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Fuel reduction treatments are increasingly used to mitigate future wildfire severity in dry forests, but few opportunities exist to assess their effectiveness. We evaluated the influence of fuel treatment, tree size and species on tree mortality following a large wildfire event in recent thin-only, thin and prescribed burn (thin-Rx) units. Of the t...
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... 2006 Tripod Complex fires burned over 70 000 ha in the Okanogan-Wenatchee National Forest, Washington State (Fig. 1). The majority of the fire area burned with moderate to high severity in high elevation forests (.1300 m) dominated by lodgepole pine (Pinus contorta var. latifolia), Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa). The south-western portion of the fire burned at low to mid elevations and involved past fuel ...
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... There was significant variability in efficacy among treatment types. We found that the "thin only" treatments reduced fire severity by less than half as much as the three most effective treatments, consistent with previous studies that conclude "thin only" treatments are less effective than those that treat surface fuels with, for example, prescribed fire Kalies and Yocom Kent, 2016;Martinson and Omi, 2013;Prichard et al., 2020;Prichard and Kennedy, 2012;Raymond and Peterson, 2005). Here we found that in some cases "thin only" treatments led to a reduction in wildfire severity, especially in younger treatments, however, not treating surface fuels following thinning led to increased wildfire severity compared to controls (positive effect size) in 40% of "thin-only" observations. ...
Increased understanding of how mechanical thinning, prescribed burning, and wildfire affect subsequent wildfire
severity is urgently needed as people and forests face a growing wildfire crisis. In response, we reviewed scientific
literature for the US West and completed a meta-analysis that answered three questions: (1) How much do
treatments reduce wildfire severity within treated areas? (2) How do the effects vary with treatment type,
treatment age, and forest type? (3) How does fire weather moderate the effects of treatments? We found overwhelming
evidence that mechanical thinning with prescribed burning, mechanical thinning with pile burning,
and prescribed burning only are effective at reducing subsequent wildfire severity, resulting in reductions in
severity between 62% and 72% relative to untreated areas. In comparison, thinning only was less effective –
underscoring the importance of treating surface fuels when mitigating wildfire severity is the management goal.
The efficacy of these treatments did not vary among forest types assessed in this study and was high across a
range of fire weather conditions. Prior wildfire had more complex impacts on subsequent wildfire severity, which
varied with forest type and initial wildfire severity. Across treatment types, we found that effectiveness of
treatments declined over time, with the mean reduction in wildfire severity decreasing more than twofold when
wildfire occurred greater than 10 years after initial treatment. Our meta-analysis provides up-to-date information
on the extent to which active forest management reduces wildfire severity and facilitates better outcomes for
people and forests during future wildfire events.
... Persistence is often assumed to be the preferred means of maintaining ecosystem services, is usually the most socially acceptable, and may be mandated by land-management agencies (Lynch et al. 2021). Fuel treatments can reduce the risk of mortality due to fire Moghaddas 2005, Prichard andKennedy 2012) and/or drought (van Mantgem et al. 2016, and help post-disturbance communities to align more closely with the historical range of variation (Young et al. 2020a. Stand density or basal area reduction may also assist areas adapting to the drier and warmer climate projected for coming decades (Young et al. 2020a. ...
Ecosystems are dynamic systems with complex responses to environmental variation. In response to pervasive stressors of changing climate and disturbance regimes, many ecosystems are realigning rapidly across spatial scales, in many cases moving outside of their observed historical range of variation into alternative ecological states. In some cases, these new states are transitory and represent successional stages that may ultimately revert to the pre-disturbance condition; in other cases, alternative states are persistent and potentially self-reinforcing, especially under conditions of altered climate, disturbance regimes, and influences of non-native species. These reorganized states may appear novel, but reorganization is a characteristic ecosystem response to environmental variation that has been expressed and documented throughout the paleoecological record. Resilience, the ability of an ecosystem to recover or adapt following disturbance, is an emergent property that results from the expression of multiple mechanisms operating across levels of organism, population, and community. We outline a unifying framework of ecological resilience based on ecological mechanisms that lead to outcomes of persistence, recovery, and reorganization. Persistence is the ability of individuals to tolerate exposure to environmental stress, disturbance, or competitive interactions. As a direct expression of life history evolution and adaptation to environmental variation and stress, persistence is manifested most directly in survivorship and continued growth and reproduction of established individuals. When persistence has been overcome (e.g., following mortality from stress, disturbance, or both), populations must recover by reproduction. Recovery requires the establishment of new individuals from seed or other propagules following dispersal from the parent plant. When recovery fails to re-establish the pre-disturbance community, the ecosystem will assemble into a new state. Reorganization occurs along a gradient of magnitude, from changes in the relative dominance of species present in a community, to individual species replacements within an essentially intact community, to complete species turnover and shift to dominance by plants of different functional types, e.g. transition from forest to shrub or grass dominance. When this latter outcome is persistent and involves reinforcing mechanisms, the resulting state represents a vegetation type conversion (VTC), which in this framework represents an end member of reorganization processes. We explore reorganization in greater detail as this phase is increasingly observed but the least understood of the resilience responses. This resilience framework provides a direct and actionable basis for ecosystem management in a rapidly changing world, by targeting specific components of ecological response and managing for sustainable change.
... Decades of NWFP implementation and monitoring, science development and significant policy changes (e.g., recovery plans, 2012 Planning Rule) have resulted in considerable body of evidence for climate change adaptations. For example, in dry zone forests, a growing body of evidence highlights the effectiveness of forest treatments to alter forest stand structure (Raymond and Peterson 2005, Prichard et al. 2010, 2020, Prichard and Kennedy 2012) and landscape-scale fire spread and severity (Collins et al. 2011, Finney et al. 2008, Ager et al. 2010, Safford 2012, Tubbesing et al. 2019, Hessburg et al. 2021). In addition, stand-level effects of restoration treatments have been monitored on a wide variety of ecosystem resources (Gaines et al. 2007(Gaines et al. , 2010aNorth 2009, McIver et al. 2012 for a review, Moghaddas et al. 2010, Schwilk et al. 2009, Stephens et al. 2009a, 2009b, Stephens and Moghaddas 2005. ...
The 1994 Northwest Forest Plan signified a watershed moment for natural resource management on federal lands in the Pacific Northwest. It established clear priorities for ecologically motivated management of terrestrial and aquatic ecosystems and biodiversity conservation on nearly 10 million hectares of public lands in Oregon, Washington, and northern California. Conservation reserves were the primary means of safeguarding remaining old forest and riparian habitats, and the populations of northern spotted owl, marbled murrelet, and Pacific salmon that depend on them. As envisioned, reserves would provide habitat for the protected species during a lengthy recovery period. However, reserve strategies were grounded on two tacit assumptions: the climate is stable, and there are limited disruptions by invasive species; neither of which has turned out to be true. Managing for northern spotted owls and other late-successional and old forest associated species within the context of static reserves has turned out to be incredibly challenging. As climatic and wildfire regimes continually shift and rapidly reshape landscapes and habitats, conservation efforts that rely solely on maintaining static conditions within reserves are likely to fail, especially in seasonally dry forests. Forest planners and managers are now occupied with efforts to amend or revise Forest Plans within the NWFP area. According to the 2012 Planning Rule, their charge is to focus management on restoring ecosystem integrity and resiliency and address impacts of climate change and invasive species. Here, we integrate information from ecological and climate sciences, species recovery planning, and forest plan monitoring to identify management adaptations that can help managers realize the original Plan goals as integrated with the goals of the 2012 Planning Rule. There are no guarantees associated with any future planning scenario; continual learning and adaptation are necessary. Our recommendations include managing for dynamic rather than static conditions in seasonally dry forests, managing dynamically shifting reserves in wetter forests, where dynamics occur more slowly, reducing stressors in aquatic and riparian habitats, and significantly increased use of adaptive management and collaborative planning.
... In areas where postfire forest density is still high, mechanical treatments like mastication, cut and pile treatments, lop and scatter treatments, or harvesting are all viable options when not constrained by slope, road access or management designation (North et al., 2015a), although prescribed burning after mechanical treatment is recommended since several studies have shown mechanical treatments alone are less effective in reducing, and may even increase, future fire severity (Raymond andPeterson, 2005, Prichard andKennedy, 2012). ...
The increasing incidence of wildfires across the southwestern United States (US) is altering the contemporary forest management template within historically frequent-fire conifer forests. An increasing fraction of southwestern conifer forests have recently burned, and many of these burned landscapes contain complex mosaics of surviving forest and severely burned patches without surviving conifer trees. These heterogeneous burned landscapes present unique social and ecological challenges. Severely burned patches can present numerous barriers to successful conifer regeneration, and often contain heavy downed fuels which have cascading effects on future fire behavior and conifer regeneration. Conversely, surviving forest patches are increasingly recognized for their value in postfire reforestation but often are overlooked from a management perspective.
Here we present a decision-making framework for landscape-scale management of complex postfire landscapes that allows for adaptation to a warming climate and future fire. We focus specifically on historically frequent-fire forests of the southwestern US but make connections to other forest types and other regions. Our framework depends on a spatially-explicit assessment of the mosaic of conifer forest and severely burned patches in the postfire landscape, evaluates likely vegetation trajectories, and identifies critical decision points to direct vegetation change via manipulations of fuels and live vegetation. This framework includes detailed considerations for postfire fuels management (e.g., edge hardening within live forest patches and repeat burning) and for reforestation (e.g., balancing tradeoffs between intensive and extensive planting strategies, establishing patches of seed trees, spatial planning to optimize reforestation success, and improving nursery capacity). In a future of increasing fire activity in forests where repeated low- to moderate-severity fire is essential to ecosystem resilience, the decision-making framework developed here can easily be integrated with existing postfire management strategies to optimize allocation of limited resources and more actively manage burned landscapes.
... A few studies (e.g., Cram et al., 2015) report little difference in fire effects across a variety of treatments including thinning only and thinning followed by prescribed fire. But the majority of published studies suggest thinning that is not followed by prescribed fire is less effective at moderating fire severity than thinning combined with prescribed fire (e.g., Prichard and Kennedy, 2012;Schwilk et al., 2009). Some studies suggest that thinning without prescribed fire can increase wildfire severity by adding fine fuels to the forest floor (e.g., Raymond and Peterson, 2005). ...
Reducing fuels to better manage risk of high severity wildfire in seasonally dry, fire-prone forests of the western U.S. is an important goal of forest managers, including private landowners, non-governmental organizations, tribal, state, and local governments, and federal agencies. Managing fire risk is a critical objective of the U.S. Forest Service, which emphasizes the use of thinning to reduce tree density and ladder fuels followed by prescribed fire to reduce surface fuel. But the area of Forest Service land treated with thinning and prescribed fire is lagging far behind the area treated only with mechanical thinning due to regulatory and logistical challenges in prescribed fire implementation. Determining if mechanical thinning alone (without prescribed fire) can achieve adequate fire risk reduction has important implications for addressing the fire and fuel management goals set by Congress and the Administration, as well as the management objectives set by non-federal actors. In this study, we report on the effects of mechanical thinning and standard post-thinning fuels management but without prescribed fire on modeled fire behavior and changes in fuel loading over time in a ponderosa pine forest in Eastern Oregon. Thinning without prescribed fire significantly reduced potential crown fire immediately following thinning and also moderated surface modeled fire behavior beginning 2–3 years following thinning. Although small (<7.6 cm diameter) woody surface fuel loading increased following thinning, other ground and surface fuels (i.e., litter and duff) declined substantially, which we attribute to surface disturbance from ground-based logging, decreased deposition of litter, and increased decomposition. These results suggest that fuel reduction and fire risk management objectives can be met with mechanical thinning alone for a number of years. Prescribed fire is likely necessary to extend the effectiveness of mechanical thinning after significant tree or shrub regeneration. Continued monitoring will allow managers to use prescribed fire most efficiently to achieve fire and fuel management objectives.
... But across western North America, the combination of thinning and prescribed fire is most effective at moderating fire behavior and reducing fire severity (Fulé et al., 2012;Kalies and Yocom Kent, 2016;Martinson and Omi, 2013;Prichard et al. 2021, Schwilk et al., 2009. We also found that harvest treatments burned with higher severity than paired controls, which is consistent with previous research showing that harvest operations can increase surface fuel loads when tree limbs and tops are relocated to surface fuel beds, resulting in higher fire severity (Prichard and Kennedy, 2012;Stephens et al., 2009). Further insights may be gained from research at ecoregion scales to specifically identify the sets of conditions when thinning and prescribed fire are not effective at reducing fire severity. ...
Author direct link, available until mid-Dec, 2021: https://authors.elsevier.com/a/1d-zm1L%7EGwQxUc
We investigated the relative importance of daily fire weather, landscape position, climate, recent forest and fuels management, and fire history to explaining patterns of remotely-sensed burn severity – as measured by the Relativized Burn Ratio – in 150 fires occurring from 2001 to 2019, which burned conifer forests of northeastern Washington State, USA. Daily fire weather, annual precipitation anomalies, and species’ fire resistance traits were important predictors of wildfire burn severity. In areas burned within the past two to three decades, prior fire decreased the severity of subsequent burns, particularly for the first 16 postfire years. In areas managed before a wildfire, thinning and prescribed burning treatments lowered burn severity relative to untreated controls. Prescribed burning was the most effective treatment at lowering subsequent burn severity, and prescribed burned areas were usually unburned or burned at low severity in subsequent wildfires. Patches that were harvested and planted <10 years before a wildfire burned with slightly higher severity. In areas managed within 5 years after an initial fire, postfire harvest and planting reduced prevalence of stand-replacing fire in reburns. However, overall, postfire management actions after a first wildfire only weakly influenced the severity of subsequent fires. The importance of fire-fire interactions to moderating burn severity establishes the importance of stabilizing feedbacks in active fire regimes, and our results demonstrate how silvicultural treatments can be combined with prescribed fire and wildfires to maintain resilient landscapes.
... On most sites, thinning alone achieves a reduction of canopy fuels but contributes to higher surface fuel loads. If burned in a wildfire, these fuels can contribute to high-intensity surface fires and elevated levels of associated tree mortality (e.g., Stephens et al. 2009, Prichard andKennedy 2012). When trees are felled and limbed, fine fuels from tree tops and branches (termed activity fuels) are re-distributed over the treatment area, thereby increasing surface fuel loads (Martinson and Omi 2013). ...
We review science‐based adaptation strategies for western North American (wNA) forests that include restoring active fire regimes and fostering resilient structure and composition of forested landscapes. As part of the review, we address common questions associated with climate adaptation and realignment treatments that run counter to a broad consensus in the literature. These include the following: (1) Are the effects of fire exclusion overstated? If so, are treatments unwarranted and even counterproductive? (2) Is forest thinning alone sufficient to mitigate wildfire hazard? (3) Can forest thinning and prescribed burning solve the problem? (4) Should active forest management, including forest thinning, be concentrated in the wildland urban interface (WUI)? (5) Can wildfires on their own do the work of fuel treatments? (6) Is the primary objective of fuel reduction treatments to assist in future firefighting response and containment? (7) Do fuel treatments work under extreme fire weather? (8) Is the scale of the problem too great? Can we ever catch up? (9) Will planting more trees mitigate climate change in wNA forests? And (10) is post‐fire management needed or even ecologically justified? Based on our review of the scientific evidence, a range of proactive management actions are justified and necessary to keep pace with changing climatic and wildfire regimes and declining forest heterogeneity after severe wildfires. Science‐based adaptation options include the use of managed wildfire, prescribed burning, and coupled mechanical thinning and prescribed burning as is consistent with land management allocations and forest conditions. Although some current models of fire management in wNA are averse to short‐term risks and uncertainties, the long‐term environmental, social, and cultural consequences of wildfire management primarily grounded in fire suppression are well documented, highlighting an urgency to invest in intentional forest management and restoration of active fire regimes.
... For example, forest fragmentation in the tropics can lead to biomass drying, increasing forest flammability and resultant tree mortality during drought (Brando et al. 2014). Human fire-exclusion in seasonally-dry temperate forests has increased the amount of biomass available to burn, causing an increase in the chance of overstory tree mortality when fire occurs (Stephens and Moghaddas 2005, Prichard and Kennedy 2012, Pausas and Paula 2012, Moreno et al. 2014). The interaction of drought and beetle-caused tree mortality can also influence biomass availability. ...
Globally, terrestrial ecosystems are responsible for taking up approximately 25% of anthropogenic carbon dioxide emissions. Forests are a major contributor to global carbon uptake and storage and are largely responsible for the consistency of the global carbon sink; with semiarid ecosystem response to precipitation and temperature accounting for the interannual variability in carbon uptake. Forest carbon source–sink dynamics and the total amount of carbon stored vary regionally and are heavily influenced by human land-use, human-caused climate change, and disturbance. The relative influence of these drivers and the effects of their interaction vary geographically. The success of the strategies that we employ to leverage forest-based climate mitigation will be dependent upon a thorough understanding of local ecological, social, and climate contexts.
... The interested reader is referred to Kalies and Kent [16] for a compilation of results pertaining to canopy structure, soil, carbon, wildlife and human values from previous fuel treatment operations. As summarized by Kalies and Kent [16], a large number of studies reported lower fire intensity and fire severity in thinned only forests compared to untreated forests, although thinning combined with prescribed burns were found most effective [17][18][19][20][21][22][23][24][25][26][27][28][29][30]. ...
Key message: We have explored the impacts of forest thinning on wildland fire behavior using a process based model. Simulating different degrees of thinning, we found out that forest thinning should be conducted cautiously as there could be a wide range of outcomes depending upon the post-thinning states of fuel availability, fuel connectivity, fuel moisture and micrometeorological features such as wind speed. Context: There are conflicting reports in the literature regarding the effectiveness of forest thinning. Some studies have found that thinning reduces fire severity, while some studies have found that thinning might lead to enhanced fire severity. Aims: Our goal was to evaluate if both of these outcomes are possible post thinning operations and what are the limiting conditions for post thinning fire behavior. Methods: We used a process based model to simulate different degrees of thinning systematically, under two different conditions, where the canopy fuel moisture was unchanged and when the canopy fuel moisture was also depleted post thinning. Both of these scenarios are reported in the literature. Results: We found out that a low degree of thinning can indeed increase fire intensity, especially if the canopy fuel moisture is low. A high degree of thinning was effective in reducing fire intensity. However, thinning also increased rate of spread under some conditions. Interestingly, both intensity and rate of spread were dependent on the competing effects of increased wind speed, fuel loading and canopy fuel moisture. Conclusion: We were able to find the limits of fire behavior post thinning and actual fire behavior is likely to be somewhere in the middle of the theoretical extremes explored in this work. The actual fire behavior post thinning should depend on the site specific conditions which would determine the outcome of the interplay among the aforementioned conditions. The work also highlights that policymakers should be careful about fine scale canopy architectural attributes and micrometeorological aspects when planning fuel treatment operations.
... Delayed post-fire tree mortality can also be affected by structure and spatial patterns at multiple scales. Delayed mortality, which can represent over 40% of total fire mortality, occurs following fires of all severities and usually takes place within 4-5 years after fire (Youngblood et al. 2009;Fettig et al. 2010;Hood et al. 2010;Van Mantgem et al. 2011;Prichard and Kennedy 2012;Miller et al. 2016). Delayed mortality can occur after nonlethal fire-caused damage to a tree's cambium and/or crown initiates a death spiral, ultimately killing the tree some years later. ...
... Most mortality on the YFDP occurred in the first year (62% of mortalities compared to 55-90% in the literature) and subsequent years showed monotonically decreasing amounts of tree death (Fig. 4) (Keyser et al. 2006;Hood and Bentz 2007;Hood et al. 2010;Prichard and Kennedy 2012). First-year mortality occurred disproportionately in small trees and mortality in subsequent years advanced through larger diameter classes (Fig. 4) (Breece et al. 2008;Youngblood et al. 2009;Van Mantgem et al. 2011). ...
ContextPost-fire tree mortality is a spatially structured process driven by interacting factors across multiple scales. However, empirical models of fire-caused tree mortality are generally not spatially explicit, do not differentiate among scales, and do not differentiate immediate from delayed mortality.Objectives
We aimed to quantify cross-scale linkages between forest structure—including spatial patterns of trees—and the progression of mortality 1–4 years post-fire in terms of rates, causes, and underlying demography.Methods
We used data from a long-term study site in the Sierra Nevada, California to build a post-fire tree mortality model predicted by lidar-measured estimates of structure. We calculated structural metrics at scales from individual trees to 90 × 90 m neighborhoods and combined them with metrics for topography, site water balance, and burn weather to predict immediate and delayed post-fire tree mortality.ResultsMortality rates decreased while average diameter of newly killed trees increased each year post-fire. Burn weather predictors as well as interactive terms across scales improved model fit and parsimony. Including landscape-scale information improved finer-scale predictions but not vice versa. The amount of fuel, fuel configuration, and burning conditions predicted total mortality at broader scales while tree group-scale fuel connectivity, tree species fire tolerance, and local stresses predicted the fine-scale distribution, timing, and agents of mortality.Conclusions
Landscape-scale conditions provide the template upon which finer-scale variation in post-fire tree mortality is arranged. Post-fire forest structure is associated with the etiologies of different mortality agents, and so landscape-level heterogeneity is a key part of ecosystem stability and resilience.
