Summary of Fire in the Colorado Front Range
(Information compiled by G. David)
The composition and structure of a forest ecosystem is dependent on the dynamic interplay of climate, fire, and biodisturbance in the form of beetles and wind. Forest fires are essential for the renewal and growth of forest ecosystems. Fires aid in the renewal of the forest by exposing the mineral soil, increasing the amount of light and moisture, and releasing plant nutrients (Macdonald and Stednick, 2003). Stands of lodgepole and ponderosa pine are the most fire-dependent trees. Lodgepole pines need moderate to high severity fires to release seeds from their cones. Both lodgepole and ponderosa pine forests need fires to keep more shade-tolerant trees from establishing and replacing them (Macdonald and Stednick, 2003).
Fire intensity, severity and frequency are dependent on climate, vegetation type and disturbance history of a stand, which are all related to elevation. Figure 1, below, shows the vegetation zones for the Colorado Front Range and the expected fire regime. Generally, the fire severity increases with elevation and the frequency decreases (Sheriff and Veblen, 2007).
The geomorphic and hydrologic changes in a basin after a fire are dependent on the severity of the fire, severity of the rainfall event that follows the fire, and relaxation time between fires (Brunsden and Thornes, 1979; Moody and Martin, 2001). High, moderate and low severity fires are characterized by the percentage bare surface exposed after the fire. The amount of bare soil, soil water repellency, particle cohesion and surface roughness are all affected by the fire severity, which subsequently affects the sediment production in a basin. After high severity fires, 85 to 95 % of the surface is either bare mineral soil or bare soil covered in ash (Benavides-Solorio and Macdonald, 2005; Macdonald and Larsen, in press). Generally, the decrease in vegetative cover decreases interception, infiltration and storage, increasing the effects of raindrop impact and the likelihood of overland flow. Expected changes after a high severity fire include, decreased infiltration capacity, increased soil water repellency, overland flow, sheetwash, rill development and subsequently increased sediment production and peak flows in streams (Macdonald and Stednick, 2003; Shakesby and Doerr, 2006). This page describes the major vegetation zones and the expected fire regime for each, the history of fire in each of these zones and the expected hydrologic and geomorphic changes from fire in a basin.
Click a link below to learn more about fires in the Colorado Front Range:
Fire Regime and Major Vegetation Zones
There are six major vegetation zones in the Colorado Front Range: alpine, subalpine, upper montane, lower montane, lower ecotone and plains grasslands. The vegetation communities change with changes in precipitation regime and climate as you go up in elevation. The intensity, severity and frequency of fires also differ with each vegetation community. The differences in fire severity are important to consider because the geomorphic responses, e.g., increases in runoff and sediment yield, will differ depending on the severity and intensity of the fire. The frequency of fires is important because the effect of the fire may differ depending on the length of the mean relaxation time between fires (Moody and Martin, 2001). Figure 1 below shows the major vegetation zones along with the historical differences in fire severity and intensity for each zone and the effect fire suppression has had on each of these zones.
Figure 1: Vegetation zones in Colorado Front Range and fire severity in each zone (modified from Kaufmann et al., 2000)
Fire Scarred trees along Meadows Trail, Grey Rock, Colorado
(Photo by G. David)
The subalpine zone consists of lodgepole pine and spruce/fir forests. Historically, both stands of lodgepole pine and spruce-fir forests were dense and burned infrequently with a recurrence interval of 100 to 500 years. Today, stands of lodgepole pine and spruce-fir forests are dense and burn infrequently (Romme et al., 2003). The removal of overstory cover when high severity and intensity fires occur can increase water yields that slowly decay over 60 to 70 years (Macdonald and Stednick, 2003).
The lodgepole pine and ponderosa pine forest in the upper and lower montane zones are the most fire-dependent forest types. Lodgepole pine forest will be replaced with more shade-tolerant species in the absence of periodic high intensity fires. During high intensity crown fires, the seeds are released from the serotinous cones and the forest is re-seeded with lodgepole pines. In the absence of crown fires the lodgepole pine will eventually be replaced by Douglas fir, subalpine fir and Engelmann spruce.
Ponderosa pine forests are dependent on fires to kill the seedlings of more shade-tolerant species. Aspen can also replace ponderosa pine after stand-replacing fires. The change in forest type from ponderosa pine to an aspen or Douglas fir forest may cause a decrease in water yield, especially in more mesic sites with deeper soils (Macdonald and Stednick, 2003).
Douglas fir forests in the lower montane zone have a mixed severity fire regime. This fire regime is characterized by short interval surface fires and long interval crown fires (Jenkins et al., 2008).
The lower ecotone, historically, has had low to moderate severity fires that occurred every 10 to 30 years. Fire suppresion in this region has created a higher tree density, leading to a potential for higher severity fires.
The plains grasslands region is predominantly non-forested, but contains some patches of both ponderosa pine woodland and piñon-juniper woodland (Kaufmann et al., 2000). The plains grasslands region historically had frequent, low severity, fires. Much of the litter and ground cover remain, allowing for high infiltration and rainfall storage post-fire (Shakesby and Doerr, 2006).
Fire History in the Colorado Front Range
Historically, the fire regime in the Colorado Front Range was a mixed severity, or variable severity, among all forest types. Variations in weather conditions, fuels, and topography cause fires to burn in a complex fashion (Kaufmann et al., 2000). The historic fire regime is most often divided into three periods: pre-settlement (pre-1850), the European settlement period (1850 - 1920) and fire-suppression or exclusion period (1920 to present) (Macdonald and Stednick, 2003). In the pre-settlement period, fire was either human-induced or caused by a lightning strike. Native Americans used fire for a number of purposes including for land clearing, hunting, wildlife habitat improvement, defense and signals (Macdonald and Stednick, 2003). Despite these many uses, Native American influence on the forest structure is thought to have been small (Kaufmann et al., 2000). Beetles have long been a part of forest ecosystems in the Front Range. Beetles have long been a part of forest ecosystems in the Front Range. The connection between fires and beetles is discussed on the biodisturbance page.
The historic fire regime and the changes in that fire regime with time are best described by considering each forest type individually:
Remnants of fire on trail to Grey Rock, Colorado
(Photo by G. David)
Plains/Grasslands and Lower Ecotone
During the pre-settlement period, fires in the plains/grassland region were mainly moderate or low severity with a frequency of 10 to 30 years. These low to moderate severity fires tended to kill juvenile trees and maintained open mature stands of ponderosa pine (Kaufmann et al., 2000). Severe fires may have occurred in the lower ecotone zone where patches of shrubland exist. Grazing during the settlement period may have facilitated increases in tree density in the grasslands region. During the period of fire suppression, these low elevation zones have experienced a substantial increase in tree density (Kaufmann et al., 2000).
Lower Montane and Upper Montane - Ponderosa Pine and Douglas Fir Forests
Historically, the frequency of fires in this region ranged from over 30 to 100 years (Romme et al., 2006; Sheriff and Veblen, 2007). The fire frequency in the Front Range for ponderosa pine forests is much longer than further south in Arizona or New Mexico. Tree-ring data indicate that most of the ponderosa pine zone was characterized by a variable-severity fire regime that included a component of high-severity fire, which kills canopy trees and results in a dense regeneration period (Romme et al., 2006). Historically, dense ponderosa pine stands existed in the Front Range because of the relatively infrequent high-severity fires. The high-severity fires would most often occur during severe droughts after a wetter period when more fuels accumulated. Large fires would burn areas up to thousands of hectares in extent, but within that burned area there would be a patchwork of high severity, low severity and unburned areas (Romme et al., 2003; Macdonald and Larsen, in press). The combination of a mixed-severity fire regime and heterogeneous environmental conditions, allowed for a patchwork of naturally dense stands and open stands in this region (Kaufmann et al., 2000).
During the period of European settlement the lower and mid-elevation forests were characterized by more frequent high severity fires. Grazing, timber harvesting and clearing for pastures were common practices during the settlement period. These practices caused a decrease in forest density in many areas. After 1900, when fire suppression began to be practiced, the density of forest increased and the frequency of fires decreased. Some of this reduction in the fire frequency is because of the cessation of widespread burning by early settlers and the removal of fine fuels from grazing (Macdonald and Stednick, 2005; Macdonald and Larsen, in press). The reduction of fire frequency in these lower and mid-elevation forests has led to some denser stands. Historically, larger fires occurred when exceptionally dry periods followed some wet years. The table below shows fires that occurred during a severe drought in Colorado during the late 1990s and early 2000s. Larger, higher severity fires occurred during this time when there was a higher fuel loading from previous wet years followed by some extremely dry years (Macdonald and Stednick, 2005).
Subalpine Zone - Lodgepole Pine and Spruce-Fir Forests
Historically the subalpine zone had the highest severity and intensity fires that resulted in active crown fires. Although the severity and intensity is high, the frequency of these fires is about every 100 years or more for the spruce-fir forests. Large proportions of lodgepole pine stands in Colorado are more than 100 years old, because of large severe fires during the late 1800s. These fires occurred because climatic conditions were conducive to fires in the subalpine zone. Similar pulses were found to have occurred during the 1600s and 1700s, indicating that there is high variability in fire extent and stand initiation over periods of 100 years. Because large fires occur so infrequently in this zone, it is not likely that a few decades of fire suppression have significantly changed these stands of lodgepole pine and spruce-fir forests. Historically these stands were dense and had infrequent fires and these conditions continue today (Romme et al., 2003).
Table 1: Below is a description of fires that occurred during a severe drought in Colorado.
| Fire |
Location |
Date |
Area Burned |
Vegetation Type |
Fire Type |
Fire Severity |
Effects |
Reference |
| Hourglass |
Pingree Park, CO |
July 1994 |
5 km2 |
Lodgepole Pine |
Wildfire, Crown Fire |
High |
6 years after fire, erosion rates on burned hillslopes not significantly different, but channel incision and downstream deposition persist |
Benavides-Solorio and Macdonald, 2005; Larsen and Macdonald, 2007 |
| Buffalo Creek Fire |
South Platte River Basin, southwest of Denver |
1996 |
48 km2 |
Ponderosa Pine and Douglas Fir |
Wildfire, Crown Fire |
High |
86% of sediment yield from rill and channel erosion |
Moody and Martin, 2001 |
| Crozier Mountain |
Larimer County |
Sep. 1998 |
10 km2 |
Lodgepole Pine |
Prescribed Fire |
High to Moderate |
Patchy distribution of fire severity led to decreased sediment production rates in comparison to wildfires. Weak Water repellancy 22 months after burning. |
Macdonald and Stednick, 2003; Benavides-Solorio and Macdonald, 2005; Larsen and Macdonald, 2007 |
| Lower Flowers |
Larimer County |
Nov. 1999 |
3 km2 |
Ponderosa Pine |
Prescribed Fire |
Moderate |
Patchy distribution of fire severity led to decreased sediment production rates in comparison to wildfires. |
Benavides-Solorio and Macdonald, 2005; Larsen and Macdonald, 2007 |
| Dadd Bennett |
Larimer County |
Jan. 2000 |
2 km2 |
Ponderosa Pine |
Prescribed Fire |
Moderate to Low |
Patchy distribution of fire severity led to decreased sediment production rates in comparison to wildfires. |
Benavides-Solorio and Macdonald, 2005; Larsen and Macdonald, 2007 |
| Hi Meadows Fire |
near Pine, CO |
June 2000 |
40 km2 |
Ponderosa Pine |
Wildfire |
High |
Infiltration rates reduced to 45 ± 16 mm/hr from 120 ± 130 mm/h |
Martin and Moody, 2001 |
| Bobcat Fire |
west of Fort Collins |
June 2000 |
40 km2 |
Ponderosa Pine |
Wildfire |
High |
Sites burned at high severity produced 10 - 30 times as much sediment as unburned sites and sites burned at moderate severity produced 2 - 6 times as much sediment as unburned sites |
Benavides-Solorio and Macdonald, 2005 |
| Hewlett Gulch |
west of Fort Collins |
April 2002 |
3 km2 |
Ponderosa Pine |
Wildfire |
High |
|
Larsen and Macdonald, 2007 |
| Schoonover |
near Decker |
May 2002 |
16 km2 |
Ponderosa Pine |
Wildfire |
High |
|
Larsen and Macdonald, 2007 |
| Hayman |
southwest of Denver |
June 2002 |
557 km2 |
Ponderosa Pine |
Illegal Fire |
High |
Significant increase in flooding, runoff and erosion |
Larsen and Macdonald, 2007 |
| Big Elk |
near Pine Wood Springs |
Aug. 2002 |
18 km2 |
Lodgepole Pine |
Wildfire |
Mixed |
|
Larsen and Macdonald, 2007 |
|
Fire, Erosion, and Runoff
Fires change the structure and hydrologic properties of the soil as well as reducing the vegetative cover. The magnitude of the change to soil properties and vegetation depends on the severity of the fire. Changes to the soil structure depend on soil type and intensity of the fire. High severity burns can result in combustion of all organic material on the forest floor, the presence of a deep ash layer, alteration of minerals in soil and induce water repellency in non-repellent soils (Martin and Moody, 2001; Moody and Martin, 2001; Shakesby and Doerr, 2006). Water repellency has been found to prevent soils from wetting over periods ranging from seconds to months (Shakesby and Doerr, 2006). In the Colorado Front Range, strong water repellency was found in ponderosa and lodgepole pine forest that burned at high or moderate severity (Macdonald and Stednick, 2003). Infiltration rates can also be reduced in a forest ecosystem by disaggregated soil particles and ash clogging up the larger soil pores. Changes in the soil properties can result in a reduction of infiltration and increase in runoff. Summer convective storms can increase runoff by one to two orders magnitude for the first two to three years after a high severity fire (Macdonald and Stednick, 2003).
Martin and Moody’s (2001) study of the Hi Meadow fire near Pine, Colorado showed infiltration rates reduced to 45 ± 16 mm/hr from 120 ± 130 mm/h in the ponderosa pine forest. This reduction in infiltration increases the potential for erosion by overland flow and rainsplash (Shakesby and Doerr, 2006). Post-fire erosion can also increase from channel incision resulting from an increase in runoff and decrease in surface roughness and mass movements. Moody and Martin (2001) measured interrill erosion, rill erosion, sediment storage and channel sediment transport for two watersheds in the Colorado Front Range. They found that more sediment was eroded from north-facing versus south-facing slopes during the first year after the wildfire. The sediment fluxes neared the pre-fire rates three years after a high severity burn.
Studies in Colorado Front Range have shown that post-fire erosion rates are a result of fire severity and rainfall intensity (Macdonald and Stendick, 2003). Benavides-Solorio and Macdonald (2001) found that sites burned at high severity produce 10 to 30 times more sediment than unburned sites. Moderate severity burn sites produced 2 to 6 times more sediment. They also noted that this increase in post-fire erosion generally lasts longer than the fire-induced soil water repellency, implying that post-fire erosion is due to other factors. Benavides-Solorio and Macdonald (2001) also found that the best predictors for post-fire sediment yield were percentage bare soil and rainfall intensity.
Topography is also important in predicting post-fire sediment yield. Moody and Martin (2001) found in Buffalo Creek that 20% of sediment was eroded from hillslopes and 80% from channels. Benavides-Solorio and Macdonald (2001) found that swales produced a greater amount of sediment than planar hillslopes. The size of the sediment is also important in these studies. Granitic soils were found to have a higher rate of rilling because of the shear strength and soil aggregation is smaller in these soils than in metamorphic soils. Also, re-vegetation may be more difficult on sites with coarse soils because of the lower water-holding capacity, leading to a longer period of increased sediment yields (Benavides-Solorio and Macdonald, 2001; Moody and Martin, 2001; Shakesby and Doerr, 2006).
Other indirect effects of fire include increased mass movements, wind erosion, and bioturbation. The mass movement processes which are more likely to occur post-fire are dry ravel, debris flows, shallow landsliding and soil creep. Dry ravel is the "rapid dry particle-to-particle sliding of debris under the force of gravity" (Shakesby and Doerr, 2006). Unchannelized low-order drainages which were incised during the post-fire increase in runoff subsequently infilled from dry ravel, small alluvial fans and freeze-thaw processes over a much longer period of time than the fire frequency. The colluvium in these unchannelized drainages and hollows has a basal age of 1550 to 13 500 BP, indicating that the geomorphic features from a previous fire may persist through the next fire-flood sequence (Moody and Martin, 2001). The size and location of the drainage basin is important to consider when discussing the geomorphic and hydrologic effects of fire. Therefore, the effects of fires are discussed for each process domain.
