Snag Dynamics in Sierra Nevada Mixed-Conifer

Kestrel Grevatt
Aug 12, 2021
15 min read

“Dead wood forms an important yet relatively poorly documented forest carbon stock that is strongly impacted by forest management.”

– M.T. Moroni, Canadian Forest Service

“Altering conditions so that disturbance processes can act to increase, rather than reduce, forest heterogeneity may provide ecosystems with the ecological flexibility to withstand and persist through future changes in climate and climate-related processes.”

– Michelle Coppoletta, US Forest Service

All photos are from the King Fire scar on Eldorado National Forest and prescribed burns at Blodgett Forest Research Station, both located on the unceded lands of the Nisenan and Washoe. This post mostly discusses snags and fire from a modern science perspective, but there is a vast amount of cultural knowledge and holistic understanding held by Native peoples. Some of this knowledge was lost due to genocide, land theft, and forced assimilation, but much of it persists and whenever possible we should be listening to the original stewards of these forests.

Wildfire & Snags

In 2014 the King Fire burned 97,717 acres on the Eldorado National Forest and private timberlands. In the month it burned the King Fire cost $117 million in suppression and forced the evacuation of nearly 3,000 people. 12 residences and 68 other residential structures were destroyed, and another 500 homes in Georgetown and Pollock Pines were threatened by the fire. The fire came within a few tenths of a mile of Blodgett Forest Research Station property. Besides the Happy Camp Complex, the King was the largest blaze of 2014. Our scale for wildfires has become drastically distorted in the past few years as we’ve watched fires ravage record-breaking acreages at high severity and in 2020 watched multiple complexes each burn over 300,000 acres. However, at the time in 2014 the King Fire was a massive incident which offered a foreshadowing of the high intensity, fast moving, extreme fire behavior which has become all too normal since, with a 15-mile run on September 17 which burned 50,000 acres [7]. 50 percent of the King Fire burn area was considered high severity, with greater than 90 percent tree mortality.

This proportion of high severity acreage is devastating (but becoming increasingly common) and left the Eldorado National Forest reeling with how to address post-burn response. High-severity patches of thousands of acres are becoming increasingly common and force managers to make difficult decisions. Erosion control and hazard tree falling were two of the first considerations, but ultimately there simply were not adequate resources to restore as needed post-wildfire, something we are seeing play out all over the state post-2020 fire season. The total burned area included 63,536 acres of National Forest System lands, which contained or partially contained 30 watersheds [7]. The restoration efforts focused on priority areas covering only 26% of this acreage. The King Post-Fire Burn Area Emergency Response (BAER) Program was awarded just over $3 million for restoration efforts.

From Forest Service King Fire Restoration Project [8, 9]:

Salvage harvest of trees killed and other fuels treatments on 15,739 acres
- Methods include mechanical or other ground based logging on approximately 11,800 acres, skyline or helicopter logging on approximately 700 acres, hand treatments on approximately 700 acres, and mastication or machine piling on approximately 100 acres.
- Within Strategic Fuels Management Zones, WUI Defense Zones, and Forest Resiliency Areas, remove dead conifer trees using in excess of soil cover needs and wildlife snag retention levels needs. In the Forest Resiliency Areas, snags will generally be retained in two to five acre patches covering 15 to 20 percent of a treatment area and incorporating the largest snags available. No standing snags will be retained in WUI Defense Zones, and four large snags per acre up to 12sq. ft./acre basal area in a grouped configuration will be retained in Strategic Fire Management Zones.
- Hazard trees to be removed are dead and dying trees that have potential to reach the road or property and live trees that are sufficiently damaged or defective to pose a risk of falling within the next 5 years.
Roadside hazard tree removal along 198 miles of roads
Erosion control and rehabilitation treatments on 227 acres of watershed sensitive area
Reforestation on 10,609 acres
Prescribed burning of 2,100 acres in the Rubicon Canyon
Aerial mulching treating of 1,265 acres with rice straw for erosion control

The King Fire left behind significant patches of standing dead trees, or snags. Salvage harvests and biomass utilization are limited by transport costs and facility processing capacities. Much of the private industrial timberlands that burned were salvage logged and replanted within a few years (with slightly more significant fuel breaks). Currently many mills are still backlogged trying to process salvaged timber from 2020. Due to fire suppression and mismanagement, when fire is reintroduced into western US forests it often burns with more homogeneity and less patchiness than it would in the natural fire regime. The King Fire is an example of this, where much of the interior burned at higher intensity and the largest high-severity patch was over 10,000 acres. This severe disturbance on a large scale is not what the Sierra Nevada ecosystem is adapted to and creates opportunity for severe erosion, soil loss, and long-term forest loss due to shrub colonization if not restored. It is important to acknowledge that prior to colonization of this region Native tribes burned frequently at low intensity to maintain the understory, reduce wildfire risk, and cultivate food sources, amongst numerous other positive impacts. The conditions for the King Fire were created by fire suppression and mismanagement after extensive land theft from Indigenous peoples.

Looking at the King Fire restoration plan, mechanical and ground-based salvage logging and reforestation are the priority for the largest amounts of acres: to prevent long-term forest loss, create income from this catastrophe, and prevent future fire risk. Much of the ongoing replanting is utilizing a technique known as “cluster planting” with gaps between small groups of seedlings [10]. This spatial arrangement more closely mimics the structure frequent low-severity fire would have created and may increase wildfire resiliency compared to plantation forests. Restoration efforts are ongoing, but seven years out from this fire the many untreated acres are beginning to have potential for reburn with the added factor of sizeable patches with significantly elevated percentages of standing dead trees.

Many of these snags will still stand for years and possibly decades. Snag decay rates and fall rates are dependent on snag size, species, cause and season of mortality, and the micro-environment. The time it takes for snags to fall is highly variable and average fall rates can be misleading as many more will fall once they reach a critical decay point. For example, one study found that approximately 85% of snags killed by a wildfire in California fell within a five-year period 18 to 23 years later [4]. This means you have a large initial pulse within five to ten years of standing dead fuel as living trees die, and another pulse of heavy surface fuels around 20 years later – but there are many factors that can impact this.

Snags are inventoried and categorized into five standard Decay Classes: 1 being the freshest, 5 being the most decayed. Generally snags are discussed as being sound and hard (Decay Classes 1, 2, and 3) or soft (Decay Classes 4 and 5).

* Left: Hard snag of lower decay classes - still has branches and twigs, wood is sound.

* Right: Soft snag of higher decay classes - no bark, only branch stubs, beginning to decay.

As size and severity of wildfires increase there is potential for soft snag deficits (and a surplus of hard snags) in large areas for extended periods of time, as snags created by a severe fire will likely die and decay at similar rates. Species will impact decay rates and can be a helpful factor to ensure existing soft snags that will last the longest are kept as well as hard snags that will enter the soft snag phase the soonest and fill that gap. Large Douglas-fir snags have been shown to have increased longevity compared to other conifer species and thus should be preserved to improve likelihood of snag continuity post disturbance [4]. The importance of species when determining which snags to keep is one component that the King Fire restoration guidelines don’t appear to incorporate.

Specifically high- to moderate-severity fire causes a significant increase in standing snags and shrub vegetation which results in increased likelihood of a high-severity reburn [2]. Studies have shown that snag density is one of the most important biophysical factors for reburn severity – potentially a larger influence than shrub cover [3]. Reburn severity is dependent on initial fire severity, time since initial burn, temperature, relative humidity, shrub cover, and snag basal area (a measure of stand density using stem cross-sectional area). Due to the impacted fire and disturbance regime in the Sierra Nevada dense pre-fire stands may be converted to high-density snag stands by fire reentry – many areas of the El Dorado NF which burned in the King Fire demonstrate that [2].

As you can see in the photos, there is enough regrowth in many areas of the King Fire scar for potential for reburn. Studies have shown that the window for low reburn potential closes within five to ten years, as fuels can recover to their pre-burn levels in nine to fifteen years. Areas reburned more than nine years after the initial fire are more likely to reburn at higher severity, due to snag recruitment from tree mortality and standing snags falling and increasing surface fuel loads in the form of coarse woody debris (CWD) [2]. CWD is defined as any dead wood (branches, trunks, pieces of stumps) that is larger than three inches in diameter (although this exact threshold may vary somewhat) and may also be referred to by the less formal term “heavy fuels” or 1000-hr time lag class. Delayed mortality occurs after wildfires of all severities, but generally damaged trees will die within four to five years post-fire, hence an increase in dried fuels [5]. Snags, stumps, and logs create “localized pockets of long-duration heating” which can ignite adjacent fuels and high densities of snags and CWD throughout an area can thus contribute to large-scale high-severity impacts [3]. Snag basal area also has decreased impact on reburn severity as more time passes since the initial fire [2].

This past summer of 2020 there was a small fire in the Rubicon near the edge of the King Fire burn scar. While the burned area would’ve offered less fuel in some areas and could potentially have slowed the fire down enough to allow crews to get ahead of it, the high density of snags, CWD, and brush in many areas (six years out from initial burn at that point, seven years out in these photos) means that under the right weather conditions much of the burned area could have reburned.

Looking at the Restoration Project guidelines for the King Fire there are set standards for snags for differently designated areas (Forest Resiliency Areas - retain snags in 2 to 5 acre patches covering 15 to 20 percent of a treatment area and incorporating the largest snags available; WUI Defense Zones - no standing snags; Strategic Fire Management Zones - 4 large snags per acre up to 12 sq. ft./acre basal area). These are based off forestry and wildlife principles and incorporate many different considerations. However, they are being set at the acre and multiple acre scale which is not representative of the heterogeneity of a healthy forest. This needs to increase by orders of magnitude to allow for realistic spatial variation, although this is complicated by variation in land ownership and hard to implement across federal, state, and private lands. Analysis of post-fire tree mortality has shown that landscape-scale heterogeneity in forest structure also allows for resistance to a range of mortality agents and can decrease mortality in initial fire as well as aid survival of impacted trees in following years [5].

Increased flexibility is needed for standards and guidelines regarding snag management overall. In the Sierra San Pedro Martir (SSPM) National Park in northwestern Mexico the Jeffrey pine-mixed conifer forests have been less anthropogenically impacted on a landscape scale and have not experienced systematic fire suppression or harvesting. It shares adequate similarities with north-central Sierra Nevada forests that it can be used as a comparison and offers critical data for unimpacted baselines. The average snag density in the SSPM varies greatly, and overall snag distribution is very patchy, which shows that small-scale snag targets should be reconsidered [1]. This finding is repeated across multiple studies.

Especially after stand-replacing events such as high-severity burns like the King Fire, achieving continuity (i.e. no gap in habitat and nutrient availability) in soft snags requires a landscape-scale perspective rather than small-scale basal area requirements. A basal area standard also does not address what size class snags are bring retained. Snag size is an important factor in decay rate and habitat quality – many smaller, harder snags will not offer equivalent ecosystem value as a few larger soft snags but could have an equal basal area. Overall post-fire treatments need to be focused on fuel reduction rather than cost-recovery, which means keeping larger legacy snags [2]. A salvage harvest that creates more profit (through larger trees) can allow for more acres to be treated, so managers must be intentional when deciding what to prioritize. Thinning treatments can also increase surface fuels through physical damage and machinery impacts and should be followed up with additional treatment such as pile or broadcast burning [2]. This presents an additional cost and is not always the priority – after the King Fire only 2,100 acres were slated for prescribed burning.

Prescribed Fire & Snags

In wildfire suppression you are taught that snags are hazards. Heads up when you’re walking through a snag patch. Never stand under a snag, never let your guard down. When falling a snag, you must keep your eyes up and always be ready for branches or the top to break out due to vibrations. The lack of weight and potential for rot presents additional risk as you must cut longer and have less ability to know where the snag will fall. Snags present challenges and require increased situational awareness whenever crews are working in their vicinity. When there was a small start in the King Fire scar this past summer it created a very hazardous situation to send firefighters into the area to perform suppression work. The Forest Service crews utilized drones to try to minimize some of this exposure. Utilizing drones, aerial resources, or falling crews can all help to reduce risk due to snags but sometimes it is unavoidable.

*Post prescribed burn in a mixed-conifer plantation. Torching and pockets of higher intensity burning means some of these trees will become snags in the next few years.

*Short snag which caught fire and became a chimney after a prescribed burn. Spewing embers and very hazardous if near line.

When putting controlled fire on the ground, whether a prescribed burn or back burn on a wildfire to create a thick blacked out buffer, snags near the edge are hazardous. They can become chimneys, spewing embers into the green and creating spot fires. Weakened by fire even hard snags can rapidly become structurally unsound. The tops may break, or they can fall on or across the line which is a massive hazard for ignition and holding crews and can cause a slop over. CWD and snags can increase chances of crowning, torching, and spot fires due to high heat generation and preheating during combustion [6]. Snags act as both a source and receptive fuel for embers, can increase the duration of smoldering and burning, and contribute to more smoke production and reduced air quality/visibility. Prolonged burning and residence times can also mean soil heating and impacts on soil communities are more intense [6]. After a prescribed burn snags, stumpholes, and logs are generally the biggest concern for ember production and ongoing burning and often end up being the focus of patrols and mop up.

For folks operating around snags, it can be tempting to remove them to minimize your crew’s exposure, and sometimes falling particularly hazardous ones is necessary. However, like most components of land management there are multiple priorities to be considered and working to mimic a natural disturbance regime and prioritize long-term forest resiliency doesn’t necessarily align with the easiest or safest practices. Research has shown that larger legacy snags should be retained whenever possible, especially if of a species of slower decay and fall rate, but sometimes those are also the most hazardous snags.

Although they can present hazards and complications to managers, the ecosystem value of snags and CWD is well documented. They are critical for wildlife habitat, the maintenance of soil organic matter, and long-term site productivity [4]. For woodpeckers snags offer a food source and nesting site. Woodpeckers are primary cavity nesters who create new cavities and rarely reuse nests, thus creating refuges that will be utilized by other birds and small mammals who cannot excavate nests themselves. Snags also provide habitat for larger mammals and insect populations (which are a food source for avian species). Over 100 species of birds, mammals, reptiles, and amphibians require snags for shelter and food in the west. While large, soft snags are important for cavity creation smaller, hard snags also play a critical role in nutrient cycling [4]. After they topple over snags continue to provide essential services as logs – as a food resource of fungi and arthropods for mammals, offering cover for fish in streams, and creating natural barriers. CWD also stabilizes soil and prevents erosion and up to forty percent of all forest fauna is dependent on it. Eliminating snags and CWD can have severe impacts on wildlife and the forest ecosystem.

From a practical sense prescribed burning is often the cheapest option per acre for getting forest treatment and wildfire risk reduction accomplished and can reduce CWD and fuels build ups while also recruiting new snags. However, it is necessary to be realistic about what prescribed burning can and cannot accomplish and be willing to accept a range of mortality and consumption impacts. Additionally, snags created by wildfires or prescribed burns will generally topple sooner than snags created by drought and may not provide equivalent habitat value [1]. In a fire-suppressed ecosystem there is often an unnatural accumulation of litter and duff as well as sloughing bark at the base of trees that can contribute to hotter burning and longer residence times which can cause heat girdling of the roots or burn through the trunk and previously living trees can die and fall quickly. Thus, even moderate intensity prescribed fires can contribute to snag and CWD recruitment.

* Potential impact of rx burning: living trees burned out at base will become snags and subsequently CWD faster than naturally occurring snags. Hazard to crews if hung-up/or weakened at base by fire.

* Strip of orange ash is the remains of large snag that caught during a prescribed burn, fell over, and consumed entirely -- an important fuel reduction during a first entry burn.

The ongoing National Fire and Fire Surrogate study compares the outcomes from prescribed fire and other treatment combinations (fire only, mechanical only, mechanical plus fire, and controls with no treatment). Data collected at Blodgett Forest Research Station found that fire only and mechanical plus fire treatments created an increase in snag density of larger fresh snags (Decay Classes 1-3) whereas just mechanical treatment (thinning from below plus rotary mastication for this study) significantly decreased the density of smaller snags (<15 cm size class) [6]. These smaller snags are not as significant for habitat and can increase fire risk both initially and longer term, because once on the ground they contribute to surface fuels loading. Additional prescribed fires may be necessary to consume this influx of surface fuels. Prescribed burning was also shown to significantly reduce the amount of rotten CWD (which is movement towards alignment with pre-suppression structure, as current levels of rotten CWD would not have been sustained under a natural fire regime), but sound CWD of the lower decay classes was not significantly reduced by any of the treatments [6].

Decades of fire suppression has resulted in high fuel continuity that makes it challenging to retain decayed snags and CWD during initial entry prescribed burns. In subsequent burns more large woody material and snags will avoid consumption due to less continuous fuel. Essentially, repeated burning does not result in more snags burning each time, over time if you are burning at an appropriate intensity the snag density will balance out. Fall burns as compared to spring burns will likely have higher snag recruitment (mortality) and CWD consumption due to higher severity of fall burning and lower fuel moistures exiting the summer months.

As we stare down another fire season with wildfire activity already far surpassing climate and fire model expectations, snags and woody debris are perhaps just one piece being added to the puzzle. Many fire models do not account adequately for density levels of CWD and snags nor their significant impacts on fire severity [3]. Even as research continues and models improve, creating implementable guidelines and treatments remains challenging. Generally, standards are set on small acreage scales, yet hazards, snags, and downed trees are not necessarily evenly distributed. There is a need to be setting these guidelines for areas of hundreds of hectares.

As we attempt to reintroduce fire to areas that once experienced frequent, low-moderate intensity fire regimes but for the past hundred years have experienced fire suppression, minimal management, and/or high-grade logging, managing for snags is not exactly the priority or an easily accomplishable goal. Furthermore, lack of intact disturbance regimes in Western North America makes it difficult to know what we should be aiming for when managing snags and CWD. Thus research from areas like the less anthropogenically impacted forest of the SSPM National Park in northwestern Mexico is very important and offers context of what to manage for. Current low numbers of large soft snags throughout the Sierra Nevada are likely a result of timber harvests throughout the 20th century and make avoiding gaps in snag availability post-wildfire even more challenging. Current smaller and less decayed snag and CWD densities are likely elevated compared to historical levels due to fire suppression, and high fuel loading overall translates to increased consumption and worse impacts from a wildfire [6].

Prescribed fire and other fire surrogates (e.g. mastication, thinning, and hand pile burning) are never going to perfectly simulate the disturbance of frequent low intensity wildfire. Whether discussing snags or any other component of a forest ecosystem we must be realistic and consider the larger picture. As much as we try to break our forests up into discrete parcels of ownership and responsibility that is not how forests and ecosystems function. Like most components of natural resource management, nuance is required when considering and communicating about snags and CWD. Snags offers a metaphor for the complex interconnections between all elements of forest ecosystems and the inherent value of each. Through one lens snags are hazards that increase wildfire risk, can be a threat to personnel, and must be removed; through another lens they offer critical ecosystem services. We must consider fire reintroduction as a landscape ecosystem process and the importance of dead wood management as we decide how to apply our array of management tools to create forests that can experience repeated wildfires without high levels of tree mortality.