Fires Need Fuel

In forest fires, vegetation is fuel. Different kinds of vegetation provide different types of fuel that burn at different rates and intensities. Fine fuels such as grasses, needles, and leaves dry out more quickly, ignite more easily, and burn rapidly at relatively low heat intensity. Fuels are commonly classified by land managers according to the time period required to equilibrate to the surrounding moisture content. Grasses and needles are referred to as 1-hour fuels because within a one hour time period their moisture content responds to the environmental moisture content. In a wildfire, these grasses and needles dry out quickly and can assist the spread of a fire in its early stages. Coarse fuels such as fallen branches, logs, and trees dry out more slowly and are more difficult to ignite, but can burn for longer periods and at relatively high intensity. These coarser fuels are referred to as 1000-hour fuels when they are greater than 3 inches (10 cm) in diameter and respond to surrounding moisture conditions more slowly than grasses and needles. During years of severe drought and in high intensity fires, these coarse fuels burn more readily.


fine fuels1000 hour fuels

Photo (left): Fine fuels in a low density Ponderosa pine stand. Photo credit: Meredith Gartner Photo (right): Coarse fuels in a high density lodgepole pine stand. Photo credit: Meredith Gartner

Photo (left): Fine fuels in a low density Ponderosa pine stand. Photo credit: Meredith Gartner

Photo (right): Coarse fuels in a high density lodgepole pine stand. Photo credit: Meredith Gartner


Mosaics of Severity

Burn severity is measured by the effect of the fire on the vegetation and ecosystem and typically reflects the percentage of canopy tree mortality due to the fire. Just like hurricanes and earthquakes, forest fires burn across a continuum of severities: from low to moderate or high.  One fire might exhibit a mosaic of different severities in different areas of the landscape.

low severity fire in the fourmile fire


Low-severitysurface fires” generally burn at low intensity and can occur relatively frequently.  These fires kill grasses, small shrubs, and tree seedlings but rarely kill thick-barked mature trees.  These fires burn the forest understory, including young tree seedlings and saplings, making resources available for new trees and understory plants to begin growing. 

Photo left: Low-severity fire in ponderosa pine stand in Fourmile fire. Photo credit: Meredith Gartner. Photo below: Areas of surface fire in the Fourmile Fire killed the smallest seedlings. Photo credit: Meredith Gartner

surface fire damaging seedlings


moderate severity fire



Moderate-severity fires kill some mature trees, but many trees survive.





Photo left: Photo below: Moderate-severity fire in ponderosa pine stand in Fourmile fire. Photo credit: Meredith Gartner


high severity fire


High-severitycrown fires” burn through tree crowns (i.e. the branches, leaves, and trunks), killing nearly all of the trees in a forest. These fires burn with very high intensity (heat) and can be dangerous and difficult for fire fighters to control.



Photo left: High-severity fire in ponderosa pine stand in Fourmile fire. Photo credit: Meredith Gartner


What affects fire activity?

Climate, weather, human activity, ecosystem characteristics, and land qualities all affect the chances of ignition, how a fire will spread, and which areas will burn more severely.


Weather and Climate Patterns

- When weather has been very hot and dry, fire risk is higher.

- Areas with frequent lightning have higher risks of ignition.

- During a fire, wind allows fire to spread much faster and across further distances.

- In dense forests with abundant woody fuels, extreme drought increases the risk of fire.


dry grass near the flat irons, Boulder Colorado

Ecosystem characteristics

In drier ecosystems, such as the lower montane zone of the Front Range, where grasses are prevalent, fire risk is highest when there is a very wet year followed by a very dry year. The rainy wet year creates a lot of grassy vegetation. The next year is hot and dry, and the vegetation dries out. The area becomes more flammable. If dry grasses catch on fire, they make it more likely for trees to ignite and for the fire to spread.

Photo right: Abundant dry grasses in a ponderosa pine stand near the Flatirons. Photo credit: Meredith Gartner

Dense forests burn more severly. Although it may be more difficult to get a fire to ignite in a dense forest, once it does, it burns at high temperatures killing most trees in the area. Dense forests can form naturally (e.g. following a natural high-severity fire) or as a result of human activity (e.g. due to the suppression of frequent low-severity fires.)

Grasslands and open woodlands burn more easily.  These areas dry out more quickly and ignite more easily, but mature trees are generally not killed, so they burn less severelythan dense forests.


Trees burn differently.  Some types of trees like pine burn more easily than trees like aspen.


Land Qualities in the Colorado Front Range

Topographic features can influence fire activity.


South-facing slopes burn more easily. In Earth's northern hemisphere, south-facing slopes receive more direct sunlight, become drier, and burn more easily. Fires are typically less severe because the drier conditions support fewer trees.

North-facing slopes often burn more severely.They receive more moisture, which creates denser vegetation. During very hot and dry conditions, these dense forests usually burn with higher severity.

View looking west into Boulder Canyon.

Photo left: View looking west into Boulder County taken in 1909. Photo credit: J. P. Sturtevant 1909. Photo right: The south-facing slope (seen on the right) supports abundant grasses and fewer trees because it receives more direct sunlight and is drier than the north-facing slope that supports denser forests (seen on the left slope in the background). Photo credit: TT Veblen and DC Lorenz 1986.

Steep slopes often burn more severely. As flames climb up the slope, heat and smoke preheat vegetation, making it burn more quickly and severely.

Higher elevations burn more severely. Due to moister conditions, forests are naturally dense at mid- and high-elevations, leading to abundance of coarse fuels for severe fires to occur.


Human Activity

Humans are often responsible for the ignition of forest fires. Overall, fires in Colorado are not limited by ignition (i.e. there is ample lightning), but humans have a detectable influence historically even though the broadscale pattern (the spread and extent) is climate controlled.

In areas where people have suppressed fires, forests have become denser and can lead to more severe, dangerous fires. 

Historically, areas of low elevation experienced frequent, low-severity surface fires that killed tree seedlings and helped maintain the open, woodland ecosystem, as exemplified by the foothills surrounding the Chautauqua auditorium in 1905 in Boulder (below). At the lowest elevations near Boulder, fire suppression during the 20th century has trees to encroach further into grasslands, resulting in dense forests with increased fire hazard. Past land-use practices such as mining, logging, and cattle grazing, can also create opportunities for dense forests to grow. By 1988, dense forests of ponderosa pine have edged further into the prairie grasslands. The resultant woody encroachment to formerly grass dominant understories increases potential for higher severity fires. Much of the area affected by this increased tree density is in the wildland urban interface. Repeat photography is one of the methods used to document the conversion of from open, park-like stands to densely forested stands.

Chautauqua Park, Boulder Colorado in 1905



Photo left: Chautauqua auditorium in 1905 surrounded by sparse cover of ponderosa pine. Photo credit: LC McClure 1905.Photo right: Chautauqua auditorium in 1985 showing surrounding areas affected by fire suppression. Photo credit: TT Veblen and DC Lorenz 1985.


Historic fires were severe in high elevations surrounding Boulder.

In the late 1800s, the northwest slope of Sugarloaf mountain in Fourmile Canyon had been cut for logging and been burned by a high-severity fire (left) which made abundant new growth possible.

By 2011, the forest had receovered to dense forests of mainly Douglas fir. The slope in the upper left shows an area burned by the Fourmile Fire.


Sugarloaf Mountain in 1905Sugarloaf mountain in 2011.

Photo left: Evidence on an extensive fire in 1894 on Sugarloaf Mountain in Boulder County. Photo Credit: J.P. Sturtevant 1905. Photo right: Regrowth of the forest following the severe fire on Sugarloaf Mountain. Photo credit: TT Veblen 2011.

Healthy Ecosystems Need Fire

Forest fires are normal for most forest ecosystems.  
sunlight pouring through burned forest For thousands of years, many ecosystems have experienced regular fires.   New resources (sunlight, soil, nutrients, water) become available and new vegetation can grow.  When plants burn, the nutrients contained in them are recycled and made available to an increasing diversity of animals and new plants. 


Photo right: Sunlight pours intot he recently burned areas in the Fourmile fire. Photo credit: Andres Holz


Dominant tree types the Colorado Rockies include: ponderosa pine, Douglas fir, Englemann spruce, subalpine fir, lodgepole pine, limber pine, and aspen.  Some plants require fire.  Certain trees, like the lodgepole pine, need very hot temperatures to break the resin seals in their cones to release their seeds and reproduce. Aspen trees reproduce quickly after fires because they can sprout from their roots.


Photo left: Douglas fir growing in the shade of a ponderosa pine stand. Credit: Meredith Gartner. Photo center: Aspen canbe found in sunny locations and following fires. Credit: Meredith Gartner  


Photo left: Dense lodgepole pine stand. Credit: Meredith Gartner. Photo center: Serotinous pine cone of lodgepole pine. Credit: Meredith Gartner Aspen seedling growing from underground root sucker. Credit: Teresa Chapman







Teresa Chapman. Last updated 12.12.2011. Contact with questions about this site.