and observed that perennial forbs and grasses reached cover values similar to pre-fire values after three years. The description of vegetation recovery under proper grazing management by Bates and coworkers [31] indicated more uncertainty about Blaisdell’s recommendations.

Rangelands in the United States vary greatly in plant species composition, fuel load potential, precipitation, topography, and geology. Bates and coworkers [31] recommend considering the timing and duration of grazing for post-fire grazing management, rather than a specific time of rest. This is supported by findings from various post-fire defoliation studies [34,35,36,37]. One paper reports that fall defoliation did not affect future plant production for Idaho fescue (Festuca idahoensis) and bluebunch wheatgrass (Pseudoroegneria spicata) [34]. However, in the Utah sagebrush steppe, West and Yorks [35] reported long-term depression of plant species from post-fire grazing. Gates and coworkers [37] reported an increase in diversity for northern mixed-grass prairie species, but there was no effect on plant productivity or ground cover. This variability in plant response makes Blaisdell’s blanket recommendation difficult to accept for all rangelands without further investigation [24,33,38].

6. Ecological Impacts and Management of Post-Fire Rangelands

Though most of the literature has been focused on the impact of large wildfires on forest ecosystems, there has been an increasing amount of work investigating fires occurring on rangeland ecosystems [20,24,39,40,41]. It is widely accepted among ecologists that some North American grasslands have evolved under the influences of grazing animals and ecological disturbances like drought and fire [13,14,42,43]. The adaptive responses of perennial grasses reflect a coevolutionary relationship between fire and grasses [42], as well as between herbivores and grasses [44]. To discourage defoliation, grasses have evolved to grow underground perennating tissues and silica in stalk and leaf epidermal cells, but that adaptation leads to the production of plant material that exceeds average annual decomposition [44]. Gleason (1922) reports that perennial grass can regenerate through underground tissues, even if the aboveground biomass was completely burned. In addition, Gleason (1922) argues that this indicates perennial grass species are more resilient than originally perceived within the Great Plains.

Donovan and others (2020) investigated the resilience of plant communities that underwent large-scale wildfires across the Great Plains. Immediately after a fire, perennial grass and forb density declined, while annual species production was minimally depressed [41]. Bare ground and shrub cover both increased immediately post-fire but decreased to normal levels within three years. Although immediate responses were seen among all functional groups, Donovan and others (2020) report finding no evidence of persistent shifts in Great Plains vegetation caused by wildfires. This research is supported by other work in the Great Plains [39,40,45]. Though total standing forage decreased after a fire, this depression is attributed to the absence of standing dead material [40]. While Vermeire and coworkers (2011) did see a shift in plant species composition within two years, the work conducted by Donovan and others (2020) suggests that communities will return to pre-fire composition after several years, even with the presence of grazing.

In Wyoming big sagebrush steppe, Bates and coworkers (2020) evaluated the long-term (2002–2018) succession after a fire. For the duration of the study, they reported that total herbage and perennial bunchgrass yields were approximately two times greater in the burned than unburned sites [46]. However, plant composition was altered within this timeframe: native perennials consisted of 90–95% of total plant composition prior to the fire but were reduced to 78% due to a post-fire increase in desert alyssum (Alyssum desertorum) and cheatgrass (Bromus tectorum) [46,47]. Other invasive plants, such as cheatgrass (Bromus tectorum), have been recorded to increase in arid and semi-arid rangelands following wildfires and further contribute to the increase in wildfire activity [48,49,50]. In the tallgrass prairie, the expansion of native woody species is considered one of the greatest concerns [51]. Native woody species such as juniper (Juniperus spp.) continue to expand despite wildfire [51]. Differences in plant community resiliency to wildfire between the sagebrush steppe and Great Plains suggest that post-fire management needs to be rangeland-ecosystem-specific.

Research into the impacts of wildfires on soil characteristics and microbial communities has been conducted in Australia’s semiarid farmland [52]. Initial depressions in fungal and bacterial populations were exhibited in burned pastures, and the shift in bacterial and fungal structures was greater in burned pastures than unburned sites. This difference is attributed to increases in soil pH and nitrogen pools because of wildfire [52]. The severity of a fire also has an impact on bacterial and fungal communities [53]. Whitman and coworkers [53] report that as fire severity increases, microbial communities become increasingly dissimilar to unburned communities.

7. Herbivores on Post-Fire Rangelands

Based on historians’ knowledge of Native American burning to alter herbivory, several studies have been conducted to determine the strength and mechanisms of the fire–grazing interaction [45,54,55,56]. Native bison, domestic cattle, and elk exhibit a strong preference for recently burned areas [55,57,58]. Cattle reduced standing biomass in burned areas by 78% compared to 19% in unburned areas in the same pastures [57], suggesting cattle consume more forage in recently burned sites compared to unburned sites. However, this preference decreases as forages mature and decrease in quality [55,56]. Elk consistently used burned forage stands more than control stands in the spring, but they used both stands equally by the summer [58].

Further work has shown that grazing the year after a fire does not have significant impacts on plant production or vigor parameters when grazed at a light (17%) to moderate (34%) utilization [37,39]. Vermeire and coworkers [39] did report a depression in biomass for needle-and-thread (Hesperostipa comata), but only at high (50%) utilization. Gates and others [37] saw an initial depression in standing forage, 35% in year one and 19% in year two, but attributed this to the 53% more standing dead found in the ungrazed pastures. For both studies, summer grazing had minimal or no impact on plant communities in the northern mixed-grass prairie.

Most post-fire grazing research has been conducted in the Great Plains of North America, which supports the northern mixed-grass ecosystem. However, vegetation communities characterized by perennial bunchgrass species like bluebunch wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca idahoensis), and needle-and-thread occur outside of the Northern Great Plains biome. Although these bunchgrass species are found across various ecotypes, most post-fire management research on bunchgrass-dominated rangeland has been conducted in the Northern Great Basin [31,32,46,59,60,61,62,63].

In the Northern Great Basin, the potential impact of grazing on vegetation succession was investigated with seven years of cattle grazing in Wyoming big sagebrush steppe after a prescribed fire [32]. Three different grazing treatments were used; neither the low (15–30%) nor moderate (30–50%) levels of grazing impacted native herbaceous species composition and productivity [31,32]. Reproductive stalk weight and canopy cover for these two treatments were comparable to those of the control site, indicating similar vigor and production levels. However, the high stocking rate (50–70%) had negative effects on reproductive stalk weight and canopy cover but did not impact basal cover and density [32].

Conrad and coworkers [47] focused their post-wildfire research on two key bunchgrass species: bluebunch wheatgrass and Idaho fescue in Northern Oregon. After a wildfire in July, considerable growth was seen by the following June. They reported lower survival rates of existing Idaho fescue plants compared to bluebunch wheatgrass: 82% and over 95%, respectively. Bluebunch wheatgrass decreased in basal area, indicating reduced vigor, but plant density remained similar to pre-burn values. Conrad and coworkers [47] concluded that Idaho fescue mortality and vigor was more affected by fire than bluebunch wheatgrass.

Conversely, Mueggler [64] states that bluebunch wheatgrass is more susceptible than Idaho fescue to defoliation. Mueggler [64] estimates the average time for bluebunch wheatgrass to recover to normal vigor is six years, but only three years for Idaho fescue. However, damage to these plants depends on intensity, frequency, and time of use; bluebunch wheatgrass defoliated to 28% original plant height is more susceptible than Idaho fescue defoliated by 75% during flower development [64]. Timing seems to be a key factor when grazing bluebunch wheatgrass where the greatest physiological impact of defoliation is before and during the boot stage of phenology [34,65]. When defoliation was conducted on bluebunch wheatgrass after a fire, Jirik and Bunting [34] saw no significant difference in biomass production, basal area, tiller number, or mortality between plants defoliated in the fall and control sites. However, early (boot stage) defoliation reduced these parameters [34,66].

When sheep grazed a recently burned sagebrush steppe in Idaho, Roselle and coworkers [63] reported slight impacts on vegetation recover. Bluebunch wheatgrass cover continued to increase in grazed areas despite drought conditions. This increase was slightly suppressed by sheep grazing at 40% utilization, but a general increase in cover was reported. In the same trial, sheep showed a high preference for early-growing forbs, reducing overall forb production by 73%. This research supports resting for one growing season to avoid altering plant species composition, specifically the reduction in forbs. Roselle and others [63] argue that fall grazing may not have a negative impact on sagebrush steppe plant communities.

Fuhlendorf and others [45] support the idea of using fire to manage livestock grazing in the Great Plains. They suggest that the use of pyric herbivory, or “herbivory shaped by fire”, be implemented in rangeland ecosystems to reintroduce the evolutionary disturbance regimes that promote heterogeneity and biological diversity [45]. Allred and coworkers [55] further support the use of fire to modify grazing behavior in cattle, citing a greater use of recently burned steep slopes. One paper even reports that the available biomass does not affect selection: cattle preferred burned areas over unburned sites with significantly more forage [56]. From a soil erosion perspective, however, Stavi and coworkers [67] caution that livestock grazing after a fire can increase soil erosion and land degradation at all utilization levels used in their study (6, 17, and 35%) [67].

While rangelands are crucial for the livestock industry, these lands also serve as an important habitat for wildlife species such as elk and deer [68]. There has been concern regarding the competition between cattle and these wildlife species. However, Damiran and coworkers [68] argue that cattle grazing mixed-conifer rangelands in the spring and summer did not have a negative effect on the diet quality of elk or deer. As concentrate selectors, elk and deer compensated for the decrease in grass and grasslike forage by consuming higher protein diets, such as that found in forbs and shrubs [68]. They reported that both cattle and elk were able to adapt to changing grazing environments by altering diet selection.

Cattle are considered to display similar foraging behavior to bison and have been recorded to consume diets with comparative quality when grazing northern mixed-grass species [69,70]. However, when investigating vegetation changes between cattle and grazing tallgrass prairie, Towne and coworkers (2005) observed lower grass cover and higher forb cover in bison-grazed paddocks compared to those grazed by cattle. When evaluating the diets of bison and cattle grazing in burned tallgrass prairie, Allred and coworkers [68] reported significant differences in species grazing behavior. Cattle preferred areas with trees and other wooded vegetation, while bison avoided such areas [55]. Additionally, when grazing burned patches of various sizes, bison preferred smaller areas to larger ones. Allred and coworkers attribute this preference to the increased grazing pressure that maintained the forage at an earlier stage of phenology associated with higher quality [55]. When provided access to burned areas of various ages, both cattle and bison showed high preference for more recently burned areas, and in some cases, bison avoided older burned areas [55,70].

8. Statement of the Problem

Gates and coworkers [51] address the need for reconsidering the two-year deferment post-wildfire. In the northern mixed-grass prairie, plant vigor and productivity remain generally unaffected by fire or increase [35,51]. Grazing post-fire has few impacts on vegetation in this area and even improves diversity, indicating rest is not required to maintain productivity and cover [51]. Bates and coworkers [29] further support this recommendation by claiming that grazing one growth cycle after a fire will not negatively affect herbaceous plant recovery in Wyoming big sagebrush steppe. However, there has been little to no research conducted to evaluate the impacts of post-fire grazing on the perennial bunchgrass rangelands within the Rocky Mountain Lower Montane, Foothill, and Valley ecosystem.

A confounding source of variation in wildfire recovery is diet selection and grazing behavior of livestock and wildfire using the area post-fire. Determining what forages animals prefer after a fire provides valuable insight into how herbivores will utilize the specific plants within the vegetation community. In turn, this information can be used to model the potential impacts grazing herbivores have on plant communities after a wildfire event (Figure 2).

9. Conclusions

With the projected increase in wildfire frequency and intensity, as well as its ecological impact on arid and semi-arid rangelands, it is important to reassess post-fire management strategies for maintaining or enhancing vegetation communities and soils in these ecosystems. While current paradigms promote a two-year deferment to minimize stress on native plant species, there is little research validating or dismissing this recommendation. Rangelands, comprising 30% of the total U.S. land cover, are crucial for livestock, wildlife, water, and recreation. Historically, fire has shaped these landscapes, and understanding the relationship between post-wildfire grazing and vegetation recovery can prove useful for future management. Existing studies primarily focused on the Great Plains ecosystem indicate that post-fire grazing can have minimal impacts on plant communities, potentially improving diversity and productivity. However, the diverse ecosystems of the Rocky Mountain region necessitate research to tailor post-fire management strategies to ensure the sustainable use of these rangelands. Based on the information in this review, the perennial bunchgrass species in arid and semi-arid rangelands may have greater resilience to post-fire grazing than initially thought, implying that the need for a two-year grazing deferment on burned rangeland may be reconsidered.