The Historic 2020 Wildfire Year in Colorado and Wyoming:

 ^{Title Picture: Calwood Fire in 2020. Photo Credit: Mark Kohn, Colorado Forest Restoration Institute.} 

The areas burned in the 2020 wildfires span a wide range of elevations (ca, 5,300 - 12,000 ft) which drives variation in typical environmental conditions. Forest communities also vary along these gradients, with lower montane forests ranging 5,500 - 8,000 ft, upper montane forests ranging 8,000 - 9,000 ft, and subalpine forests ranging 9,000 - 11,500 ft. These three forest types have a range of tree species and natural disturbance regimes to which they are adapted.

Lower montane forests are primarily composed of fire-tolerant species such as ponderosa pine (Pinus ponderosa var. scopulorum) and are adapted to survive frequent, low-severity fire.  In upper montane forests, thick-barked ponderosa pine and Douglas-fir (Pseudotsuga menziesii var. glauca) co-exist with lodgepole pine (Pinus contorta var. latifolia) and aspen (Populus tremuloides); these diverse forests are adapted to mixed-severity fire, where high-severity fire shaped forest dynamics and created some persistent openings and meadows. Subalpine forests of the study area are primarily composed of Engelmann spruce (Picea engelmannii var. engelmannii) and subalpine fir (Abies lasiocarpa var. lasiocarpa), with some lodgepole pine and aspen at lower elevations in this forest type. Infrequent, high-severity fires are a natural occurrence in subalpine forests. Much of the area affected by the 2020 wildfires burned under extreme winds and weather, with 42.8 - 58.5% of each fire burning at high severity (see table above).

The species in these forest types have a range of strategies to recover following fire. Most coniferous tree species in the western United States require seed dispersal from live trees to successfully regenerate in severely burned areas. For many species, the majority of seed dispersal is limited to 650 ft from a live tree.  Thus the limited availability of live trees in large, high-severity patches raise potential concerns about natural forest recovery in some portions of the 2020 fires. 

However, the pre-fire composition of the forest plays an important role in ecosystem response.   For example, species such as ponderosa pine have larger, heavier seeds that frequently tend to fall around parent trees, with fewer long distance dispersal events, which has resulted in researchers finding limited or delayed regeneration of this species within large, high severity burn areas. Some Rocky Mountain species such as lodgepole pine and aspen have fire-adaptive traits like serotiny (cones that open following heating) or resprouting (the ability to reproduce from belowground root tissues after burning), respectively, that may facilitate rapid post-fire recovery of large high-severity patches. Temperature, precipitation, and topography also influence patterns of post-fire tree regeneration for most species, with cooler, wetter sites often having more abundant tree establishment in comparison to warm, dry areas. Therefore, it could be expected that the way in which forests will recover following the 2020 wildfires will vary based on the severity of the fire in a particular area, as well as the species present before the fire.  

Because of the limited availability of live trees in large, high severity patches of the 2020 fires, conversions from forests to grasslands or shrublands may occur, as well as associated changes to ecosystem services. Such changes in ecosystems may not be desirable for landowners, land managers, or other stakeholders, which motivates potential management intervention. After fire, reforestation is a key management strategy that can help to accelerate forest recovery. However, reforestation is expensive and time-consuming, and may not be feasible, successful, or desirable everywhere. Assessments of postfire landscape conditions may help to plan management efforts and allocate reforestation resources following the 2020 wildfires. To address this need, Rodman et al. 2022 used spatial data, together with statistical models developed from west-wide surveys of post-fire tree regeneration, to conduct a post-fire landscape assessment for areas affected by five 2020 wildfires. For each fire, Rodman et al. 2022 quantified spatial patterns of live trees, the effects of pre-fire bark beetle outbreaks, the potential for natural post-fire recruitment of the dominant conifer species, and site moisture (a proxy for planting suitability).  

Satellite-derived maps of wildfire severity and pre-fire tree height were used to quantify distances to live trees within each of the five wildfires. Distances to live trees give an estimate of viable seed dispersal to aid in natural recruitment; beyond 650 feet from live trees, natural seed dispersal becomes difficult for most the tree species in the US Rocky Mountains.

• Calwood- Of total fire area, 42.8% was high severity. 40.7% of this high-severity area was within 300 ft of a live tree, as compared to 29.9% of the area within 300 - 650 ft, and 29.4% beyond 650 ft.

• Cameron Peak- Burned 208,913 ac, becoming the largest fire in Colorado’s recorded (i.e., since the early 1900s) history. Of the total fire area, 54.3% burned at high severity. Overall, 38.5% of this high-severity area was within 300 ft of a live tree, as compared to 28.8% of the area within 300 - 650 ft and 32.8% beyond 650 ft of a live tree.

• East Troublesome- 57.4% burned at high severity. 37.9% of this high-severity area was within 300 ft of a live tree, as compared to 32.2% of the area from 300 - 650 ft to a live tree, and 29.9% of the area > 650 ft.

• Mullen- 49.7% of total fire area burned at high severity. 50.4% of the high-severity area was within 300 ft of a live tree, as compared to 31.0% of the area from 300 - 650 ft to a live tree, and just 18.7% being > 650 ft.

• Williams Fork- The highest proportion of high-severity fire (58.5% of the total area). Still, 49.2% of the high-severity area was within 300 ft of live trees, 27.8% was between 300 - 650 ft, and only 23.0% was beyond 650 ft.

After 650 feet, natural seed dispersal becomes difficult. Below is an example of the importance of surviving trees along edges of high severity burn areas, where post-fire regeneration is occurring near parent trees. The foreground shows areas that have not established regeneration due to distances from surviving trees.

To estimate the likelihood of natural forest recovery in the 2020 wildfires, statistical models were developed to relate post-fire recruitment across thousands of field plots in the US West to recent climate, topography, fire severity, distance to live trees, post-fire canopy cover, and pre-fire bark beetle outbreaks. These statistical models were then used to predict post-fire recruitment probability for each of the dominant conifer species in the 2020 wildfires.

In the figures below, P(recruit) represents the probability that at least 40 trees/ac of a given species will establish within 10 years of fire occurrence. Recruitment probabilities varied by tree species and landscape, with most species predicted to have greatest recruitment at higher elevations in their range, in valley bottoms, and on north-facing slopes. This information can be used to target future monitoring activities and help to plan early reforestation efforts. Where natural recruitment is likely to meet the desired conditions for a site, post-fire reforestation activities such as planting seedlings are probably not needed.

• Calwood- At higher elevations, lodgepole pine had the highest probabilities of natural recruitment. Ponderosa pine and Douglas-fir had low to moderate probabilities of natural recruitment in many portions of the fire, with the highest probabilities near live trees; Douglas-fir also had higher recruitment probabilities on northeast-facing slopes.

• Cameron Peak- Lodgepole pine had high probabilities of recruitment throughout much of the fire. Subalpine fir and Engelmann spruce had relatively low recruitment probabilities overall. Probability of ponderosa pine recruitment is relatively low throughout the fire but is moderate in areas near live trees. Douglas-fir had high recruitment probabilities at intermediate elevations, in areas with surviving canopy cover, and on northeast facing slopes.

• East Troublesome- Recruitment probabilities were relatively low for many tree species. However, lodgepole pine had high recruitment probabilities throughout much of the fire, and Engelmann spruce had high probabilities in wet, high-elevation areas. Subalpine fir, ponderosa pine, and Douglas-fir had low predicted recruitment probabilities in most portions of the fire.

• Mullen- Predictions of post-fire recruitment by species suggest that lodgepole pine and Engelmann spruce are likely to recover well throughout much of the fire. Ponderosa pine and Douglas-fir also show high potential for natural recruitment, though these species were relatively limited in extent before the fire.

• Williams Fork- The dominant species was lodgepole pine, with Engelmann spruce being dominant at higher elevations. Models of post-fire recruitment suggest that abundant lodgepole pine recruitment is likely in the eastern half of the fire. Engelmann spruce and subalpine fir had high recruitment probabilities in low-severity portions of the fire, particularly at high elevations. Ponderosa pine and Douglas-fir are unlikely to recruit in many portions of the fire because of low pre-fire abundances.

To inform potential reforestation activities, recent climate data (ca., 1981 - 2020) and topography were compared to the environmental tolerances of each species. Restricted to high-severity portions of each wildfire where planting is most likely, these maps provide an estimate of how suitable different parts of the 2020 wildfires would be for reforestation using a given tree species. Like maps of natural recruitment, these predictions indicate that site moisture varied among species and landscapes, suggesting that a range of site-specific management strategies will be needed across the 2020 wildfires.

• Calwood- Ponderosa pine and Douglas-fir seedlings had high planting suitability in higher elevations, valley bottoms, and northeast-facing slopes. Planting of ponderosa pine and Douglas-fir may be best suited at intermediate to high elevations throughout the fire.

• Cameron Peak- Planting suitability varied substantially for each species in this fire. Ponderosa pine and Douglas-fir had high moisture availability at intermediate elevations in the fire, particularly in valley bottoms and northeast-facing slopes. Suitability for subalpine fir was relatively low in most areas, but was moderate at the highest elevations and on northeast-facing slopes. These same sites had suitability for Engelmann spruce and lodgepole pine. Plantings of ponderosa pine and Douglas-fir at intermediate elevations in the fire, and plantings of lodgepole pine and Engelmann spruce at higher elevations may be most suitable based on these models, particularly on northeast-facing slopes and valley bottoms.

• East Troublesome- Planting suitability across this fire was relatively high, suggesting that reforestation activities may be successful. Reforestation may be best suited to areas where initial site assessments indicate limited post-fire recruitment of lodgepole pine from serotinous cones. In particular, areas occupied by Engelmann spruce, ponderosa pine, and Douglas-fir before fire occurrence had high planting suitability. At lower elevations in the fire (e.g., < 9,000 ft), limited planting of ponderosa pine and Douglas-fir may be appropriate, whereas planting of lodgepole pine and Engelmann spruce may be more appropriate at higher elevations in the fire, with a particular focus on northeast-facing slopes.

• Mullen- Site moisture was high for many of the coniferous tree species. Lodgepole pine had moderate to high site moisture in many areas, particularly at intermediate to high elevations. High-elevation areas throughout the fire had high site moisture for Engelmann spruce, whereas ponderosa pine and Douglas-fir had some isolated areas with high site moisture in protected topographic settings.

• Williams Fork- Localized areas of moderate or high site moisture were present for each coniferous tree species. Overall, high elevations and northeast-facing slopes were most suitable for subalpine fir, lodgepole pine, and Engelmann spruce, while low-elevation areas were considered suitable for ponderosa pine and Douglas-fir. Limited planting of ponderosa pine and Douglas-fir would both be appropriate at the lowest elevations, while Engelmann spruce and lodgepole pine would be appropriate at the highest elevations.

The 2020 wildfire year has accentuated the need for strategic post-fire planning across large, forested landscapes in the western US. As the number of severely burned acres at risk of converting to non-forest continues to expand, there is increased urgency to effectively direct limited funds and expertise to high-priority areas. Predictions of natural recruitment following wildfire will help to identify locations where natural forest recovery will meet the desired conditions for a site.

Alternatively, where a site is likely to convert to a grassland or shrubland and the public desires a return to forested conditions, maps of site moisture may help to prioritize reforestation activities where they are most likely to be successful. Reforestation of all areas affected by the 2020 wildfires (or even high-severity areas within them) is likely to be infeasible in the coming years due to the time and money required for active reforestation. Therefore, targeting areas in which natural recruitment is unlikely but site moisture is high for one or more tree species, may help in the initial prioritization of limited resources for post-fire reforestation. However, such considerations should be included in a broader decision framework that also incorporates policy mandates, local site visits, expert knowledge, and desired conditions.

Below are examples of natural forest recovery from the Colorado Forest Restoration Institute field monitoring in 2021 - 2022 in the Cameron Peak and Mullen Fires. Use the slider to switch between photos.