Grass-Dominated Landscapes
This content is part of a series on grassland fuels management prepared in collaboration by Colorado State Forest Service with support from Southern Rockies Fire Science Network (SRFSN) and Great Plains Fire Science Exchange (GPFSE).
Contents
At a Glance
Grasses represent a critical component of many landscapes. In landscapes where grasses are the predominant vegetative fuel type, these regions are characterized by an abundance of fine fuels and can be particularly vulnerable to wildfire due to their rapid fuel accumulation and high fire frequency. Complications in these landscapes include changes associated with dynamic and complex processes such as altered wildfire regimes, invasive species, including woody encroachment and conifer expansion, human land use and modification, and impacts resulting from a changing climate. These impacts have led to a loss of at least 80% of these grasslands, including a loss of 99% of tallgrass prairie (Blair et al., 2014; U.S. Fish & Wildlife, 2024), resulting in grasslands being the most imperiled terrestrial ecosystem on the planet, experiencing a far greater rate of conversion than forests (Scholtz et al., 2022).
Increasingly arid conditions, modifications to grass-dominated landscapes and invasive species have contributed to elevated wildfire behavior and risk (Ford et al., 2021). Wildfires occurring in areas predominantly populated by fine rapidly igniting fuels, like those found in grass-dominated landscapes, can quickly impact communities within the wildland-urban interface (WUI) with little forewarning. In such instances standard predictions of fire spread may not be capable of issuing timely emergency warnings in advance of a fast-moving fire (Cruz et al., 2022). Consequently, communities situated in grass-dominated landscapes may require different guidelines concerning defensible space, home hardening, and other fire risks and hazards (Wells et al., 2021).
Grass-Dominated Landscapes
Grasses, while diverse and occasionally difficult to define, include three distinct types, shortgrass, mixed grass and tallgrass, and differ primarily in their vegetation structure, composition (which is mostly defined by particle size and fuel moisture) and ecological characteristics. A fundamental characteristic of grass-dominated landscapes is their dominance or co-dominance by grass-like vegetation which includes true grasses and grass-like plants such as sedges and rushes (Higgins et al.,1989). Where pure grasslands are distinguished by a substantial presence of grasses and other grass-like vegetation within an open and undulating terrain, typically lacking significant tree and shrub cover (Blair et al., 2014).
Fuel loads are primarily composed of fine fuels that consist of plant material less than ¼ inch in diameter but may also include plant material greater than 1 inch in diameter. In general the main differences between shortgrass, mixed grass and tallgrass lie in their vegetation height, composition and ecological adaptations to different climatic and environmental conditions.
Grassland Types
Shortgrass is characterized by their minimal height and typically grow up to around 10-20 inches in height. These grasses are often found in regions with semi-arid climates, such as the Great Plains of North America. Shortgrass prairies are adapted to tolerate drought and grazing pressure.
Mixed-grass prairies contain a combination of grass species, resulting in a more diverse vegetation structure compared to shortgrass prairies. Mixed-grass prairies are typically found in regions with slightly higher precipitation levels than shortgrass prairies.
Tallgrass prairies are characterized by their tall grasses which can grow several feet in height. These grasses are typically found in regions with higher rainfall amounts and wetter conditions compared to shortgrass and mixed-grass prairies.
Fire in Grass-Dominated Landscapes
Historically, fires once spread across vast expanses of open grass-dominated landscapes and played a significant role in shaping these regions. The accumulation of highly combustible vegetation combined with arid, windy and open landscapes creates favorable conditions for large-scale fires. Although fires can occur at any time of the year the frequency and intensity of these fires depend on factors like precipitation and aboveground productivity. In grass-dominated landscapes including arid ones like shortgrass steppe and desert grasslands, fires can occur at various frequencies (Blair et al., 2014).
Fires in grass-dominated landscapes have shown to be capable of rapid fire spread and extreme fire behavior. Driven by complex physical and chemical processes operating on vastly different scales, processes that drive wildfire depend on interactions between the atmosphere, topography, fire and fuels. Fire can occur in any type of vegetation and under the right conditions, all fuels have the potential for combustion. Fire affects ecosystem structure, composition and function, where different types of fuels will interact with fire in different ways (Wells et al., 2021).
The composition of vegetation can affect fire behavior and surface fires can vary in intensity (Wragg et al., 2018). The intensity of fire is subject to various factors, including fuel load, fuel condition (such as compaction and moisture content), relative humidity, wind speed and topography (Vogl, 1979). Grass fires can be intense and capable of generating significant heat aboveground. Because grassfires are often rapid spreading surface fires, temperatures peak quickly as fire passes, often killing the tops of living plants including trees leaving belowground vegetation unimpacted (Blair et al., 2014).
Fire in grass fuels is characterized by its rapid combustion and high rate of spread. The rate at which fire spreads is influenced by vegetative fuel moisture, arrangement, continuity, fuel load and environmental conditions. Finer fuels like grasses can promote fire spread and intensity, connect fuels and more readily carry fire than coarser woody fuels like shrubs and forbs (Wragg et al., 2018). The high surface area-to-volume ratio of a grass leaf, its high thermal conductivity, low density and vertical orientation allows for a blade of grass to ignite rapidly and just as rapidly burn out (Cruz et al., 2022).
At a basic level, fuels can be identified as fine fuels, which are comprised of leaf litter, grasses, other herbaceous plants and small shrubs, or large/coarse fuels which are larger shrubs, trees and dead woody debris (Cruz et al., 2022). Grass-dominated landscapes predominantly consist of fine “flashy” fuels often referred to as 1-hour fuels, explained below in Figure 1, characterized by their rapid combustibility, ease of ignition and contribution to fire spread. Vegetative embers are generally smaller and lighter; embers from grass are the smallest and lightest, followed by shrubs then larger vegetative fuels like trees (Zhou 2019).
Fuel Categories
Most grasses are less than ¼ inches in diameter, where the responsiveness of the fuel-moisture and how quickly it responds to environmental conditions is very rapid. In contrast with forest litter fuels which are often coarser, of higher fuel-loading, higher density and have burnout times in the order of minutes to hours, grass fuels have burnout times around 5–15 seconds (Sullivan, 2010). One-hour fuels can burn under a broader range of environmental conditions than other fuel types due to their moisture of extinction.
The amount of live and dead vegetation over a landscape is often a mix of fine and coarse fuels and is subject to annual variations in the abundance and distribution (Leis, 2013). Even seemingly simple fuel beds like continuous grasses can have multiple types of fuels in multiple states or conditions, resulting in dynamic fuels (Cruz et al., 2022). These factors are important considerations for mitigation and management efforts concerning fire in grass-dominated landscapes as they can significantly influence fire behavior and spread, especially during periods of high fire danger and extreme weather conditions.
Fuels Variations
- Can change over a relatively short timeframe due to factors such as weather, season or human activities.
- Can cure quickly and become more combustible during dry conditions.
- Often have a faster rate of extinction due to higher surface-to-volume ratio, which allows them to ignite and burn more quickly.
- Remains relatively consistent in characteristics over time, where properties generally do not change rapidly compared to dynamic fuels.
- Typically have a slower rate of extinction compared to dynamic fuels because they require more heat and energy to ignite and sustain combustion.
In general, fine fuels with low moisture content tend to burn more readily and contribute to faster rates of extinction compared to larger fuels or those with higher moisture content. The rate of extinction refers to how quickly a flame is extinguished and is influenced by various factors, including the fuel type and moisture content, fire intensity, and environmental conditions such as wind speed and humidity. The Scott and Burgan’s Standard Fire Behavior Fuel Models are integral in assessing fire behavior by considering factors such as fuel moisture, type and structure, height, density and dead-to-live ratio, with variables like terrain and wind speed, to aid in understanding of grassfire dynamics (Scott et al., 2005).
Weather and Seasonal Effects
The following insights were derived from discussions with Subject Matter Experts, facilitated by Chad Julian at the Colorado State Forest Service (CSFS).
Vegetative fuels undergo seasonal variations due to the cumulative effects of weather cycles, where wet periods often lead to heightened production of grass fuels (surface fuels), contributing to an increased risk of extreme fire behavior in subsequent dry years. When critical fire weather patterns occur, grass fuels have the potential to experience extreme fire behavior (Werth et al., 2023). Most grass fuels are very fine-sized fuel particles with high surface area-to-volume ratios and light fuel loads when compared to other vegetation fuel types. When fully cured (grasses are dry and have low moisture content), grass fuel-beds are highly combustible and grassfires become exceptionally responsive to wind speed, wind direction and relative humidity in comparison to other wildland fuels (Cruz et al., 2022; 79, Vogl, 1979).
The rate of fire spread in grass fuels is influenced by various factors, including topography, vegetative fuel moisture, arrangement, continuity and environmental conditions such as wind, relative humidity and dew point. Many grass-dominated regions have topography that includes hills, valleys and slopes, which influences wildfire behavior by affecting wind patterns, fuel continuity, fire spread rates and the difficulty of suppression efforts. Furthermore, it is important to recognize that grasses are not confined to pure grasslands and prairies; many other landscapes rely on grass fuels as the primary source of fuel connectivity.
Influential Factors
Vegetation that is dry, fine, well-aerated and continuously connected, like many of fuel types found in grass-dominated landscapes, increases:
- fire spread
- fire intensity
- temperature
- rate of spread
- flame length
Wind speed has the most critical influence on fire behavior, affecting the speed at which a fire spreads, flame length and fire intensity. Wind speed and direction is a major driver in direction of fire spread and the size of the fire front, increasing the overall area of the fire. Sudden and unpredictable changes in wind direction and speed are some of the primary causes behind people and assets getting caught in a grassfire and is one of the most unpredictable aspects influencing fire behavior.
Grass-dominated landscapes are generally more open to the wind than forested landscapes and as a result grassfires are highly responsive to changes in the direction and speed of the wind (Sullivan, 2010). Under normal burning conditions, factors such as fuel loads, composition and continuity play major roles in fire behavior, often requiring connectivity of fuels for fire to spread. Under extreme conditions, defined by high wind speeds, low fuel moisture content, cured fuel-beds and low relative humidity, wind is the dominant effect on fire behavior. In these conditions, the fire front quickly spreads as topography, fuel loads, and wind speeds and direction can drastically affect fire behavior (Vogl, 1979).
Wind strength and direction can shift abruptly throughout the day. This increase in wind can fuel the fire by providing more oxygen, making the fire more intense. Additionally, the wind reduces moisture levels in vegetation, resulting in drier fuel beds and further increasing flame intensity. By moving flames closer to new fuel sources, the wind accelerates the preheating of the fuel due to increased radiant thermal energy (Ghodrat et al., 2021). This leads to faster horizontal spread of the fire across the landscape. The rate of fire spread can be directly proportional to the wind speed, where greater wind speeds result in increased fire expansion and rate of spread. High winds can lead to extremely fast-moving fires that cover large areas in a short period, especially in grass-dominated landscapes.
When winds flow over mountaintops like the Rocky Mountains, and an inversion layer is present just above the peaks-when a layer of warmer air traps cooler air near the surface-a mountain wave pattern can develop that can produce hurricane strength winds. Wind passing between the inversion level and the mountaintops will accelerate when traveling downslope on the leeward side towards the base of the mountain, reaching its highest winds as it reaches the base of the foothills. In North America, downslope wind events occur in four regions; along the eastern slope of the Rocky Mountain from Alberta to New Mexico, along the eastern slope of the Sierra Nevada and Cascade Range, on the west slope of the San Bernardino, Santa Ana, and San Gabriel Mountains, and on the leeward side of the Alaskan and Chugach mountains in Alaska (Durran, 2022). In Colorado, high wind events are predominantly associated with the Front Range. Although high winds can occur in other parts of the state, those along the Front Range, extending from Wyoming to New Mexico, are distinctive due to their alignment with the topography and their position upwind of major Values at Risk in developed areas. Wind events along the Front Range are predominantly observed between September to April and coincide with a time-period that aligns with plant dormancy and fuel curing in fall and winter (C. Julian, personal communication, May 30, 2024).
Rocky Mountain winds include the Bora Winds and Chinook Winds. The Bora Wind is a widespread wind across the Front Range, characterized by a northwest component. It is colder and occurs after a frontal passage, typically with higher relative humidity. This wind pattern consistently maintains medium to high wind speeds and can produce gusts of varying intensity. Chinook wind events are more destructive and often accompany Mountain Wave Winds. These downslope winds are warmer with lower relative humidity and tend to have a narrower geographic impact compared to the Bora Wind. They exhibit variable wind speeds with a very high potential for gusts (Durran, 2022). The Marshall Fire occurred during such a wind event. In the Rockies, both wind events occur mainly between September and April, coinciding with plant dormancy and fuel drying in autumn and winter.
Fire is an important natural disturbance in grass-dominated landscapes, where new plant growth is generally very rapid, while recovery in more arid areas may take considerably longer. Much of western North America is typified by an extended summer dry season, which contributes to the region’s unique fire regimes. These differences highlight the diverse and complex nature of fire ecology across the west, underscoring the need for region-specific fire management strategies. While these regions quickly recover fuel continuity and are characterized by high fire frequency, how fast fuels accumulate and the prevalence of ignition sources differs significantly by region and ecosystem. Within these regions, the potential for fire can vary year to year, influenced by global circulation patterns such as the El Niño–Southern Oscillation (ENSO), where major conflagrations can be common during La Niña episodes, when monsoons are either delayed or weak (Ryan et al., 2013).
In addition to wind, the rate of fire spread in grass fuels is influenced by vegetation fuel moisture, which responds to environmental conditions such as relative humidity and dewpoint and can influence the type and intensity of fire effects in grass-dominated landscapes. Grasses are heavily impacted by ambient weather conditions, gaining and losing moisture more rapidly than heavier fuels, and can return to ambient moisture levels within a few hours after heavy rainfall (Trauernicht at al., 2021). This rapid change can lead to fires spreading quickly through grasses saturated by heavy rains hours earlier or as soon as snow cover has melted (Ford et al., 1996).
Dewpoint is the temperature at which air becomes saturated with moisture and dew forms. A high dewpoint has ample moisture in the air, which can increase the moisture of fine fuels like grasses. Conversely, a low dewpoint has drier air, leading to fast drying grasses, thereby increasing their combustibility. Thus, lower dewpoint levels can create conditions that are more conducive to fire ignition and spread in grass environments.
Relative humidity (rH) measures the amount of moisture in the air as a percentage, relative to the saturation point at a specific temperature. When the air holds all the moisture possible at a given temperature the relative humidity is 100%. Conditions with higher air temperatures and low humidity can produce fuels that more readily burn, where strong winds can result in rapid moving fire (Cruz et al., 2022). Conversely, conditions with high humidity can increase surface fuel production, which can result in dry grasses by early spring, raising the risk of extreme fire behavior, a cycle not uncommon in the western U.S. (Ford et al., 1996).
The moisture of extinction, or the moisture level at which vegetative fuels cannot ignite, varies among types of vegetation. When vegetation is green and actively photosynthesizing its fuel moisture content is typically high and remains relatively unaffected by atmospheric or soil humidity.
Regional variations in ambient moisture content can significantly impact grass fuels and fire behavior by influencing the moisture content of the fuels. Higher humidity levels and higher dewpoints increase the moisture content in grass fuels making them less combustible and slowing the rate of fire spread. Conversely, low humidity and dewpoints result in drier grass fuels, which ignite more easily and burn more intensely. Dry conditions facilitate rapid fire spread, particularly in fine fuels like grass, which are highly responsive to changes in moisture content. These factors are crucial in fire behavior prediction and management strategies as they directly affect the potential for ignition and the intensity of wildfires in grass-dominated landscapes.