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The National Fire and Fire Surrogate Study: effects of fuel reduction methods on forest vegetation structure and fuels
Abstract
Changes in vegetation and fuels were evaluated from measurements taken before and after fuel reduction treatments (prescribed fire, mechanical treatments, and the combination of the two) at 12 Fire and Fire Surrogate (FFS) sites located in forests with a surface fire regime across the conterminous United States. To test the relative effectiveness of fuel reduction treatments and their effect on ecological parameters we used an information-theoretic approach on a suite of 12 variables representing the overstory (basal area and live tree, sapling, and snag density), the understory (seedling density, shrub cover, and native and alien herbaceous species richness), and the most relevant fuel parameters for wildfire damage (height to live crown, total fuel bed mass, forest floor mass, and woody fuel mass). In the short term (one year after treatment), mechanical treatments were more effective at reducing overstory tree density and basal area and at increasing quadratic mean tree diameter. Prescribed fire treatments were more effective at creating snags, killing seedlings, elevating height to live crown, and reducing surface woody fuels. Overall, the response to fuel reduction treatments of the ecological variables presented in this paper was generally maximized by the combined mechanical plus burning treatment. If the management goal is to quickly produce stands with fewer and larger diameter trees, less surface fuel mass, and greater herbaceous species richness, the combined treatment gave the most desirable results. However, because mechanical plus burning treatments also favored alien species invasion at some sites, monitoring and control need to be part of the prescription when using this treatment.
... Studies investigating how the understory plant community responds to mechanical restoration treatments in dry conifer forests have yielded mixed results with respect to the direction, magnitude, and timing of effect (Schwilk et al. 2009;Abella and Springer 2015;Willms et al. 2017). Changes in overall understory abundance and diversity were variable one to three years after treatment, with both increases and decreases commonly reported (Collins et al., 2007;Dodson et al., 2008;Wayman and North, 2007). ...
... Mechanical restoration treatments in dry conifer forests have been implemented in many different places with varying conditions throughout the western US (Schwilk et al., 2009;Stephens et al., 2012), yet little is known about how biologically meaningful environmental gradients affect understory responses to treatments Jang et al., 2021;Kane et al., 2010). It is possible that variations in topography and climate may modify understory responses following treatment; cooler and wetter sites may be able to support more understory vegetation post-treatment than their warmer, drier counterparts (North et al., 2005). ...
... In this study, gains in native species were accompanied by gains in non-native richness and cover in treated plots by 4-6 years after treatment (Figs. 2 and 3; Table 3). While restoration treatments are critically needed to increase dry conifer forest resilience to wildfire and other disturbances, preventing non-native species proliferation in the process is also a high priority (Collins et al. 2007;Schwilk et al. 2009;Jang et al. 2021). Monitoring in treated areas should continue for non-native species like cheatgrass (a prolific seeder) and Canada thistle (a strong vegetative spreader) which showed a clear affinity for treated areas. ...
Restoration efforts are underway in dry conifer forests across the western United States to increase their resilience to wildfire and other disturbances. Because such treatments typically decrease overstory density and homogeneity, they can also drive changes in the understory plant community. Past studies of post-treatment changes in understories have found variable results over short time frames and across regions, highlighting the need to study longer-term, region-specific responses. We investigated whether mechanical restoration treatments benefited understory plants in dry conifer forests of the Colorado Front Range, and what biotic and abiotic variables modified understory plant responses in treated areas. We analyzed data collected 1–2 years pre-treatment, 1–2 years post-treatment, and 4–6 years post-treatment in 168 plots, which were distributed across 8 sites and 16 pairs of treated and nearby untreated areas. Treatments were implemented by removing trees with heavy machinery or hand tools (i.e., thinning). By the time treatments were 4–6 years old, native understory plant cover was 1.7 times higher in treated compared to untreated plots, and native richness was 1.1 times higher. Heightened cover and richness values in treated plots were not driven by a single native plant functional group, but by a large portion of the native community; long-lived, graminoid, vegetatively spreading, and non-vegetatively spreading plants all had higher cover in treated plots, while short-lived, long-lived, forb, graminoid, and non-vegetatively spreading plants had higher richness in treated plots. Non-native plants showed 3.1 times higher cover and 4.4 times higher richness in treated compared to untreated plots at 4–6 years post-treatment, but were present at very low levels (e.g., ≤ 0.5% mean cover in either treatment). Greater native plant cover and richness at 4–6 years post-treatment were associated with lower overstory basal areas that resembled 19th-century forest structural conditions for the landscape. Contrary to expectations, a long-term measure of moisture availability (i.e., 30-year average climatic water deficit) was not a strong predictor of native cover or richness in treated plots 4–6 years post-treatment; rather, they were better predicted by moisture availability during the spring months prior to sampling. Overall, the consistent and enduring stimulation of cover and richness of native understory plants after mechanical treatments, with only limited invasion from non-native species, illustrates the important benefits of ongoing restoration activities in dry conifer forests of the Colorado Front Range.
... Perhaps the broadest and most comprehensive study of fuel treatment effectiveness was the National Fire and Fire Surrogates Study (NFFS) supported by Forest Service R&D and the Joint Fire Science Program and led by scientists from Forest Service R&D, USGS, and numerous university partners (McIver et al. 2012, Schwilk et al. 2009). The 12-site NFFS was a multivariate experiment that evaluated landscape and ecological consequences of alternative fuel management treatments in seasonally dry forests of the United States. ...
... Mechanical treatments did not serve as surrogates for fire for most ecological factors, suggesting that wildland fire must be maintained whenever possible. Restoring and maintaining resilient landscapes will require repeated treatments over time, with eastern forests requiring more frequent applications (McIver et al. 2012, Schwilk et al. 2009). During the past several years, the Joint Fire Science Program has had an active line of work investigating fuel treatment effectiveness (see http://www.firescience.gov ...
... regimes are usually expressed as wildfire frequency, extent, pattern, and severity(Agee 1993, Bowman et al. 2009, Brown and Smith 2000, Keeley et al. 2009, Morgan et al. 2001. Note that in the context of fire ecology, fire severity refers to the effects of wildland fires on landscape and ecosystem components, including the human dimensions (Hardy 2005). ...
This report focuses on wildland fire management and response outcomes—in the context of fire and fuels research and development (R&D) at the Forest Service, an agency of the U.S. Department of Agriculture (USDA)—that have transformed the business of wildland fire in the United States. This research has resulted in knowledge, data, and applications that have contributed greatly to the following outcomes.
Outcome 1. Enhanced Physical Fire Science
Wildland fire managers and responders are able to predict the occurrence, extent, and severity of fires with more precision and accuracy resulting in better decisionmaking, resource allocation, and firefighter and public safety. National systems that characterize fire danger and risk are maintained and innovated.
Outcome 2. Better Access to Smoke and Emissions Tools
Practitioners and regulators have access to information and tools for estimating smoke and emissions from wildland fire to support decisions about suppression, managed wildfires, and prescribed fire implementation, primarily to address firefighter and public health and safety.
Outcome 3. Advanced Wildland Fuels Management
Wildland fuels are characterized using consistent and comprehensive science and technology to meet diverse objectives and ensure effective investment strategies to restore resilient landscapes, mitigate wildfire risk, and deliver benefits to the public.
Outcome 4. Improved Social and Economic Context for Wildland Fire Management and Response
The public is aware of the costs and benefits of wildland fire management and response, and socioeconomic research and development are foundations for large landscape collaboration, organizational performance and effectiveness, and firefighter and public safety. Fire management organizations have access to information and tools to improve performance through learning and innovative leadership.
Outcome 5. Strengthened Wildland Fire Ecology Practices Supporting Landscape Restoration
Diverse and comprehensive information and tools that characterize the ecologic costs and benefits of fire are avail¬able to support the restoration of resilient landscapes; deliver clean, abundant water; and strengthen communities. Accessible science and technology exist for adaptive and collaborative fire management in the face of changing baseline conditions.
... Although improved resilience and protection of biodiversity are goals of proposed adaptation management, active management may, in some cases, have little effect on future stand resistance (Morris et al., 2022), is often unnecessary for natural forest resilience (e.g., Cansler et al., 2022;Hart et al., 2015) and biodiversity (Thom & Seidl, 2016;Viljur et al., 2022), and is generally counterproductive to carbon storage, structural complexity, tree diversity, and resistance to invasive species. (Donato et al., 2013;Miller et al., 2018;Patton et al., 2022;Schwilk et al., 2009;Young et al., 2017; Table 1). Moreover, conservation evidence for the effectiveness of management interventions is often lacking or has mixed results (Sutherland et al., 2021), resources for interventions are limited, and management incurs substantial financial and other costs to society (Houtman et al., 2013). ...
... In some cases, thinning has been shown to reduce subsequent tree death from insects and drought compared to untreated areas, thereby promoting stand resistance and maintaining an existing species composition, while procuring sound timber (Hood et al., 2016;Knapp et al., 2021). However, in other cases prescribed burn treatments increased subsequent tree mortality (Knapp et al., 2021;Stark et al., 2013;Youngblood et al., 2009), and thinning and burn treatments generally promote the spread of invasive plants relative to controls (Schwilk et al., 2009;Willms et al., 2017). Additionally, loss of tree basal area and carbon storage from thinning and prescribed burning is often equal to or considerably greater than tree mortality and carbon loss from the disturbances themselves Hood et al., 2016;Knapp et al., 2021;Powers et al., 2010;Yocom-Kent et al., 2015). ...
... Management strategy and outcome (+ positive; À negative; ? unknown)Schwilk et al., 2009, Donato et al., 2013Miller et al., 2016;Young et al., 2017;Stiers et al., 2018, Shell et al., 2021Patton et al., 2022 ...
Abstract Forests are critical to the planetary operational system and evolved without human management for millions of years in North America. Actively managing forests to help them adapt to a changing climate and disturbance regime has become a major focus in the United States. Aside from a subset of forests wherein wood production, human safety, and experimental research are primary goals, we argue that expensive management interventions are often unnecessary, have uncertain benefits, or are detrimental to many forest attributes such as resilience, carbon accumulation, structural complexity, and genetic and biological diversity. Natural forests (i.e., those protected and largely free from human management) tend to develop greater complexity, carbon storage, and tree diversity over time than forests that are actively managed; and natural forests often become less susceptible to future insect attacks and fire following these disturbances. Natural forest stewardship is therefore a critical and cost effective strategy in forest climate adaptation.
... High-severity wildfires kill large numbers of trees, creating dead woody fuels that are deposited on the forest floor over time as the dead trees decay, thereby increasing potential fire severity (Peterson et al. 2015, Ritchie et al. 2013). In contrast, low-severity fires-and most prescribed fires-consume surface fuels without generating large amounts of new woody fuels, thereby limiting future fire severity (Safford et al. 2012, Schwilk et al. 2009). ...
... Forest restoration treatments-particularly those with fuel reduction and fire resilience objectives-often seek to modify overstory stand structure and surface woody fuels with the goal of reducing future wildfire severity Skinner 2005, Allen et al. 2002). These treatments typically employ mechanical thinning of overstory trees and prescribed burning, alone or in combination (Allen et al. 2002, Schwilk et al. 2009). As noted earlier, prescribed fires are generally low-severity fires that consume surface fuels and thereby limit future fire severity for 10 to 15 years (Prichard et al. 2010, Safford et al. 2012. ...
... As noted earlier, prescribed fires are generally low-severity fires that consume surface fuels and thereby limit future fire severity for 10 to 15 years (Prichard et al. 2010, Safford et al. 2012. Mechanical thinning treatments can reduce potential fire severity by reducing the likelihood of crown fire occurrence but may also increase surface woody fuels if not combined with prescribed fire (Schwilk et al. 2009). ...
... Timber harvesting may sometimes be used as a fuel management technique, e.g., thinning (Agee and Skinner, 2005;Proctor and McCarthy, 2015;Keenan et al., 2021), but more generally it is a process to extract timber. Timber harvesting influences fine fuel both directly via addition of harvesting residue and indirectly by changing stand structure and fine fuel accumulation (Schwilk et al., 2009;Stephens et al., 2009;Hood et al., 2020). Post-harvest burning is often applied following harvesting to reduce the fine and coarse fuels created by the harvesting operation and promote regeneration of canopy species (Florence, 2004). ...
... Post-harvest burning is often applied following harvesting to reduce the fine and coarse fuels created by the harvesting operation and promote regeneration of canopy species (Florence, 2004). The type of harvesting operation and equipment used will influence forest regeneration, the growing conditions (e.g., light availability) and future fine fuel inputs (McCaw et al., 2002;Schwilk et al., 2009;Campbell et al., 2016). For example, selective harvesting where some mature trees are retained may have a different effect on fine fuel than clear-felling, as litter inputs from some mature trees are maintained and fewer gaps are created. ...
... The effect of different combinations of thinning (a form of timber harvesting) and prescribed burning on fine fuel has been studied extensively for North American forests. Generally, thinning followed by burning gives the best results for wildfire risk reduction, as it produces stands with fewer, larger trees and less surface litter (Schwilk et al., 2009;Waldrop et al., 2010). However, this reduction in wildfire risk may be offset by increased shrub cover following thinning and burning in some forest systems (Odland et al., 2021). ...
Forests are managed for a range of ecological and economic values. Prescribed burning and timber harvesting are two widely used and often co-occurring forest management activities. Both may alter wildfire risk by changing fuel characteristics. Prescribed burning alters the amount and arrangement of fuel, which changes the probability of ignition and rate of fire spread. Timber harvesting may influence future fuel composition, amount and structure directly via the addition of harvesting residue and indirectly by changing stand structure and fine fuel accumulation. Whilst there is reasonable understanding of how fuel changes immediately after a single prescribed burn or harvest in temperate eucalypt forests, the long-term effects of frequent prescribed burning combined with timber harvesting are poorly understood. We used a long-term (∼25 year) prescribed burning experiment conducted in ∼1000 ha of temperate eucalypt forest in southeastern New South Wales, Australia to determine the effects of a single selective timber harvest and 0–6 subsequent prescribed burns on the accumulation and composition of fine fuel (<25 mm thickness). It was found that fine fuel loads differed between harvested and unharvested stands, with 30 % less leaves and fine twigs (<6 mm) in the litter bed but 24 % more green understorey fuel up to 10 years post-burn in harvested stands. Frequent prescribed burning did not change total fine fuel load in harvested stands but did change fine fuel composition, with 4 times more leaves in the litter bed and 6 times less green understorey fuel in plots burnt at high frequencies (>4 fires in 25 years) versus low frequencies (0 fires in 25 years). Similar trends were observed in unharvested stands following frequent burning, however, there was 45% more leaves in the litter bed of frequently burnt, unharvested stands compared to frequently burnt, harvested stands. Our results suggest that irrespective of harvesting, frequent burning (>4 fires over 25 years) in temperate eucalypt forest may be a suitable strategy for lowering wildfire risk as it can reduce the amount of green understorey fuel, which may reduce flame heights and the likelihood of canopy fire. However, the decrease in green understorey fuel may be offset by the increase in leaves in the litter bed, which support fire ignition and spread. Our results highlight how the presence of timber harvesting changes the accumulation and composition of fine fuel post-fire, yet timber harvesting is currently not well integrated into many landscape fuel models, which often focus solely on time since last fire. Overlooking key attributes of disturbance history (timber harvesting, number of past fires), may jeopardise the accuracy of fuel models, and therefore the reliability of fire behaviour predictions.
... Past management (Agee and Lolley, 2006;Agee and Skinner, 2005;Hessburg et al., 2015;Schwilk et al., 2009) and prior fires (Parks et al., 2015a;Prichard et al., 2017;Stephens et al., 2009;Stevens-Rumann et al., 2016) influence the spread, intensity, and severity of subsequent fires. Silviculture and fire, including both prescribed and wildfire, are thus the main tools that managers can use to restore ecological resilience and resistance, and direct ecosystems toward conditions that will be more resilient under future climates Millar et al., 2007;Schuurman et al., 2020). ...
... We found that management treatments that include prescribed fire were most effective at decreasing the severity of subsequent fires. These results are consistent with previous research in our study area (Agee and Lolley, 2006;Lyons-Tinsley and Peterson, 2012;Prichard et al., 2020;Prichard and Kennedy, 2014), and elsewhere in western North America (Kalies and Yocom Kent, 2016;Schwilk et al., 2009;Stephens and Moghaddas, 2005), but our sampled area encompasses > 7,000 sampled 30-m areas in the entire reburned landscape (51,919 ha; Table 6), and all 3,086 ha within prescribed fire areas that were subsequently burned by a wildfire, instead of field plots Fig. 5. Two-variable partial dependence surfaces showing probability of high severity fire, as measured by the Relativized Burn Ratio, in areas that were longunburned before the fire. The most important predictor variables was the fire resistance score (FRS) captures community-scale tree species fire resistance traits (x-axis of each panels; Stevens et al., 2020). ...
... These results too are supported by previous research indicating that thinning alone can moderate fire behavior, in our study area (Prichard et al., 2020;Prichard and Kennedy, 2014). 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). ...
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.
... Several studies of ponderosa regeneration patterns after thinning in Montana (Fajardo et al. 2007), New Mexico (Thomas and Waring 2015), Colorado (Shepperd et al. 2006), and Arizona (Bailey and Covington 2002;Puhlick et al. 2012;Flathers et al. 2016) reported significantly greater regeneration densities in thinned stands versus unthinned stands (e.g., Fig. 2). Mechanical thinning and thinning plus prescribed fire increased seedling density over time but there was high variability among sites (Schwilk et al. 2009). Disturbances such as thinning and burning can lead to increased ponderosa pine regeneration by creating microsites for germination, or can reduce regeneration by causing direct injury or mortality (Bailey and Covington 2002). ...
... Disturbances such as thinning and burning can lead to increased ponderosa pine regeneration by creating microsites for germination, or can reduce regeneration by causing direct injury or mortality (Bailey and Covington 2002). Regeneration can also be linked to variables such as stand density, light availability, soil moisture, disturbance, masting, and site productivity (Gray et al. 2005;Zald et al. 2008;Schwilk et al. 2009). Burn-only treatments created site conditions favorable to ponderosa pine seedling establishment two years following burning (Ffoliott and Guertin 1990). ...
... Thinning and burning treatments have impacts on understory vegetation (Abella and Springer 2015), wildfire behavior (Fulé et al. 2012), wildlife diversity and abundance (Kalies et al. 2010), and change existing overstory spatial structures (Schwilk et al. 2009). Understory vegetation can reduce seedling mortality by protecting seedlings from wind and direct sunlight which may cause desiccation, but can also negatively affect regeneration via competition for soil moisture (Pearson 1942;Heidmann et al. 1982). ...
Understanding naturally occurring pine regeneration dynamics in response to thinning and burning treatments is necessary not only to measure the longevity of the restoration or fuels treatment, but also to assess how well regeneration meets forest sustainability guidelines and whether natural regeneration is sufficient for maintaining a sustainable forest structure and composition. A synthesis review was carried out on the effects of mechanical thinning and prescribed burn treatments on natural pine regeneration response in frequent-fire ponderosa pine forests across the western United States. The focus was on site-specific variability in pine regeneration dynamics, temporal trends in regeneration presence and abundance, and response to treatment as described in the current literature using 29 studies that met our evidence-based review protocols. Data showed that the effects of thinning and burning treatments on regeneration depended on time since treatment. Mechanical thinning, prescribed burning, and thinning plus burn treatments all increased seedling density, but there was high variability among sites and studies. There were mixed results in the short-term (< 10 years) with both increasing and decreasing regeneration, and a general increase in regeneration 11 − 20 years post-treatment. Some long-term studies (> 20 years) concluded that stands can return to pre-treatment densities in terms of total trees per hectare and forest floor duff levels when there are no maintenance treatments applied. Several studies showed the average ponderosa pine seedling presence, survival and growth found in today’s forests to be at a high density; this combined with missed fire cycles could contribute to future fire risk and reduce the efficacy of maintaining fuel reduction goals.
... Fire-scarred trees are particularly vulnerable to subsequent fire damage, as exposed wood and thinner bark can lead to greater cambium damage (Shive et al. 2022). Prescribed fire alone typically does not significantly change overstory conditions (Schwilk et al. 2009, Stephens et al. 2012), but the reduction of surface and ladder fuels (Figs. 3 and 4) through repeated small prescribed fires over decades achieved conditions associated with forest resilience. This outcome is highlighted through the trends in tree density and basal area in the Wawona area. ...
... Local incident management personnel stated that "without the debris removal already completed [along Wawona Road], I do not believe we would have been able to keep fire from crossing the road. " Mechanical thinning treatments primarily manipulate and reduce crown connectivity, tree density, and basal area, forest characteristics that strongly influence crown fire potential and long-range spotting (Schwilk et al. 2009, Stephens et al. 2009), which are components of fire behavior that are highly resistant to control. By requiring fewer firefighters in the grove and along Wawona Road, additional firefighters could be committed to structure protection in Wawona and handline construction in heavily fuel-loaded areas that were important for holding (pers. ...
Background
The Washburn fire started on July 7, 2022 in the lower Mariposa Grove in Yosemite National Park, posing immediate threats to the iconic giant sequoias ( Sequoiadendron giganteum ), critical Pacific fisher ( Pekania pennanti ) habitat, and the community of Wawona. The wildfire quickly gained national attention and the public followed Yosemite’s firefighting efforts closely. In the aftermath of the Washburn fire, we evaluate how decades of fire management and recent roadside thinning in the Park shaped the tactics and outcomes of this wildfire. Fuels reduction and prescribed fire have been at the core of Yosemite’s fire management program since the 1970s and much of this activity has been concentrated in the two areas where firefighting operations were most successful in protecting key resources. We use long-term fire effects monitoring data, airborne light detection and ranging data, and operational perspectives to link together science, management, and fire operations.
Results
Successful initial attack and suppression efforts kept fire out of the Mariposa Grove and the community, where topography and the distribution of heavy fuels largely drove fire behavior throughout the course of the fire. We demonstrate the cumulative effects of decades of fuels treatments in reducing two major drivers of fire behavior – tree density and fuel load – and highlight substantially lower ladder fuels and fire severity in treatment units compared to untreated areas within the Washburn fire footprint.
Conclusions
We show how repeated prescribed fire and fuels treatments can promote forest structure indicative of frequent-fire regimes, moderate extreme fire behavior, improve the ability to protect key resources of concern, and increase human safety in the face of unplanned wildfire. Strategically placed prescribed fire and fuels treatments surrounding the Washburn fire were key considerations in wildfire suppression tactics and were instrumental in protecting the Mariposa Grove.
... 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). ...
... Results across these twelve diverse study sites were somewhat mixed. One study that summarized results of treatments across sites found that the mechanical thinning plus fire treatment was best suited for the creation of stands with fewer and larger trees, reduced surface fuel mass, and greater herbaceous species richness, but that the mechanical thinning plus fire treatment sometimes resulted in invasion of sites by invasive species (Schwilk et al., 2009). Another comprehensive summary of FFS results suggested that all treatments were relatively effective at moderating modeled fire behavior . ...
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.
... Regardless, fine fuel loading in control units was similar in 1998 and 2015. The high variability of fuel loading across landscapes makes the use of experimental controls somewhat speculative in all but the most homogenous study areas [21]. ...
... However, fuel reduction treatments do not have a lasting impact on most categories of dead woody fuel loading, highlighting the need for maintenance treatments in areas where surface fuels are a concern. The combination of thinning and burning has consistently been highlighted as the most effective method to accomplish forest restoration, fuel reduction, and resilience to disturbances [21,44,51]. To re-create the heterogeneity produced by a patchwork of fires across the historical dry forest landscape, treatments could be applied every 5-25 years, with more intense treatments following longer treatment-free intervals. ...
Fire exclusion and a lengthening fire season has resulted in an era of megafires. Fuel reduction treatments in forested ecosystems are designed to guard against future extreme wildfire behavior. Treatments create a heterogenous landscape and facilitate ecosystem function and resilience in fire-adapted forests of the western United States. Despite widespread recognition that repeated fuel treatments are needed to maintain desired stand characteristics over time, few field studies have evaluated treatment longevity. The Blue Mountains Fire and Fire Surrogate site in northeastern Oregon presented an opportunity to investigate woody fuel loading 15–17 years after four treatments: mechanical thin, prescribed burn, both thin and burn, and no treatment control. The principal findings were: (1) fine fuel load 15 years post-burn remained slightly below pre-treatment values; (2) rotten coarse fuel load was reduced post-burn, but sound coarse fuel was not altered by any active treatment; and (3) total woody fuel load 15–17 years post-treatment was similar to pre-treatment values. Understanding surface fuel loading is essential for predicting fire behavior. Overall, the effects of fuel reduction treatments on woody surface fuels were transitory in dry mixed conifer forests. Frequent maintenance treatments are recommended to protect values at risk in areas with high fire hazards. Quantifying the persistence of changes in forest conditions aids in the planning and analysis of future fuel treatments, along with scheduling maintenance of existing treated areas.
... Although restoring natural ecosystem processes has long been understood as an important goal, early emphasis was on reducing fire hazard that could destroy timber resources (Mutch 1997). However, it has long been recognised that this can be accomplished with the use of mechanical thinning of forests (Schwilk et al. 2009, Safford et al. 2012, North et al. 2015a, which is an acceptable alternative to prescription burning on USFS lands, but not something that was compatible with restoring natural ecosystem processes on NPS lands. Reliance on mechanical treatments of fuels contributed to the limited use of Rx burns from 1972 to 1998 on these USFS lands, which resulted in very little area burned and they are reasonably interpreted as experimental. ...
... The outcome was the Sierra Nevada Conservation Framework (Pyne 2015) and in 2004, Rx burning continued on an annual basis on these Sierra Nevada national forests. Several factors account for this renewed interest in Rx burning, including greater efficiency of Rx burns on steep terrain and less severe impacts on soil structure relative to mechanical treatments (North et al. 2015a), and reduced costs of burning when the goal is to reduce ingrowth from ladder fuels, a treatment that has limited commercial value (Schwilk et al. 2009). Additionally, a national comparison of mechanical treatments with prescription burning showed that the former was not an appropriate surrogate for restoring many ecosystem processes requiring fire (McIver et al. 2013) Historical changes in fire policy have most certainly been influenced by biophysical characteristics that influence fire activity. ...
History of prescription burning and wildfires in the three Sierra Nevada National Park Service (NPS) parks and adjacent US Forest Service (USFS) forests is presented. Annual prescription (Rx) burns began in 1968 in Sequoia and Kings Canyon National Parks, followed by Yosemite National Park and Lassen Volcanic National Park. During the last third of the 20th century, USFS national forests adjacent to these parks did limited Rx burns, accounting for very little area burned. However, in 2004, an aggressive annual burn program was initiated in these national forests and in the last decade, area burned by planned prescription burns, relative to area protected, was approximately comparable between these NPS and USFS lands. In 1968, the NPS prescription burning program was unique because it coupled planned Rx burns with managing many lightning-ignited fires for resource benefit. From 1968 to 2017, these natural fires managed for resource benefit averaged the same total area burned as planned Rx burns in the three national parks; thus, they have had a substantial impact on total area burned by prescription. In contrast, on USFS lands, most lightning-ignited fires have been managed for suppression, but increasing attention is being paid to managing wildfires for resource benefit.
... Note that our study was limited to national parks, where such treatments do not occur, and so our results cannot provide direct insight on mechanical treatments. Nevertheless, if mechanical treatments are applied with rigorous guidelines designed to maintain key habitat features (e.g., retention of large trees and dense canopy of tall trees) of old forest ecosystems and sensitive species like spotted owls, fishers (Pekania pennanti), and others, the benefits of reducing severe fire through mechanical thinning may outweigh the adverse effect on spotted owl habitat, yet these areas need to be closely monitored because of high scientific uncertainty Schwilk et al., 2009;Tempel et al., 2015;Jones, 2019). Increasing the use of fire as a management tool in fire-suppressed forests may increase the feasibility and spatial extent of restoration efforts compared to mechanical treatments alone (North et al., 2012). ...
... Increasing the use of fire as a management tool in fire-suppressed forests may increase the feasibility and spatial extent of restoration efforts compared to mechanical treatments alone (North et al., 2012). However, fire used in combination with mechanical treatments (e.g., removal of small and medium-sized trees) may be more effective in restoring vegetation structure with lower fuel loads than currently present, particularly in forests where the risks from prescribed or managed fire are now high (Schwilk et al., 2009). Thus, while much uncertainty and many obstacles remain, our study reinforces previous findings that owl conservation may benefit from restoration of frequent fire regimes in dry forests (Roberts et al., 2011;Jones et al., 2016Jones et al., , 2020Eyes et al., 2017;Stephens et al., 2019). ...
Wildland fire is a disturbance that shapes frequent-fire forest ecosystems and the life-histories of wildlife species that inhabit them. The California spotted owl (Strix occidentalis occidentalis) is an iconic old-forest species that evolved under a frequent-fire regime in western North America. While recent studies have focused on owl response to large, severe fire events, relatively little is known about how owls might respond to prescribed fires and wildfires managed for resource benefit. Therefore, understanding how owls use landscapes that are managed using fire may offer insight into how owls respond to fire management. We studied the breeding season nocturnal foraging habitat selection of 22 GPS-tagged California spotted owls in three national parks (Yosemite, Sequoia, and Kings Canyon) in the Sierra Nevada, California, USA where natural fires have largely been allowed to burn during the past 50 years and controlled burning has been used to target additional areas. Consistent with other studies of this species, owls selected forests dominated by medium and large trees and avoided areas with smaller trees within their home ranges based on step selection analysis. Owls neither selected nor avoided forests burned by low- and moderate-severity, or high-severity fires, yet avoided larger patches of severely-burned forest (odds of selection decreased by 20% for every 10 ha increase in severely-burned patch area). These results indicated the importance of patch characteristics, suggesting that larger patches reflected either lower quality foraging habitat or increased predation risk, even in these frequent-fire landscapes where “large” severely-burned patches were small compared to those common after megafires. Additionally, selection strength increased for areas burned recently by lower-severity fire and, to a lesser extent, by older fires (largely of lower severity) as the extent of these burned areas increased within individual home ranges. These results suggested that lower-severity fire benefitted spotted owls and that these benefits declined over time. Thus, our findings are consistent with the hypothesis that California spotted owls are adapted to historical frequent-fire regimes of overall lower-severity with small high-severity patches. We hypothesize that fire management, coupled with medium- and large-tree retention, likely maintains high quality spotted owl habitat and may contribute to the observed owl population stability in Sequoia and Kings Canyon National Parks, compared to declining populations on three national forests. Finally, our results indicated that fire management, as practiced in these national parks, could benefit owl conservation elsewhere if challenges to the reintroduction of frequent-fire regimes can be overcome.
... Units were thinned from below in a non-uniform pattern to a target basal area of 10-14 m 2 /ha such that thinning intensity varied with initial forest structure [44]. Targets reflected the historical structure, including clumped patterning of trees [42,45] and a goal of reducing fire hazard such that at least 80% of the basal area of dominant and co-dominant trees would survive if subjected to a head fire under 80 th -percentile fire-weather conditions [18]. On average, density was reduced by 60% and basal area by 50%, leaving a mean residual basal area of 17.4 m 2 /ha-somewhat larger than the target [8]. ...
... Units were thinned from below in a non-uniform pattern to a target basal area of 10-14 m 2 /ha such that thinning intensity varied with initial forest structure [44]. Targets reflected the historical structure, including clumped patterning of trees [42,45] and a goal of reducing fire hazard such that at least 80% of the basal area of dominant and co-dominant trees would survive if subjected to a head fire under 80th-percentile fire-weather conditions [18]. On average, density was reduced by 60% and basal area by 50%, leaving a mean residual basal area of 17.4 m 2 /ha-somewhat larger than the target [8]. ...
The long-term effectiveness of dry-forest fuels treatments (restoration thinning and prescribed burning) depends, in part, on the pace at which trees regenerate and recruit into the overstory. Knowledge of the factors that shape post-treatment regeneration and growth is limited by the short timeframes and simple disturbance histories of past research. Here, we present results of a 15-year fuels-reduction experiment in central Washington, including responses to planned and unplanned disturbances. We explore the changing patterns of Douglas-fir regeneration in 72 permanent plots (0.1 ha) varying in overstory abundance (a function of density and basal area) and disturbance history-the latter including thinning, prescribed burning, and/or wildfire. Plots were measured before treatment (2000/2001), soon afterwards (2004/2005), and more than a decade later (2015). Thinning combined with burning enhanced sapling recruitment (ingrowth) into the overstory, although rates of ingrowth were consistently low and greatly exceeded by mortality. Relationships between seedling frequency (proportion of quadrats within a plot) and overstory abundance shifted from weakly negative before treatment to positive after thinning, to neutral in the longer term. However, these relationships were overshadowed by more recent, higher-severity prescribed fire and wildfire that stimulated seedling establishment while killing advanced regeneration and overstory trees. Our results highlight the dependence of regeneration responses on the history of, and time since, fuels treatment and subsequent disturbance. Managers must be aware of this spatial and temporal complexity and plan for future disturbances that are inevitable but unpredictable in timing and severity.
... Through long-term adaptation, plants have evolved heritable fire-adapted traits to improve their fire adaptability during long-term adaptation to fire disturbance [44]. Short-term studies (<10 years) have shown that programmed burns have minimal effect on the mortality of canopy trees [45][46][47]. The mortality rate of P. yunnanensis under natural conditions was approximately 0.93% to 1.27%, while under prescribed burns, it was 1.33%, indicating that P. yunnanensis has some fire resistance and that prescribed burns on it are safe. ...
Prescribed burning is a widely used fuel management employed technique to mitigate the risk of forest fires. The Pinus yunnanensis Franch. forest, which is frequently prone to forest fires in southwestern China, serves as a prime example for investigating the effects of prescribed burning on the flammability of surface dead fuel. This research aims to establish a scientific foundation for managing dead fuel in forests, as well as fire prevention and control strategies. Field data was collected from P. yunnanensis forests located in central Yunnan Province in 2021 and 2022. The study implemented a randomized complete block design with two blocks and three treatments: an unburned control (UB), one year after the prescribed burning (PB1a), and three years after the prescribed burning (PB3a). These treatments were evaluated based on three indices: surface dead-bed structure, physicochemical properties, and potential fire behavior parameters. To analyze the stand characteristics of the sample plots, a paired t-test was conducted. The results indicated no significant differences in the stand characteristics of P. yunnanensis following prescribed burning (p > 0.05). Prescribed burning led to a significant decrease in the average surface dead fuel load from 10.24 t/ha to 3.70 t/ha, representing a reduction of 63.87%. Additionally, the average fire−line intensity decreased from 454 kw/m to 190 kw/m, indicating a decrease of 58.15%. Despite prescribed burning, there were no significant changes observed in the physical and chemical properties of dead fuels (p > 0.05). However, the bed structure of dead fuels and fire behavior parameters exhibited a significant reduction compared with the control sample site. The findings of this study provide essential theoretical support for the scientific implementation of prescribed burning programs and the accurate evaluation of ecological and environmental effects post burning.
... This effect is generally transitory, but varies with the degree of disturbance (Nave et al. 2010). Mechanical treatments to thin forest stands can cause disturbance that leads to additional soil carbon losses through increased erosion (Schwilk et al. 2009, Stephens et al. 2012. Forest harvest also creates emissions from the operation of machinery used to implement forest harvest; these emissions vary widely based upon the size and type of machinery, as well as the specifics of silvicultural treatments (box 3). ...
Forest and grassland ecosystems in the United States play a critical role in the global carbon cycle, and land management activities influence their ability to absorb and sequester carbon. These ecosystems provide a critical regulating function, offsetting about 12 to 19 percent of the Nation’s annual greenhouse gas emissions. Forests and grasslands are managed for many different objectives and a variety of goods and services, including clean water, clean air, biodiversity, wood products, wildlife habitat, food, recreation, and carbon sequestration. Although carbon may be of interest in developing management plans and options, it may not be a primary management objective. The amount of carbon absorbed by, and stored within, a particular ecosystem can be affected by many factors related to land management, including land-use change, management activities, disturbance, the use of harvested wood, and climate. The long-term capacity of forest ecosystems to absorb and sequester carbon depends in large part on their health, productivity, resilience, and ability to adapt to changing conditions. This report describes the role of forest and grassland ecosystems in the carbon cycle and provides information for considering carbon as one of many objectives for land management activities.
... Fuel reduction treatments can reduce wildfire impacts on forests (Agee and Skinner 2005), bolster suppression efforts (Agee et al. 2000), and improve firefighter safety (Moghaddas and Craggs 2007). In high fire hazard forests, the application of thinning treatments followed by prescribed fire can create fire resistant forest structures by reducing stand densities (Stephens et al. 2009), retaining larger-diameter, fire-resistant trees with greater crown base heights (Agee and Skinner 2005), and decreasing surface and ladder fuels (Schwilk et al. 2009;Hood 2010). Mainly, these changes enhance managers' ability to accommodate the effects of wildfire on the landscape and promote overall forest resilience by allowing ecosystems to maintain their basic structure and function amid disturbance and eventually return to similar structure and composition post-disturbance DeRose and Long 2014;Tubbesing et al. 2019;Steel et al. 2021). ...
Background
In California’s mixed-conifer forests, fuel reduction treatments can successfully reduce fire severity, bolster forest resilience, and make lasting changes in forest structure. However, current understanding of the duration of treatment effectiveness is lacking robust empirical evidence. We leveraged data collected from 20-year-old forest monitoring plots within fuel treatments that captured a range of wildfire occurrence (i.e., not burned, burned once, or burned twice) following initial plot establishment and overstory thinning and prescribed fire treatments.
Results
Initial treatments reduced live basal area and retained larger-diameter trees; these effects persisted throughout the 20-year study period. Wildfires maintained low surface and ground fuel loads established by treatments. Treatments also reduced the probability of torching immediately post-treatment and 20 years post initial thinning treatments.
Conclusions
Fuel treatments in conifer-dominated forests can conserve forest structure in the face of wildfire. Additionally, findings support that the effective lifespans of treatments can be extended by wildfire occurrence. Our results suggest that continued application of shaded fuel breaks is not only a sound strategy to ensure forest persistence through wildfire but may also be compatible with restoration objectives aimed at allowing for the use of more ecologically beneficial fire across landscapes.
... Studies by McKinley et al. (2011) and Ryan et al. (2010) indicate that reducing the amount of forest harvest can decrease carbon losses to the atmosphere. As stated by Schwilk et al. (2009) and Stephens et al. (2012), forest disturbances can lead to additional soil carbon losses through soil erosion inducement. All F-ratios are based on the residual mean square error a Significant at 0.05% level of significance ...
Mabira Central Forest Reserve (CFR), one of the biggest forest reserves in Uganda, has increasingly undergone encroachments and deforestation. This chapter presents the implications of a range of forest management options for carbon stocks in the Mabira CFR. The effects of forest management options were reviewed by comparing above-ground biomass (AGB), carbon, and soil organic carbon (SOC) in three management zones. The chapter attempts to provide estimates of AGB and carbon stocks (t/ha) of forest (trees) and SOC using sampling techniques and allometric equations. AGB and carbon were obtained from a count of 143 trees, measuring parameters of diameter at breast height (DBH), crown diameter (CW), and height (H) with tree coordinates. It also makes use of the Velle (Estimation of standing stock of woody biomass in areas where little or no baseline data are available. A study based on field measurements in Uganda. Norges Landbrukshoegskole, Ås, 1995) allometric equations developed for Uganda to estimate AGB.
The strict nature reserve management zone was noted to sink the highest volume of carbon of approximately 6,771,092.34 tonnes, as compared to the recreation zone (2,196,467.59 tonnes) and production zone (458,903.57 tonnes). A statistically significant relationship was identified between AGB and carbon. SOC varied with soil depth, with the soil surface of 0–10 cm depth registering the highest mean of 2.78% across all the management zones. Soil depth and land use/cover types also had a statistically significant effect on the percentage of SOC ( P = 0.05). A statistically significant difference at the 95% significance level was also identified between the mean carbon stocks from one level of management zones to another. Recommendations include: demarcating forest boundaries to minimize encroachment, enforcement of forestry policy for sustainable development, promote reforestation, and increase human resources for efficient monitoring of the forest compartments.
... We predicted that vegetation structure would respond to these disturbances. Specifically, we hypothesized that disturbances would be associated with diminished tree cover, and, due to increased light penetration, would be associated with increasing cover of bare ground, grasses, forbs, and, over time, shrubs (Goosem, 2007;Shatford et al., 2007;Schwilk et al., 2009;Stephens et al., 2012;Crotteau et al., 2013;Miller et al., 2014). We also hypothesized that disturbances would reduce soil stability and alter soil chemistry (Kutiel and Shaviv, 1989;Manley et al., 1995;Neff et al., 2005;James et al., 2021). ...
Introduction
Ecological conditions at a given site are driven by factors including resource availability, habitat connectivity, and disturbance history. Land managers can influence disturbance history at a site by harvesting resources, creating transportation pathways, introducing new species, and altering the frequency and severity of events such as fires and floods. As a result, locations with different land management histories have also likely experienced different disturbance trajectories that, over time, are likely to result in different ecological characteristics.
Methods
To understand how the presence of different management histories may shape ecological conditions across large landscapes, we examined plant and soil characteristics at matched sampling points across jurisdictional boundaries within four Protected Area-Centered Ecosystems (PACEs) in the western US. We employed Bayesian modeling to explore 1) the extent to which specific ecological variables are linked to disturbance and jurisdiction both among and within individual PACEs, and 2) whether disturbance evidence differs among jurisdictions within each PACE.
Results
Across all jurisdictions we found that disturbances were associated with ecologically meaningful shifts in percent cover of bare ground, forbs, grass, shrubs, and trees, as well as in tree species richness, soil stability, and total carbon. However, the magnitude of shifts varied by PACE. Within PACEs, there were also meaningful associations between some ecological variables and jurisdiction type; the most consistent of these were in soil stability and soil carbon:nitrogen ratios. Disturbance evidence within each PACE was relatively similar across jurisdictions, with strong differences detected between contrast jurisdictions only for the Lassen Volcanic National Park PACE (LAVO).
Discussion
These findings suggest an interaction between management history and geography, such that ecotones appear to manifest at jurisdictional boundaries within some, but not all, contexts of disturbance and location. Additionally, we detected numerous differences between PACEs in the size of disturbance effects on ecological variables, suggesting that while the interplay between disturbance and management explored here appears influential, there remains a large amount of unexplained variance in these landscapes. As continued global change elevates the importance of large landscape habitat connectivity, unaligned management activities among neighboring jurisdictions are likely to influence existing ecological conditions and connectivity, conservation planning, and desired outcomes.
... The combination of mechanical thinning and prescribed burning appears to be a potential good management approach for fire risk and drought stress mitigation. Previous studies have experimentally proved short-term benefits of combining prescribed burning with thinning in reducing potential fire hazard (Piqué and Domènech, 2018;Schwilk et al., 2009). Hood et al. (2020) reported absence of long-term benefits after treatments in terms of fuel reduction, but they described a long-lasting change in species composition towards less flammable mixtures. ...
Vegetation structure affects the vulnerability of a forest to drought events and wildfires. Management decisions, such as thinning intensity and type of understory treatment, influence competition for water resources and amount of fuel available. While heavy thinning effectively reduces tree water stress and intensity of a crown fire, the duration of these benefits may be limited by a fast growth response of the understory. Our aim was to study the effect of forest structure on pine forests vulnerability to extreme drought events and on the potential wildfire behaviour after management, with a special focus on the role of the understory. In three sub-Mediterranean sites of NE Spain dominated by Pinus nigra, two intensities of thinning (light: aiming at 70-75% canopy cover; and heavy: aiming at 50-60% canopy cover) followed by two understory treatments (mechanical only and mechanical plus prescribed burning) were applied, resulting in four differently managed stands plus an untreated control per site. Four to five years after management, we measured forest structure (overstory in one 314 m 2 circular plot and understory in 20 quadrats of 1 m 2 per treatment unit) and fuel load (in two 10 m transects per treatment unit) and simulated water balance and fire behaviour under extreme weather conditions. Understory contribution was assessed comparing the real structure with a virtual forest stand where understory vegetation equalled the one of the untreated control. Our results suggest that the resulting mid-term structure following treatments effectively reduced water stress and fire behaviour compared with untreated control, and that the most effective treatments were the ones where prescribed burning was applied after light or heavy thinning. While understory clearing contributes to increase the resistance to both disturbances, an additive effect of burning the debris reduced the vulnerability to drought and wildfires after treatments. Our study highlights the importance of managing the understory to further increase forest resistance to both disturbances.
... Given that the shrubs in their experiment were 80-100 years old and as large as 23 cm in diameter, the lack of a greater watershed response suggests that soil moisture pools or canopy tree water use may have increased. The short-lived effects of the shrub removal reported by Johnson and Kovner (1956) suggests that the combination of mechanical, chemical (herbicide) and burning treatments (Schwilk et al., 2009;McIver et al., 2013), similar to those employed in our study, are required to improve forest structure over the long term. ...
Forest disturbance to overstory vegetation can result in increased abundance of understory vegetation, potentially affecting various ecosystem processes. Habitat expansion of a native evergreen woody shrub, Rhododendron maximum (rosebay rhododendron), in southern Appalachian Mountain riparian forests of the eastern United States has occurred following the mortality of Castanea dentata (American chesnut) in the early 20th century and more recently Tsuga canadensis (Eastern hemlock). Increased abundance of R. maximum has been associated with reduced tree seedling recruitment and survival, as well as shorter overstory tree height compared to where R. maximum understory was not present. As part of a larger investigation into the removal of R. maximum to restore vegetation structure and composition, this study examined the effects of removing R. maximum understory (by cutting) and soil O-horizon (by prescribed fire) on microclimate, whole tree and plot-level transpiration during 2016 and 2017 growing seasons. We found increased soil moisture in the wetter year (2017) and greater variation in light transmission to the forest floor after the treatment. Larger trees were able to increase their water use in the drier year (2016), resulting in similar plot-level transpiration of treatment and reference plots. Removal of R. maximum understory is not expected to significantly alter streamflow or stream chemistry during the growing season; however, it can help restore forest structure through improving tree seedling survival and recruitment as well as increasing growth of established trees.
... For example, changing fire regimes drive corresponding shifts in ecosystem carbon and nutrient cycles by changing plant communities, removing plant biomass (both live and dead), direct combustion of soil organic legacies including carbon (C) and nitrogen (N), altering fuel loads, and impacting soil microbial communities (e.g., Baird et al. 1999;Kauffman et al. 1995;Mack et al. 2021;Muqaddas et al. 2015;Ojima et al. 1994;Pellegrini et al. 2015). Drivers that may underlie the responses of fungal communities are complex and likely include (i) direct heatinduced mortality from fire; (ii) abrupt changes to substrates and nutrient availability (Kaye and Hart 1998;Wang et al. 2012); (iii) long-lasting indirect changes in environmental conditions (Chen and Cairney 2002;Reazin et al. 2016); and (iv) altered physiology or mortality of plant hosts on which some fungi depend (Haase and Sackett 1998;Schwilk et al. 2009). Simultaneous shifts in plant responses (from individual plant physiology to entire communities), substrates for fungal decomposers, and soil nutrient availability (including immobilization, losses through combustion and volatilization, and release through organismal mortality) all complicate generalizing fungal fire ecology, particularly across widespread variation in fire regimes and ecosystems. ...
Fires occur in most terrestrial ecosystems where they drive changes in the traits, composition, and diversity of fungal communities. Fires range from rare, stand-replacing wildfires to frequent, prescribed fires used to mimic natural fire regimes. Fire regime factors, including burn severity, fire intensity, and timing, vary widely and likely determine how fungi respond to fires. Despite the importance of fungi to post-fire plant communities and ecosystem functioning, attempts to identify common fungal responses and their major drivers are lacking. This synthesis addresses this knowledge gap and ranges from fire adaptations of specific fungi to succession and assembly fungal communities as they respond to spatially heterogenous burning within the landscape. Fires impact fungi directly and indirectly through their effects on fungal survival, substrate and habitat modifications , changes in environmental conditions, and/or physiological responses of the hosts with which fungi interact. Some specific pyrophilous, or "fire-loving," fungi often appear after fire. Our synthesis explores whether such taxa can be considered cosmopolitan, and whether they are truly fire-adapted or simply opportunists adapted to rapidly occupy substrates and habitats made available by fires. We also discuss the possible inoculum sources of post-fire fungi and explore existing conceptual models and ecological frameworks that may be useful in generalizing fungal fire responses. We conclude with identifying research gaps and areas that may best transform the current knowledge and understanding of fungal responses to fire.
... Thus, a perceived lack of effectiveness for these two approaches may reflect respondents' expertise in fire behavior and forest management; this is confirmed by the finding that "lack of information regarding fuel treatments" is not strongly limiting. In contrast, prescribed burning following tree cutting and livestock grazing were also considered less effective by respondents although they have proven effective in other jurisdictions (Schwilk et al., 2009;Prichard et al., 2021;Rouet-Leduc et al., 2021). These approaches are not common practice for reducing fire risk in BC, so respondents may not be familiar with the effectiveness of these approaches, which can be a barrier to uptake (McGee, 2007;McCaffrey et al., 2013). ...
Wildfires in the wildland-urban interface (WUI) are increasingly threatening lives and livelihoods. These growing impacts have prompted a paradigm shift toward proactive wildfire management that prioritizes prevention and preparedness instead of response. Despite this shift, many communities remain unprepared for wildfires in the WUI due to diverse individual and social-political factors influencing engagement with proactive management approaches. The catastrophic fire seasons of 2017, 2018, and 2021 in British Columbia (BC), Canada, highlighted just how vulnerable communities continue to be and the urgent need to understand the factors limiting engagement to future resilience to wildfire. Our study, conducted prior to the catastrophic fire season in 2017, surveyed 77 community leaders across BC to better understand the factors driving engagement, including risk perception, preferences and support for approaches, and key barriers limiting progress. We demonstrate that wildfire risk is an urgent issue facing communities across BC, but a range of factors drive variable community engagement with proactive wildfire management. First Nations and smaller (≤5,000 residents) communities were less likely to have developed a community wildfire plan, even though First Nations were significantly more concerned than municipalities/regional districts about certain values (such as drinking water and biodiversity) that were at risk from wildfire. In general, proactive approaches that were considered effective were also the most supported. The most highly supported approaches included enforcement of regulations and education, both of which are considered provincial responsibility in BC and are unlikely to alter community values in the WUI. In contrast, approaches involving prescribed burning of the understory had the highest levels of opposition. Despite variability in these individual factors, social-political barriers related to financial and social (time and expertise) capacity primarily limited engagement with proactive wildfire management, including provincial and federal funding programs. However, these barriers are not equally felt across community groups; First Nations identified social capacity (such as expertise on government-sponsored approaches and awareness of funding programs) as significantly more limiting than municipalities/regional districts. Our study illustrates the limitations of implementing a “shared responsibility” of proactive wildfire management in the WUI in BC without targeted supports to address unequal capacity barriers.
... 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. Based on study findings, there is evidence that properly designed treatments can reduce burned area and wildfire severity and improve forest resilience to climatic changes, but there are tradeoffs in terms of some kinds of wildlife (e.g., spotted owl nesting, roosting) habitats (Barros et al. 2018, Ager et al. 2020. ...
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.
... The other two sites (2A and 2B) were part of the nationwide Fire and Fire Surrogate (FFS) study conducted by the U.S. Forest Service and each included a thinned plot, a thinned-and-burned plot and a control plot. Lowintensity prescribed fire was applied to the thinned-and-burned plots as part of the restoration treatments (Faiella & Bailey, 2007;Schwilk et al., 2009). Prescribed burn methods were consistent across the study sites as all were conducted as part of the FFS project. ...
Thinning of semi‐arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration, and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent, and the spatial and temporal variation in soil moisture at four experimental paired treatment‐control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3‐dimensional lidar data to determine the relationship between vegetation structure, snow, and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8‐11 years before this study. However, we did not find evidence that snow water equivalent was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R2 < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow, and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties. This article is protected by copyright. All rights reserved.
... In the western USA, fuel treatments designed to reduce wildfire intensity and the severity of effects are rarely implemented at scales that are large enough to mitigate undesirable wildfire impacts (Hiers 2017;Kolden 2019). Efforts to reduce fuel loads at meaningful scales are critical but may promote invasive plants in some cases (Schwilk et al. 2009). ...
Higher temperatures, lower snowpacks, drought, and extended dry periods have contributed to increased wildfire activity in recent decades. Climate change is expected to increase the frequency of large fires, the cumulative area burned, and fire suppression costs and risks in many areas of the USA. Fire regimes are likely to change due to interactions among climate, fire, and other stressors and disturbances; resulting in persistent changes in forest structure and function. The remainder of the twenty-first century will present substantial challenges, as natural resource managers are faced with higher fire risk and the difficult task of maintaining ecological function in a rapidly changing biophysical and social landscape. Fuel treatments will continue to be important for minimizing the undesirable ecological effects of fire, and for enhancing firefighter safety; however, treatments must be implemented strategically across large areas. Collaboration among agencies, private landowners, and other organizations will be critical for ensuring resilience and sustainable forest management.
... However, fire may improve growing conditions by removing competing vegetation [15], and stimulate the development of structures that may improve resistance to bark beetle attack [16]. Prescribed fire is designed to disproportionally kill small trees, so that large reductions in stand basal area are uncommon [17]. Yet, prescribed fire may remove a substantial number of small trees [18], potentially reducing the competitive environment for surviving trees. ...
Prescribed fire reduces fire hazards by removing dead and live fuels (small trees and shrubs). Reductions in forest density following prescribed fire treatments (often in concert with mechanical treatments) may also lessen competition so that residual trees might be more likely to survive when confronted with additional stressors, such as drought. The current evidence for these effects is mixed and additional study is needed. Previous work found increased tree survivorship in low elevation forests with a recent history of fire during the early years of an intense drought (2012 to 2014) in national parks in the southern Sierra Nevada. We extend these observations through additional years of intense drought and continuing elevated tree mortality through 2017 at Sequoia and Kings Canyon National Parks. Relative to unburned sites, we found that burned sites had lower stem density and had lower proportions of recently dead trees (for stems ≤47.5 cm dbh) that presumably died during the drought. Differences in recent tree mortality among burned and unburned sites held for both fir (white fir and red fir) and pine (sugar pine and ponderosa pine) species. Unlike earlier results, models of individual tree mortality probability supported an interaction between plot burn status and tree size, suggesting the effect of prescribed fire was limited to small trees. We consider differences with other recent results and discuss potential management implications including trade-offs between large tree mortality following prescribed fire and increased drought resistance.
... Adaptations to wildfire center on accommodating forest fire in fireadapted ecosystems primarily by managing fuel loads through prescribed burning and silvicultural practices. Where fire suppression has resulted in hazardous levels of fuels, re-introducing fire may require first altering structure by mechanical means followed by a series of carefully planned controlled burns (Boerner et al., 2008;Schwilk et al., 2009;Phillips et al., 2012) that may include increased harvest to reduce old and susceptible stands (Schelhaas et al., 2010). These treatments, particularly prescribed fire, result in soil C losses that take decades to recover (Nave et al., 2011;James and Harrison, 2016), but may be favorable when compared to wildfire soil C losses . ...
Until recently, the processes of litter decomposition and soil organic matter formation in forests have been studied in isolation, which has hindered the development of a comprehensive understanding of the entire process. The last decade has brought considerable progress in this scientific endeavour in response to the challenge to sequester atmospheric C in forest soils. In this paper we review key recent developments in this field and describe our current collective understanding of litter decomposition and transformation processes and pathways in forest ecosystems.
Compelling evidence that most slow-cycling SOM has been microbially transformed forces us to rethink the standard technique of measuring mass remaining in litterbags during incubation to indicate litter decomposition rates. Rather than indicating the mass of litter that remains undecomposed, these measurements reflect the net outcome of two simultaneous processes: decomposition of plant material and accumulation of microbial and faunal transformation products. Measurement of both of these pools, rather than just the total mass of material in litterbags is necessary to understand decomposition processes. For example, the apparent retarding effect of available N on mass loss during late-stage decomposition may actually result from N promoting the production of microbial biomass and necromass, thereby increasing the accumulation of transformation products during late-stage decay. We recommend referring to the mass of material in litterbags as ‘net mass remaining’ or ‘residue mass’ rather than ‘litter mass’, to acknowledge its changing composition as decomposition proceeds.
Decomposition pathways in forests with abundant detritivorous meso- and macrofauna remain poorly understood as a consequence of the inability of the litterbag technique to capture their influences (even with differing mesh sizes). Long-term studies monitoring the transformation of litter to faunal faecal material and subsequent transformations of this material are urgently needed.
Roots and mycorrhizal fungal hyphae are important sources of SOM, including stable SOM. Fine roots (orders 1 and 2) decompose particularly slowly, as do some mycorrhizal hyphae, which has been attributed to cell-wall constituents such as lignins, melanin and glycoproteins. Convergence of mass loss curves of litters that initially decompose quickly and slowly indicates that leaf litter, root litter, and fungal residues with large labile contents can generate as much SOM as recalcitrant litters.
Transformation of litter into SOM can follow many pathways, depending on characteristics of the site. Key site and soil properties influence the biotic community present and together determine the pathway that decomposition follows on that site. As such, litter transformations occur along a continuum between situations in which aboveground litter is mainly transformed into humus that accumulates on the soil surface, and situations in which partially decomposed litter is transferred to the mineral soil via bioturbation. Predicting the most likely decomposition pathway should inform decisions on how to measure and interpret the transformations that occur on a particular site.
... For this study, we focused on a pure ponderosa pine (Pinus ponderosa) forest near Flagstaff, AZ where we could make use of previously collected ALS and TLS lidar datasets ( Figure 2) [18]. These sites were managed by the US Forest Service as part of the "Fire and Fire Surrogate" study network [29] with original treatment objectives including fuels reductions [30]. In brief, the study design presented an ideal tree density gradient resulting from four treatments: thinning from below (thin-only), broadcast burning (burn-only), a combination of thinning from below with subsequent broadcast burning (thin-and-burn), and an untreated control ( Figure 2). ...
Applications of lidar in ecosystem conservation and management continue to expand as technology has rapidly evolved. An accounting of relative accuracy and errors among lidar platforms within a range of forest types and structural configurations was needed. Within a ponderosa pine forest in northern Arizona, we compare vegetation attributes at the tree-, plot-, and stand-scales derived from three lidar platforms: fixed-wing airborne (ALS), fixed-location terrestrial (TLS), and hand-held mobile laser scanning (MLS). We present a methodology to segment individual trees from TLS and MLS datasets, incorporating eigen-value and density metrics to locate trees, then assigning point returns to trees using a graph-theory shortest-path approach. Overall, we found MLS consistently provided more accurate structural metrics at the tree- (e.g., mean absolute error for DBH in cm was 4.8, 5.0, and 9.1 for MLS, TLS and ALS, respectively) and plot-scale (e.g., R2 for field observed and lidar-derived basal area, m2 ha−1, was 0.986, 0.974, and 0.851 for MLS, TLS, and ALS, respectively) as compared to ALS and TLS. While TLS data produced estimates similar to MLS, attributes derived from TLS often underpredicted structural values due to occlusion. Additionally, ALS data provided accurate estimates of tree height for larger trees, yet consistently missed and underpredicted small trees (≤35 cm). MLS produced accurate estimates of canopy cover and landscape metrics up to 50 m from plot center. TLS tended to underpredict both canopy cover and patch metrics with constant bias due to occlusion. Taking full advantage of minimal occlusion effects, MLS data consistently provided the best individual tree and plot-based metrics, with ALS providing the best estimates for volume, biomass, and canopy cover. Overall, we found MLS data logistically simple, quickly acquirable, and accurate for small area inventories, assessments, and monitoring activities. We suggest further work exploring the active use of MLS for forest monitoring and inventory.
... Prescribed burning is one management technique to reduce fire severity because prescribed fires are generally lower intensity and reduce the severity of future fires by combusting accumulated fuel (Stephens et al., 2013), which may also lower soil C losses. Prescribed burning incurs C costs by reducing aboveground biomass stocks (fire reduces biomass in trees, litter, and the forest floor; Schwilk et al., 2009). But whether prescribed fire changes soil C storage in coniferous forests is less clear (Boerner et al., 2008(Boerner et al., , 2009Hatten et al., 2005;Moghaddas & Stephens, 2007) and could be one management avenue to promote soil C sequestration in forests (Jandl et al., 2007). ...
The impact of shifting disturbance regimes on soil carbon (C) storage is a key uncertainty in global change research. Wildfires in coniferous forests are becoming more frequent in many regions, potentially causing large C emissions. Repeated low‐intensity fires can mitigate wildfire severity, but repeated combustion may decrease soil C unless compensatory responses stabilize soil organic matter. Here we tested how 30 years of decadal prescribed burning affected C and nitrogen (N) in plants, detritus, and soils in coniferous forests in the Sierra Nevada mountains, USA. Tree basal area and litter stocks were resilient to fire, but fire reduced forest floor C by 77% (‐36.4 MgC ha‐1). In mineral soils, fire reduced C that was free from minerals by 41% (‐4.4 MgC ha‐1) but not C associated with minerals, and only in depths ≤ 5 cm. Fire also transformed the properties of remaining mineral soil organic matter by increasing the proportion of C in a pyrogenic form (from 3.2% to 7.5%) and associated with minerals (from 47% to 58%), suggesting the remaining soil C is more resistant to decomposition. Laboratory assays illustrated that fire reduced microbial CO2 respiration rates by 59% and the activity of eight extracellular enzymes that degrade cellulosic and aromatic compounds by 40‐65%. Lower decomposition was correlated with lower inorganic N (‐49%), especially ammonium, suggesting N availability is coupled with decomposition. The relative increase in forms of soil organic matter that are resistant to decay or stabilized onto mineral surfaces, and the associated decline in decomposition suggest that low‐intensity fire can potentially promote mineral soil C storage in pools with long mean residence times in coniferous forests.
... Regardless of the desired outcome, our results suggest that commercial thinning can be used to achieve wildlife objectives in production forests, including when thinning prescriptions call for atypical reductions in basal area. This is especially important given that recreating effects of large-scale fires is unrealistic within the pine forests of the southeastern United States, or other fire-adapted forest types [40]. Instead, efforts to restore desired understory communities should evaluate alternative management techniques [15,[41][42][43]. ...
Tree stocking and the associated canopy closure in production forests is often greater than optimal for wildlife that require an open canopy and the associated understory plant community. Although mid-rotation treatments such as thinning can reduce canopy closure and return sunlight to the forest floor, stimulating understory vegetation, wildlife-focused thinning prescriptions often involve thinning stands to lower tree densities than are typically prescribed for commercial logging operations. Therefore, we quantified the accuracy and precision with which commercial logging crews thinned pre-marked and unmarked mid-rotation loblolly pine (Pinus taeda) stands to residual basal areas of 9 (low), 14 (medium), and 18 (high) m2/ha. Following harvest, observed basal areas were 3.36, 1.58, and 0.6 m2/ha below target basal areas for the high, medium, and low basal area treatments, respectively. Pre-marking stands increased precision, but not accuracy, of thinning operations. We believe the thinning outcomes we observed are sufficient to achieve wildlife objectives in production forests, and that the added expense associated with pre-marking stands to achieve wildlife objectives in production forests depends on focal wildlife species and management objectives.
... In the southern Appalachian Mountains, prescribed fire is used for objectives such as reducing wildfire risk, creating early seral wildlife habitat, and restoring fire-adapted plant communities (Monk et al. 1985;Christensen 1993;Waldrop et al. 2016). Although the most prevalent technique for achieving these objectives in the southern Appalachian Mountains has been dormant-season burning (January through March), scientists and managers in the region have also expressed interest in using other fuel reduction methods such as mechanical treatments like mastication or chainsaw felling of ladder fuels to reverse the effects of fire suppression (Brose and Van Lear 1998;Schwilk et al. 2009). Alone or in combination with prescribed fire, these treatments have shown promise for reducing fuels and restoring a more open forest structure, but some knowledge gaps remain (Waldrop et al. 2016). ...
Background
Decades of fire exclusion in the southern Appalachian Mountains, USA, has led to changing forest structure and species composition over time. Forest managers and scientists recognize this and are implementing silvicultural treatments to restore forest communities. In this study, conducted at the southern Appalachian Fire and Fire Surrogate Study site in Green River Game Land, North Carolina, USA, we assessed the effects of four fuel-reduction methods (burned four times, B; mechanical treatment two times, M; mechanical treatment two times plus burned four times, MB; and control, C) on the changes in understory community from pre-treatment to post-treatment years (2001 to 2016). We used non-metric multidimensional scaling (NMDS) to determine overall understory community heterogeneity, agglomerative hierarchical cluster analyses (AHCA) to determine finer-scale changes in understory community structure, and indicator species analyses (ISA) to identify the species that were associated with the different fuel reduction treatments over time.
Results
The NMDS ordination showed little separation between treatment polygons. The AHCA resulted in two main categories of understory species responses based on how treatment plots clustered together: (1) species apparently unaffected by the treatments ( i.e., no treatment pattern present within cluster); and (2) species that responded to B, M, or MB treatments ( i.e., pattern of treatment plots present within cluster). Nearly half (49.2%) of tree-species plots clustered based on treatments; 60% of shrub-species plots clustered based on treatments; and 64% of herbaceous-species plots clustered based on treatments. Many plots clustered similarly in response to fire-related treatments (B and MB). The ISA identified 11 total tree species: three in B, one in M, and seven in MB; six total shrub species: two in M, and four in MB, and 17 total herbaceous species or genera: one in C, and 16 in MB.
Conclusion
Fire and fire surrogate treatments did not dramatically shift understory composition after 15 years. However, certain ruderal and early seral species responded positively to MB, which was the most intensive treatment. Modest understory community changes were also observed in B, suggestive of early signs of shifting composition toward a more open forest community after four burns.
... In a system characterized by infrequent fire, prescribed burning neither promoted nor reduced tree establishment. Although bare soil was exposed, providing seedbed conditions that favor conifer germination (Kilgore 1973;Zald et al. 2008;Schwilk et al. 2009), the effect was transient and overshadowed by the more pervasive, soil-disturbing activities of gophers (Jones et al. 2008;Case et al. 2013). ...
Tree encroachment threatens the biodiversity and ecological functioning of grasslands worldwide. Reversing effects of encroachment requires not only removing trees, but limiting subsequent invasions, which can stall grassland reassembly. We quantified rates and patterns of conifer reinvasion of mountain grasslands (meadows) in western Oregon, 7 and 15 years after experimental tree removal with or without burning. We assessed frequency (percentage of 100 m² subplots) and density of reinvading trees in six 1‐ha plots as a function of burn treatment, abundance in the adjacent forest (a proxy for seed availability), and distance to or orientation of forest edges. After 15 years, frequency and density of colonists varied widely among plots (24–57% of subplots and 60–250 trees/ha, respectively), but burning had no effect on either measure. Abies grandis, which dominated adjacent forests, was the principal invader. Colonization by less‐common Pinus contorta and A. procera was correlated with species' density and basal area in adjacent forests, suggesting that establishment was seed limited. Annualized rates of invasion were similar between early and late sampling intervals (9‐10/trees/ha). Establishment was generally greater along the forested edges of plots (0–10 m) than in the core (10–50 m). Prominence of shade‐tolerant A. grandis along N‐facing edges suggests strong microclimatic controls on establishment. In sum, tree removal, with or without prescribed fire, is successful in reversing effects of conifer encroachment. Despite abundant seed rain, reinvasion is slow and spatially restricted. Periodic hand removal of colonizing trees, while they are small, can be done efficiently and economically.
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... For example, following the 2000 Cerro Grande Fire in New Mexico, contamination of aerial seeding sources was responsible for inadvertently broadcasting cheatgrass seeds across recently burned areas . Relatedly, prescribed fire and thinning of smalldiameter trees are used to reduce fuels in order to increase the resilience of forests to high-intensity wildfire (Stephens et al. 2012), but some studies have shown that these treatments promote an increase in invasive species richness (Schwilk et al. 2009). Climate change may also affect the efficacy of tools used to manage invasive species (Sect. ...
Mean surface temperatures have increased globally by ~0.7 °C per century since 1900 and 0.16 °C per decade since 1970 (Levinson and Fettig 2014). Most of this warming is believed to result from increases in atmospheric concentrations of greenhouse gases produced by human activity. Temperature increases have been greater in winter than in summer, and there is a tendency for these increases to be manifested mainly by changes in minimum (nighttime low) temperatures (Kukla and Karl 1993). Changes in precipitation patterns have also been observed, but are more variable than those of temperature. Even under conservative emission scenarios, future climatic changes are likely to include further increases in temperature with significant drying (drought) in some regions and increases in the frequency and severity of extreme weather events (IPCC 2007). For example, multimodel means of annual temperature from climate projections predict an increase of 3–9 °C in the United States over the next century combined with reductions in summer precipitation in certain areas (Walsh et al. 2014). These changes will affect invasive species in several ways. Furthermore, climate change may challenge the way we perceive and consider nonnative invasive species, as impacts to some will change and others will remain unaffected; other nonnative species are likely to become invasive; and native species are likely to shift their geographic ranges into novel habitats.
... We suggest that fire in these extremes will decrease biological activity over and above the direct chemical or biological impacts of fires, as drought preferentially affects burned soils. Because lower severity and prescribed fires generally leave the forest canopy untouched (Ryan, 2002;Schwilk et al., 2009) and can actually contribute to regeneration because of the release of nutrients into soils (Certini, 2005), prescribed fire is a possible method for moderating the negative effects of severe wildfires on soils, as shown in this study, and has been added to the forest management strategy of these Texas State Park sites and many other forests (Sparks, 2004). ...
Increasing fire severity and frequency may stress ecosystems also impacted by climate change. We studied the physical limitations to regeneration after fire in an ecosystem that already experiences high summer temperatures and drought and is therefore a possible analog of the future. We compared soil respiration as an indicator of microbial activity in burned and unburned forest soils in central Texas, where two recent wildfires have occurred (2011 and 2015). We also measured soil temperature, water content, soil water δ¹⁸O and δD values, total C, N, pH, and δ¹³C values of total organic matter. Burned soils had lower total C and N than unburned soils; however, lab‐based respiration measurements, which controlled for temperature and water content, suggest that microbial activity in burned and unburned soils are similar. Conversely, field measurements show that during hot and dry months respiration rates in burned soils were much lower than they were in unburned soils due to differences in soil temperature and water content. Soil temperature at 5 cm reached 60°C in burned soils due to the removal of canopy cover, the removal of organic matter insulation, and the deposition of black ash on the soil surface. Higher temperatures increased evaporation as indicated by significantly lower deuterium excess of water in soils burned in 2015 than unburned soils. If the disturbance of vegetation by fire is substantial enough, the resulting perturbations to soils persist for years, most importantly increased heat absorption which results in lower water contents and ultimately reduced microbial activity.
... The NRV concept has been applied to help manage wildfire fuel-loads (Schwilk et al. 2009), groundwater recharging in agricultural systems (Dunin et al. 1999), large woody debris in streams (Bisson et al. 2003), river and stream flow levels (Richter et al. 1997), marine ecosystems (Hughes et al. 2005), fisheries (Witherell et al. 2000) and rangeland vegetation dynamics (Fuhlendorf and Engle 2001). Almost 20 years ago, NRV was introduced to forest management agencies in Canada and is today variously referenced as emulation of natural disturbance (Klenk et al. 2009), natural disturbance management (Meitner et al. 2005), natural disturbance model (Hunter 1993), natural disturbance based forestry (Nielsen et al. 2008), and natural disturbance management model (Schmiegelow et al. 2006). ...
... Controlled burning is a tool in management of chirpine forest to reduce incidence of forest fire. The burning treatments are used throughout the world to reduce the fuel load (Cronan, et al., 2015, Schwilk et al., 2009and Stephens et al., 2012 to mitigate the uncontrolled fire. Various workers have conducted study on impact of burning in various landscapes of world (Alcaniz et al., 2018;Borja et al., 2016;Butnor et al., 2020;Catherine and Andersen, 2006;Eales et al., 2016;Elliot et al., 1999;Fernandes and Botelho, 2003;Hood et al., 2020;Huisinga et al., 2005;Magallanes, et al., 2020;Rawat, 1949 andZhan et al., 2020). ...
... Intentional, controlled burning is often the most effective means to decrease fuel loads and future risk from fires (Kalies and Yocom Kent 2016;Schwilk et al. 2009). Prescribed burns require careful planning and preparation, as well as permits. ...
Tree mortality is changing California’s landscape and affecting the amount and type of fuel available for wildland fires. Since the extraordinary drought of 2012 through 2015, millions of trees have died in California, particularly in the Sierra Nevada. Over the same period, fires have devastated towns and communities at forest edges, heightening concerns about future fires and the risks they pose to forests and people. Risk posed by fire depends on multiple factors, including weather and ignition sources, which are not affected by tree mortality. However, in areas that have experienced mass tree mortality, the amount and distribution of dead wood that can act as fuel has changed, altering the risk posed by fires. This guide summarizes current research on the links among tree mortality, fuels, and fire risk. It identifies resources that can help landowners and managers take appropriate action to manage fuels. In addition, it discusses examples of mass tree mortality and assesses mortality’s impacts on fuels—so that landowners and managers can understand fire risk on their own property and develop strategies to reduce it.
Uno studio che ha messo a confronto diversi tipi di trattamento (fuoco prescritto, trinciatura meccanizzata e combinazione di entrambi) per la manutenzione dei viali parafuoco. L’indagine ha stimato il carico e la struttura dei combustibili di superficie in tre anni successivi agli interventi. Attraverso l’uso di tecniche di simulazione sono state valutate le differenze tra i trattamenti nel mitigare il comportamento del potenziale incendio estivo.
Prescribed fires are an important management tool for reducing fuels and returning fire to the landscape. However, rarely are changes in fuels fully quantified using pre to post prescribed fire measurements and those studies that do exist show variable results. In the southern Rockies little literature exists on the impacts of prescribed fires, thus we examined multiple prescribed fires in northern Colorado to understand fire effects and changes in fuel complexes. Most prominently, prescribed fires influenced litter, duff, and rotten coarse woody debris but did not influence other surface fuels. Crown base height increased and tree density decreased, while basal area was relatively unimpacted. Season of burning impacted fire effects as substrate burn severity, bole char, and crown volume scorched were highest in summer and fall. Continued monitoring of prescribed fires is critical to understand the influence of prescribed fire on wildfires and ultimately improve prescribed fire outcomes.
Entender cómo la gestión forestal afecta a la resistencia de los bosques a múltiples perturbaciones es esencial para garantizar el cumplimento de los objetivos silvícolas. Muchas perturbaciones naturales, como sequías e incendios, a menudo suceden a la par y su impacto depende en gran medida de la estructura y composición del bosque, así como de la calidad de estación. En este estudio nos centramos en el efecto que tienen diferentes tipos de tratamientos selvícolas sobre la resistencia a medio plazo del pino laricio (Pinus nigra) a episodios de sequía e incendios. En 2015 y 2016 se realizaron claras por lo bajo de distinta intensidad (baja y alta), seguidas de dos tipos de gestión del sotobosque (desbroce y tratamiento mecánico o quema de restos) en tres localidades submediterráneas (Solsonès, Cataluña), resultando en cuatro rodales gestionados y un rodal control por localidad. Las simulaciones de balance hídrico y de comportamiento de un incendio sugieren que la reducción del sotobosque mediante quemas de restos en ambas intensidades de clara es el tratamiento que más resistencia confiere al bosque, mientras que la dendrocronología indica que los pinos dominantes incrementan su producción.
The social barriers and opportunities of prescribed fire management practices in the mid-Atlantic region are unknown. We hypothesized that there are mismatches between community perceptions of prescribed fire operations in the mid-Atlantic U.S. and the realities of its current and potential use in landscape management, but that these mismatches varied across the region between places with recent (Pennsylvania (PA)) and long-term (New Jersey (NJ)) active management programs. We used interdisciplinary, mixed-method approaches including trailhead surveys, focus groups, field measurements, simulation modeling, and environmental cost-benefit analysis. The goal was to triangulate the interactions among fire effects on ecosystems, community perceptions of benefits and concerns about prescribed fire, and manager perceptions of barriers and opportunities for putting managed fire safely back on the landscape.
Prescribed fire in dry coniferous forests of the western U.S. is used to reduce fire hazards. How large, old trees respond to these treatments is an important management consideration. Growth is a key indicator of residual tree condition, which can be predictive of mortality and response to future disturbance. Using a combination of long-term plot records and dendrochronological samples, we analyzed the effects of prescribed fire treatments from the early 1990 s on forest structure and individual tree growth in mixed-conifer forests of Lassen Volcanic National Park in northern California. Prescribed fire reduced stand live tree basal area and stem density at our sites up to 10 years following fire. Within two prescribed fire burn units and two adjacent unburned stands, we analyzed tree cores from 136 large (mean stem diameter > 70 cm) yellow pine (Pinus jeffreyi and P. ponderosa) and 136 large (mean stem diameter > 50 cm) white fir (Abies concolor). After accounting for annual precipitation, basal area increment for individual trees initially declined up to < 3 years post-fire for white fir and > 10 years post-fire for yellow pine, presumably in response to tree injuries. Growth improved for both species at a site that was burned twice, particularly for white fir. Recent average basal area increment was positively related to crown ratio and negatively associated with an index of local competition. Our findings suggest that forest management, such as prescribed fire and mechanical thinning, may be beneficial in terms of maintaining or improving tree growth among large residual trees. However, managers may want to balance the benefits of these treatments against inadvertent injury and mortality of large trees.
Fuels influence fire ignition, spread, intensity, and severity. Thus, fuels link fire behavior and fire effects. Fuels are central to our book, Fire science from chemistry to landscape management. We address how scientists and managers describe types of fuels, assess the amount of fuels (called fuel load) and characterize fuelbeds. The amount and type of fuel change with deposition, decomposition, disturbance, and vegetation dynamics. Fuel moisture too is dynamic. The moisture of live and dead fuels is critical to how readily fuels burn and how much fuel is consumed by fires. Lastly, we discuss fuels management, starting with strategies and principles, and continuing through different methods, including mechanical treatments and prescribed burning. Managers use fuels treatments to reduce or rearrange fuels to reduce potential fire spread and intensity. Fire fighters can use treatments in their fire suppression efforts. Fuels treatments are often part of ecological restoration to enhance resilience of forests and woodlands to future fires. Fuels management decreases burn severity, and not just when fires burn under moderate environmental conditions. Fuels treatments are less effective when challenged by fires burning under extreme conditions and as vegetation regrows. Strategic placement of fuels treatments can increase their effectiveness. Through our explanations of concepts and examples from forests, woodlands, and shrublands from around the world, readers develop a nuanced understanding of fuels dynamics and management. We discuss current issues such as where and how fuels treatments are effective, arguing that fuels treatments will still be useful in fire management even as climate changes. Readers can use the two interactive spreadsheets reinforce learning about fuel dynamics and crown fire mitigation.
The total area of land under forest plantation is increasing globally and in Patagonia. The consequences of this activity on natural ecosystems vary, depending on factors such as ecological context, history of use, and forest management. Reaching a balance between timber production and biodiversity conservation is one of the aims increasingly pursued in forest plantations, since biodiversity contributes to plantation stability while contributing to the maintenance of the structure and functions of natural communities. Through forest management, forest structure is controlled. Thus, forest management can contribute to maintaining the resources necessary for the persistence and development of natural communities. In northwestern Patagonia, exotic conifer plantations have been established since the ‘70s; first for cellulose production and particularly since the 90’s with timber production as the main objective. The most commonly used species in afforestation is the ponderosa pine (Pinus ponderosa P. Lawson & C. Lawson), which has been established in areas of ecotone and Patagonian Steppe. Although there are studies that cover various aspects of the effects plantations may cause in the region, there are others, especially associated with forest management stages, that have not been thoroughly addressed. The aim of this thesis was to evaluate ponderosa pine plantation effects on plant communities in northwestern Patagonia, and their relation with forest management stages. To achieve this aim, plant community attributes present under the canopy of several ponderosa pine forest structures, representative of the plantations in the region, were evaluated. Also, three characteristic plant species of the steppe (Berberis microphylla G. Forst, Adesmia volckmannii Phil., and Poa ligularis Nees ex Steud) were selected and their growth under three irradiance levels were studied: 20 and 60% simulating two canopy cover level situations, and 100% simulating the open sky situation. Moreover, plant community response to residual slash management generated by prunning and thinning activities, which is usually left on the forest floor, were evaluated during three years in a row. The fuel reduction treatments implemented were mastication, prescribed fire, and their combination. Finally, the potential contribution of above-ground vegetation and the soil seed bank to the persistence of plant communities in the edge and interior of plantations in the middle of their cycle were analyzed. Ponderosa pine plantations affected the steppe and the ecotone plant communities, due to variable modifications that directly influence plant development, such as solar radiation. Both total species richness and abundance decreased linearly with forest structure variables such as basal area and canopy cover, whereas the most frequent species as well as growth form and origin groups showed different responses to the radiation that penetrates the forest canopy. Berberis microphylla and Adesmia volckmannii mantained their productivity at 60% irradiance; Adesmia volckmannii, also, was seen to increase productivity at this level. Poa ligularis productivity tended to decrease at 60% irradiance. The general response of plant communities to fuel reduction treatments involved a decrease of richness and abundance, followed by an immediate increase of richness and a delayed recovery of abundance. Although site-specific conditions influenced plant community response, mastication was, in general, the fuel reduction treatment with the lowest negative impact on initial vegetation recovery. The soil seed bank was mainly composed of exotic annual herbs, whereas native perennial herbs and shrubs predominated in the above-ground vegetation. These groups in the above-ground vegetation decreased from the steppe to the plantation interior. All of these results contribute to the development of forest management guidelines that allow for the maintenance of plant community attributes. The persistence of plants in the herb-shrub stratum is necessary, since they constitute the fundamental structure of natural communities and act as propagule sources. To favor plants persistence, solar radiation availability in the herb-shrub stratum is fundamental, and it can be regulated through forest management. Generating a wide range of forest structures in plantations would favor light heterogeneity, which would promote more microsites for plant species development and persistence. In the cases where vegetation has been drastically diminished by plantations, other mechanisms for their recovery might be considered, such as seedling and planting native species. These results also reflected the importance of site particularities, including the history of use, for plant community responses to plantations and forest management. Thus, considering site particularities it is important to select the most appropriate forest management at each situation.
Background
Prescribed fire in Eastern deciduous forests has been understudied relative to other regions in the United States. In Pennsylvania, USA, prescribed fire use has increased more than five-fold since 2009, yet forest response has not been extensively studied. Due to variations in forest composition and the feedback between vegetation and fire, Pennsylvania deciduous forests may burn and respond differently than forests across the eastern US. We measured changes in forest structure and composition up to eight years after prescribed fire in a hardwood forest of the Ridge and Valley region of the Appalachian Mountains in central Pennsylvania.
Results
Within five years post fire, tree seedling density increased more than 72% while sapling density decreased by 90%, midstory density decreased by 46%, and overstory response varied. Following one burn in the mixed-oak unit, overstory tree density decreased by 12%. In the aspen–oak unit, where pre-fire harvesting and two burns occurred, overstory tree density increased by 25%. Not all tree species responded similarly and post-fire shifts in species relative abundance occurred in sapling and seedling size classes. Abundance of red maple and cherry species decreased, whereas abundance of sassafras, quaking aspen, black oak, and hickory species increased.
Conclusions
Forest composition plays a key role in the vegetation–fire relationship and localized studies are necessary to measure forest response to prescribed fire. Compositional shifts in tree species were most pronounced in the aspen–oak unit where pre-fire overstory thinning and two prescribed fires were applied and significant structural changes occurred in all stands after just one burn. Increases in fire-tolerant tree species combined with reductions in fire-intolerant species highlight the role of prescribed fire in meeting management objectives such as altering forest structure and composition to improve game habitat in mid-Atlantic hardwood forests.
The production of renewable fuels is a critical component of global strategies to reduce greenhouse gas (GHG) emissions. Moreover, the collection of raw materials for its production can provide added benefits such as reduction of wildfire risk, additional income for farmers, and decreased disposal costs. Although there is substantial literature on design and modeling of supply chains, the authors were unable to find a single reference with the information needed for the selection and cost estimation of each type of equipment involved in the supply chain. Therefore, the goal of this research is to gather information necessary for the construction and utilization of models that might drive the identification of a feasible supply chain to produce renewable fuels at a commercial scale. The primary objectives are to 1) understand the supply chain of critical feedstocks for renewable fuels production; 2) identify the equipment commercially available for collection and adequation of feedstock; and 3) consolidate information regarding equipment cost, energy consumption, and efficiency, as well as feedstock storage and transportation systems. This paper provides a compilation for five feedstock types studied for sustainable aviation fuel production: 1) agricultural residues and grasses, 2) forest residues, 3) urban wood waste, 4) oilseeds, 5) fats, oils & greases. All the technologies involved from the field to the gate of the preprocessing or conversion unit were reviewed. The information on fats, oils & greases supply chains and equipment purposely designed for forest thinning and pruning was very limited.
We used a prescribed fire study to demonstrate the concept of pyrosilviculture, defined here as a) using prescribed fire to meet management objectives or b) altering non-fire silvicultural treatments explicitly so that they can optimize the incorporation of prescribed fire in the future. The study included implementation of relatively hot prescribed burns in mixed-conifer forests that have been managed with gap-based silviculture. The fires burned through 12-, 22-, 32- and, 100-year old cohorts, thus enabling an analysis of stand age influences on fire effects. Mastication and pre-commercial thinning were assessed as pre-fire treatments in the 12-year-old stands. Post-burn mortality and crown scorch declined with stand age. There was a clear tradeoff between fuel consumption and high rates of tree damage and mortality in the 12-year-old stands. Masticated stands had higher levels of average crown scorch (78%) compared with pre-commercially thinned stands (52%). Mortality for all 12-year-old stands was high, as nearly half of the trees were dead one year after the fires. Giant sequoia and ponderosa pine had relatively high resistance to prescribed fire-related mortality. When applying the concept of pyrosilviculture, there could be opportunities to combine prescribed fire with regeneration harvests that create a variety of gap sizes in order to sustain both low fire hazard and to promote structural heterogeneity and sustainable age structures that may not be achieved with prescribed fires alone.
Abstract Increasing trends in wildfire severity can partly be attributed to fire exclusion in the past century which led to higher fuel accumulation. Mechanical thinning and prescribed burns are effective techniques to manage fuel loads and to establish a higher degree of control over future fire risk, while restoring fire prone landscapes to their natural states of succession. However, given the complexity of interactions between fine scale fuel heterogeneity and wind, it is difficult to assess the success of thinning operations and prescribed burns. The present work addresses this issue systematically by simulating a simple fire line and propagating through a vegetative environment where the midstory has been cleared in different degrees, leading to a canopy with almost no midstory, another with a sparse midstory and another with a dense midstory. The simulations are conducted for these three canopies under two different conditions, where the fuel moisture is high and where it is low. These six sets of simulations show widely different fire behavior, in terms of fire intensity, spread rate and consumption. To understand the physical mechanisms that lead to these differences, detailed analyses are conducted to look at wind patterns, mean flow and turbulent fluxes of momentum and energy. The analyses also lead to improved understanding of processes leading to high intensity crowning behavior in presence of a dense midstory. Moreover, this work highlights the importance of considering fine scale fuel heterogeneity, seasonality, wind effects and the associated fire-canopy-atmosphere interactions while considering prescribed burns and forest management operations.
One goal of fuels treatments is to limit potential fire behavior by reducing overstory tree density, but this may precipitate regeneration, which contributes to increasing potential fire behavior over time. To understand factors that influence tree regeneration in treated stands, we compared abundance of advance and post treatment regeneration in 5–14-year-old thinning and mastication treatments covering approximately 2600 ha within two National Forests in Colorado, U.S.A. The study sites were dominated by two species with complimentary regeneration niches. Ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) is less shade tolerant, but more fire and drought resistant than Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). We considered three critical features of the post treatment environment: species composition, forest structure (especially density), and site characteristics. Regeneration densities at the plot level varied considerably: 37% of the plots had no regeneration, while 15% had more than double the average density. High-density areas tended to occur in moist sites, mostly on north aspects that were dominated by Douglas-fir in the overstory. These areas are where wildfire mitigation benefits will likely deteriorate most rapidly. The vast majority (69%) of all regeneration was Douglas-fir. 31% of all regeneration occurred post treatment. In this group, ponderosa pine abundance was positively related to time since treatment but Douglas-fir was not, suggesting a stronger positive effect of treatment for ponderosa pine, although Douglas-fir was still more abundant. This is likely because treatments reduced the seed source for Douglas-fir more than for ponderosa pine while reducing total overstory density to create conditions that met the regeneration requirements of this shade intolerant species. Advanced regeneration was common throughout the study area, consisting of nearly 80% Douglas-fir and only 13% ponderosa pine. Although the abundance of advance regeneration decreased over time since treatment, likely due to mortality given that we encountered few sapling-sized trees, surviving trees reduce treatment longevity and have the potential for subsequent growth release and contribution to fuel hazard development. Conifer regeneration did not vary between mastication and thinning treatments. The impact of regeneration on treatment longevity was highly variable at smaller-than-stand scales. On the Colorado Front Range, moist sites with low overstory density and mature Douglas-fir to provide a seed source are where treatment effectiveness is likely to degrade most rapidly. These areas with abundant regeneration may be best left untreated, or managers should anticipate the need to re-treat them more frequently.
Recent increases in Pinus ponderosa Dougl. ex Laws. forest density in the southwestern United States have severely reduced understory herbaceous biomass and altered understory species composition. To examine whether changes in graminoid species composition are caused by increased shading, we studied the effects of shade on leaf gas exchange, biomass, and reproductive characteristics of five grass species native to Arizona P. ponderosa forests in a greenhouse study. Blepharoneuron tricholepis (Nash) Torr., Koeleria cristata (L.) Pers., Festuca arizonica Vasey, Muhlenbergia montana (Nutt.) Hitchc., and Sitanion hystrix (Nutt.) J.G. Smith were grown under three light levels representative of photosynthetic photon flux densities and red/far-red ratios that occur beneath P. ponderosa canopies. In general, all species grew better under unshaded conditions, but all survived and flowered even under the dense shade treatment. Reduction of net assimilation rate by shading was the strongest during early reproductive shoot growth for all species except K. cristata, whose assimilation rate was unaffected by shading. Biomass allocation and reproductive responses to shading varied among species. Biomass of S. hystrix was the least affected by shading of all species, and it showed no response in biomass allocation to reproduction but increased height and weight of individual flower stalks under shade. Overall, S. hystrix and K. cristata, species that occur in dense P. ponderosa stands, were least affected by experimental shading, which suggests that shade is a contributing factor to the distribution of grass species in Arizona P. ponderosa forests.
Question: Do rectangular sample plots record more plant species than square plots as suggested by both empirical and theoretical studies? Location: Grasslands, shrublands and forests in the Mediterranean-climate region of California, USA. Methods: We compared three 0.1-ha sampling designs that differed in the shape and dispersion of 1-m 2 and 100-m 2 nested subplots. We duplicated an earlier study that compared the Whittaker sample design, which had square clustered subplots, with the modified Whittaker design, which had dispersed rectangular subplots. To sort out effects of dispersion from shape we used a third design that overlaid square subplots on the modified Whittaker design. Also, using data from published studies we extracted species richness values for 400-m 2 subplots that were either square or 1:4 rectangles partially overlaid on each other from desert scrub in high and low rainfall years, chaparral, sage scrub, oak savanna and coniferous forests with and without fire. Results: We found that earlier empirical reports of more than 30% greater richness with rectangles were due to the confusion of shape effects with spatial effects, coupled with the use of cumulative number of species as the metric for comparison. Average species richness was not significantly different between square and 1:4 rectangular sample plots at either 1-or 100-m 2. Pairwise comparisons showed no significant difference between square and rectangular samples in all but one vegetation type, and that one exhibited significantly greater richness with squares. Our three intensive study sites appear to exhibit some level of self-similarity at the scale of 400 m 2, but, contrary to theoretical expectations, we could not detect plot shape effects on species richness at this scale. Conclusions: At the 0.1-ha scale or lower there is no evidence that plot shape has predictable effects on number of species recorded from sample plots. We hypothesize that for the mediterranean-climate vegetation types studied here, the primary reason that 1:4 rectangles do not sample greater species richness than squares is because species turnover varies along complex environmental gradients that are both parallel and perpendicular to the long axis of rectangular plots. Reports in the literature of much greater species richness recorded for highly elongated rectangular strips than for squares of the same area are not likely to be fair comparisons because of the dramatically different periphery/area ratio, which includes a much greater proportion of species that are using both above and below-ground niche space outside the sample area.
Frequent, low intensity fire was an important component of the natural disturbance regime of presettlement savannas and woodlands in the southeastern USA dominated by longleaf pine (Pinus palustris), and prescribed burning is now a critical part of the management of these endangered habitats. Fire season, fire frequency, and fire intensity are three potentially important, though still little understood, components of both natural and managed fire regimes. In this long-term (8-yr) study, we experimentally (through the use of prescribed burning) tested for effects of fire season (eight different times throughout the year) and fire frequency (annual vs. biennial burning), on population dynamics (recruitment, growth, mortality, change in density, and change in basal area [the total basal area of all stems in a plot]) and species composition of trees in two quite different types of longleaf-pine-dominated habitats (north Florida sandhills and flatwoods). Limited fire temperature and intensity data were also collected during one year to examine the relationship between fire behavior (temperature and intensity) and tree mortality. Contrary to prior hypotheses, our results showed few systematic or predictable effects of season or frequency of burning on dynamics of longleaf pine. Instead, variability in the population dynamics of this species appeared to be related largely to variation in fire behavior, regardless of the season of burning. Consistent with prior hypotheses, we found that deciduous oak species (Quercus laevis, Q. margaretta, and Q. incana) were least vulnerable to dormant-season burning and most vulnerable to burning early in the growing season. This was shown particularly by seasonal trends in the effect of burning on oak mortality (both topkill and complete kill) and, to a lesser extent, on oak recruitment. Oak densities and basal areas also declined in the spring-burned plots, resulting in a shift away from oaks and towards increased dominance by longleaf pine. Detrimental effects of spring burning on oaks were partly explained by fire behavior, but there appeared also to be an important residual effect of burning season, particularly on complete kill. Though longleaf pine population dynamics did not differ markedly as a result of burning season and frequency, we did find important differences in pine dynamics between the two habitats (i.e., sandhills and flatwoods). In general, populations of longleaf pines in the sandhills appeared to be density regulated, while flatwoods pine populations were declining regardless of the level of intraspecific competition. This suggests that long-term persistence of longleaf pine, and perhaps other fire-adapted species in frequently burned longleaf-pine-dominated communities, may be determined by complex interactions between habitat factors and fire regimes.
A conceptual model of fire and forest restoration and maintenance is presented. The process must begin with clearly articulated goals and depends upon derivation of science-driven models that describe the natural or desired conditions. Evaluating the extent to which contemporary landscapes depart from the model is a prerequisite to determining the need for restoration. Model landscapes that include the historical range of variability are commonly used as target conditions in setting restoration objec- tives. Restoration is a corrective step that ultimately must be replaced by a maintenance process. In a world of changing climate, structural targets of historical conditions will become progressively less meaningful to ecosystem maintenance. Future fire manage- ment needs to focus more on fire as a process, in particular as it pertains to proper ecosystem functioning. One area in need of much further research is the critical role of gap formation in forest regeneration. Forests of the Sierra Nevada in California, like other western coniferous forests, have had ecosystem processes greatly disturbed by fire management practices of the 20 th century. This impact has been repeatedly documented through historical studies of fire frequencies revealed in the annual growth rings of fire-scarred trees. These dendrochro- nology studies show a high frequency of fire prior to Euroamerican settlement, with fires in many mid-elevation forest stands occurring at intervals of roughly every 5-25 years (fig. 1). The fact that these estimates are based upon trees that have persisted through repeated fires demon- strates that the pre-Euroamerican fire regime was one of low intensity/severity fires over a significant portion of the landscape. Beginning in the latter half of the 19 th century, fire frequency declined and throughout the 20 th century,
During the late fall of 2002 we administered three burns in mixed conifer forest sites in the north-central Si- erra Nevada. Eight months later we measured fire-induced injury and mortality in 1300 trees. Using logistic regression, an array of crown scorch, stem damage, fuels, and fire-behavior variables were examined for their influence on tree mortality. In Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), white fir (Abies concolor (Gord. & Glend.) Lindl.), and incense cedar (Calocedrus decurrens (Torr.) Florin), smaller trees with greater total crown damage had higher mor- tality rates. Smaller stem diameters and denser canopies predicted mortality best in ponderosa pine (Pinus ponderosa Dougl. ex P. Laws. & C. Laws). Duff consumption and bark char severity increased model discrimination for white fir and incense cedar and California black oak (Quercus kelloggii Newberry), respectively. In tanoak (Lithocarpus densiflorus (Hook. & Arn.) Rehd.), greater total crown damage in shorter trees resulted in higher mortality rates. Along with tree diameter and consumption of large (>7.6 cm diameter at breast height, DBH) rotten downed woody debris, fire inten- sity was a significant predictor of overall tree mortality for all species. Mortality patterns for white fir in relation to crown damage were similar among sites, while those for incense cedar were not, which suggests that species in repli- cated sites responded differently to similar burns. Our results demonstrate actual fire-behavior data incorporated into mortality models, and can be used to design prescribed burns for targeted reduction of tree density in mixed conifer forests.
After nearly a century of fire exclusion in many central and southern Sierra Nevada mixed- conifer forests, dead and down surface fuels have reached high levels without the recurring fires that consume the accumulated organic matter. The effects of prescribed fires used to reduce fuel loads and restore fire have been monitored in Yosemite, Sequoia, and Kings Canyon National Parks for over 30 years. Ten years following prescribed fire treatments in Sequoia and Kings Canyon, mean total fuel loads accumulated to 84 percent of pre -fire levels in ponderosa pine forests, 83 percent in white fir-mixed conifer, and 66 percent in giant sequoia-mixed conifer forests. Thirty-one years after burning, mean fuel load of fine and sound woody fuels increased in ponderosa pine research plots in Yosemite. Most fuel accumulation appeared to occur within the first decade after fire, however the post-fire fuel complex was different than that pre-fire. In areas that have remained unburned, surface fuel accumulation appears to be relatively slow and may indicate that decay rates have approached accumulation rates for the mixed-conifer forest types. This long-term information has important implications for fire management planning, including scheduling fuel hazard reduction and subsequent maintenance treatments.
Decades of fire exclusion have led to hazardous fuel accumulations and the deterioration of fire-dependent ecosystems, particularly in the American West. Managers are striving to return the ecological role of fire to many ecosystems and would benefit from a much better understanding of how forest structure and composition might change if fires were reintroduced. We used a forest gap model, developed for forests in the Sierra Nevada, California, USA, that integrates climate, fire, and forest dynamics to investigate forest response to changes in the fire regime. The model simulates a spatially heterogeneous fuel bed that is responsive to changes in forest condition, making it well suited for examining alternative management approaches for restoring Sierra Nevada forests after a century of fire exclusion. Presuppression forest basal area, species composition, and spatial autocor- relation structure were restored quickly, if simulated disturbances that caused substantial tree mortality were reintroduced. Simulations of harvest induced the highest levels of mortality and, thus, most effectively restored forest structure and composition. However, prescribed fires were just as effective in restoring forest structure and composition if they were sufficiently severe.
Emulation silviculture is the use of silvicultural techniques that try to imitate natural disturbances such as wildfire. Emulation silviculture is becoming increasingly popular in Canada because it may help circumvent the political and environmental difficulties associated with intensive forest harvesting practices. In this review we summarize empirical evidence that illustrates disparities between forest harvesting and wildfire. As a rule, harvesting and wildfire affect biodiversity in different ways, which vary a great deal among ecosystem types, harvesting practices, and scale of disturbance. The scales of disturbance are different in that patch sizes created by logging are a small subset of the range of those of wildfire. In particular, typical forestry does not result in the large numbers of small disturbances and the small number of extremely large disturbances created by wildfires. Moreover, the frequency of timber harvesting is generally different from typical fire return intervals. The latter varies widely, with stand-replacing fires occurring in the range of 20 to 500 years in Canada. In contrast, harvest frequencies are dictated primarily by the rotational age at merchantable size, which typically ranges from 40 to 100 years. Forest harvesting does not maintain the natural stand-age distributions associated with wildfire in many regions, especially in the oldest age classes. The occurrence of fire on the landscape is largely a function of stand age and flammability, slope, aspect, valley orientation, and the location of a timely ignition event. These factors result in a complex mosaic of stand types and ages on the landscape. Timber harvesting does not generally emulate these ecological influences. The shape of cut blocks does not follow the general ellipse pattern of wind driven fires, nor do harvested stands have the ragged edges and unburned patches typically found in stand-replacing fires. Wildfire also leaves large numbers of snags and abundant coarse woody debris, while some types of harvesting typically leave few standing trees and not much large debris. Successional pathways following logging and fire often differ. Harvesting tends to favor angiosperm trees and results in less dominance by conifers. Also, understory species richness and cover do not always recover to the pre-harvest condition during the rotation periods used in typical logging, especially in eastern Canada and in old-growth forests. As well, animal species that depend on conifers or old-growth forests are affected negatively by forest harvesting in ways that may not occur after wildfire. The road networks developed for timber extraction cause erosion, reduce the areas available for reforestation, fragment the landscape for some species and ecological functions, and allow easier access by humans, whereas there is no such equivalency in a fire-disturbed forest. Key words: silviculture, forest management, clearcutting, forest conservation, wildfire, biodiversity.
The objective of this research was to test the hypothesis that water and nutrients, and not light, control understory plant species composition in a Pinus ponderosa forest in northeastern Oregon. The experiment was conducted as a split-plot experimental design with a 2 x 2 factorial analysis. To assess the effects of root competition of overstory trees on understory species composition, 20 plots (4 x 4 m) were trenched approximately 1.0 m in depth, and compared to 24 nontrenched plots. To increase light levels to understory vegetation, trees were thinned from 345 to 148 trees ha-1 in half of each block (2.5 ha) during the winter and early spring of 1986. Canonical discriminant analysis indicated that light accounted for the greatest environmental resource response among the treatments. The number of species (8) that increased in cover and density was 60% greater when tree root competition was reduced in the root-reduction treatment, versus 5 when tree canopy influences were reduced in the canopy-reduction treatment. Simple correlation showed that changes in species composition were significantly (P ≤ 0.05) related to both changes in aboveground attributes (light, midday air temperature, and soil temperature) and belowground attributes (soil water potential, pH, and nitrogen). Competition for limited resources, light, water, and nutrients, does affect understory species composition as evidenced by the response of individual species to increasing availability of these resources. For. Sci. 41(4):864-889.
Pre- and post-burn tree mortality rates, size structure, basal area, and ingrowth were determined for four 1.0 ha mixed conifer forest stands in the Log Creek and Tharp's Creek watersheds of Sequoia National Park. Mean annual mortality between 1986 and 1990 was 0.8% for both watersheds. In the fall of 1990, the Tharp's Creek watershed was treated with a prescribed burn. Between 1991 and 1995, mean annual mortality was 1.4% in the unburned Log Creek watershed and 17.2% in the burned Tharp's Creek watershed. A drought from 1987 to 1992 likely contributed to the mortality increase in the Log Creek watershed. The high mortality in the Tharp's Creek watershed was primarily related to crown scorch from the 1990 fire and was modeled with logistic regression for white fir (Abies concolor [Gord. and Glend.]) and sugar pine (Pinus lambertiana [Dougl.]). From 1989 to 1994, basal area declined an average of 5% per year in the burned Tharp's Creek watershed, compared to average annual increases of less than 1% per year in the unburned Log Creek watershed and in the Tharp's watershed prior to burning. Post-burn size structure was dramatically changed in the Tharp's Creek stands: 75% of trees ≤50 cm and 25% of trees >50 cm were killed by the fire. For. Sci. 44(2):341-355.
The fire suppression policy on public lands during the last century in the United States has resulted in increased fuel loadings, necessitating the use of prescribed fire and mechanical treatments to decrease hazardous fuels and risks of catastrophic wildfire. While these practices are widespread, there is great variability in project costs, making planning difficult. Although previous studies have examined the factors that influence management costs, they have grappled with the lack of consistent and reliable data. We used the FASTRACS (Fuel Analysis, Smoke Tracking, and Report Access Computer System) database from the Pacific Northwest Region of the USDA Forest Service to identify important influences on fuels management costs. Projects conducted in the wildland-urban interface consistently exhibited higher treatment costs for both prescribed fire and mechanical fuels treatments.
Nous avons examiné la richesse et la couverture en herbacées en relation avec la saison et la sévérité des feux, cinq saisons de croissance après des brûlages dirigés. Les données ont été récoltées dans six peuplements forestiers auxquels ont été appliqués au hasard trois traitements : pas de feu, un feu printanier et un feu automnal. Les feux d'automne comportaient significativement plus d'espèces exotiques et indigènes annuelles/bisannuelles (an/bi) et une plus grande couverture de cellesci (6,5 % d'exotiques; 1,7 % d'indigènes) que les feux d'automne ou que les aires n'ayant pas été brûlées. Ces patrons sont probablement dus aux effets indirects associés à la sévérité du feu et à la disponibilité de la ressource plus qu'à l'effet direct du feu associé à la période de brûlage. Les modèles CART ont indiqués que la richesse et la couverture élevées des espèces exotiques et indigènes an/bi étaient associées à des trouées dans l'étage dominant et à des aires brûlées sévèrement, des conditions courantes à la suite d'un feu d'automne. Les espèces exotiques ont peut être plus de succès à exploiter ces environnements. Aucun effet du traitement n'a été trouvé pour les espèces indigènes pérennes. L'emplacement était important pour expliquer les patrons des espèces indigènes pérennes, mais la richesse et la couverture étaient également associées positivement avec des feux de faible sévérité, une plus grande couverture arborescente et des débris ligneux grossiers. Ainsi, les attentes d'une plus grande diversité et abondance de plantes indigènes pérennes à la suite de brûlages dirigés ne sont peut être pas remplies et la propagation d'espèces exotiques peut compromettre les autres attributs de l'écosystème. La restauration dans ces forêts représente donc un défi puisque les brûlages dirigés interagissent avec les conditions environnementales actuelles très différentes des conditions historiques.
As part of a national experiment, the Fire and Fire Surrogate Project, we evaluated the effects of forest thinning on small mammal population densities and total small mammal biomass in ponderosa pine (Pinus ponderosa)–dominated forests at 2 study areas in northern Arizona and northern New Mexico, USA. We also evaluated the effects of wildfire on small mammal population densities and biomass after a wildfire burned a portion of one study area. Our statistical methods consisted of estimation of population densities in combined analyses across space and time, followed by a weighted regression analysis of treatment effects on densities. We hypothesized that habitat change postdisturbance would be the critical determinant of population responses to thinning and wildfire within 1 year of disturbances. Our results largely supported this hypothesis, as we documented predicted positive responses to thinning for deer mice (Peromyscus maniculatus), gray-collared chipmunks (Tamias cinereicollis), and least chipmunks (T. minimus). We also observed predicted positive responses to wildfire for deer mice, although our results did not support predicted negative responses to wildfire for least chipmunks. Total small mammal biomass generally increased following both thinning and wildfire. Our results suggest that fuel reduction treatments will have the largest positive impact on small mammal populations in areas where tree densities are especially high.
Although the majority of fires in the western United States historically occurred during the late summer or early fall when fuels were dry and plants were dormant or nearly so, early-season prescribed burns are often ignited when fuels are still moist and plants are actively growing. The purpose of this study was to determine if burn season influences postfire vegetation recovery. Replicated early-season burn, late-season burn, and unburned control units were established in a mixed conifer forest, and understory vegetation was evaluated before and after treatment. Vegetation generally recovered rapidly after prescribed burning. However, late-season burns resulted in a temporary but significant drop in cover and a decline in species richness at the 1 m 2 scale in the following year. For two of the several taxa that were negatively affected by burning, the reduction in frequency was greater after late-season than early-season burns. Early-season burns may have moderated the effect of fire by consuming less fuel and lessening the amount of soil heating. Our results suggest that, when burned under high fuel loading conditions, many plant species respond more strongly to differences in fire intensity and severity than to timing of the burn relative to stage of plant growth.
Patterns of native and alien plant diversity in response to disturbance were examined along an elevational gradient in blue oak savanna, chaparral, and coniferous forests. Total species richness, alien species richness, and alien cover declined with ele-vation, at scales from 1 to 1000 m 2 . We found no support for the hypothesis that community diversity inhibits alien invasion. At the 1-m 2 point scale, where we would expect competitive interactions between the largely herbaceous flora to be most intense, alien species richness as well as alien cover increased with increasing native species richness in all communities. This suggests that aliens are limited not by the number of native competitors, but by resources that affect establishment of both natives and aliens. Blue oak savannas were heavily dominated by alien species and consistently had more alien than native species at the 1-m 2 scale. All of these aliens are annuals, and it is widely thought that they have displaced native bunchgrasses. If true, this means that aliens have greatly increased species richness. Alternatively, there is a rich regional flora of native annual forbs that could have dominated these grasslands prior to displacement by alien grasses. On our sites, livestock grazing increased the number of alien species and alien cover only slightly over that of sites free of livestock grazing for more than a century, indicating some level of permanency to this invasion. In chaparral, both diversity and aliens increased markedly several years after fire. In-vasive species are rare in undisturbed shrublands, and alien propagules fail to survive the natural crown fires in these ecosystems. Thus, aliens necessarily must colonize after fire and, as a consequence, time since fire is an important determinant of invasive presence. Blue oak savannas are an important propagule source for alien species because they maintain permanent populations of all alien species encountered in postfire chaparral, and because the vegetation mosaic in this region places them in proximity to chaparral. The speed at which alien propagules reach a burned site and the speed at which the shrublands return to their former closed-canopy condition determine alien invasion. Frequent burning of this vegetation alters the balance in favor of alien invasion. In the higher-elevation coniferous forests, species diversity was a function of fire severity and time since fire. High-intensity fires create gaps that decrease canopy coverage and increase light levels and nutrients for an ephemeral successional flora. Few species have persistent seed banks, so the time since fire is an important determinant of colonization success. There was a highly significant interaction between fire severity and time since fire for understory cover, species richness, and alien richness and cover. Understory was sparse in the first year after fire, particularly in low-severity burns, and increased substantially several years after fire, particularly on high-severity burns. Both fire severity and time since fire affected alien species richness and dominance. Coniferous forests had about one-third as many alien species as the foothill oak savannas, and fewer than half of the species were shared between these communities. Unburned coniferous forests were largely free of alien species, whereas some burned sites had a significant alien presence, which presents a challenge for fire restoration of these forests.
Introduction This chapter considers the internal and external processes that affect seedling communities. Internal or endogenous drivers include the density dependence of seedling populations, as well as the relationship of parent to offspring and the competitive relationships affecting seedling populations. There are many external or exogenous drivers, but we will focus on climate, predation, and fire. We will integrate these internal and external drivers of community composition to address the questions: To what extent do seedling recruitment strategies relate to community assembly rules and do these rules dictate the potential combinations of regeneration niches to be found in any given community? This chapter will focus on long-lived woody species because the differences in life history stages and factors affecting them appear much more prominent than in some other growth forms. Internal drivers Communities of regeneration niches Plant recruitment strategies have received a great deal of attention generally to determine the environmental conditions that favor one strategy over another (Chapter 11). Although communities comprise an assemblage of different seedling strategies, relatively little attention has been paid to the community combinations or rules that limit possible combinations. A useful concept for understanding seedling communities is that of safe sites (Harper, 1977), which is a species-specific phenomenon driven by unique aspects of ecology and phylogeny. It is important to recognize the diversity of potential safe sites and how they are distributed in space and time. Also of importance is how species reach safe sites and the role of metapopulations.
Seedlings are highly sensitive to their environment. After seeds, they typically suffer the highest mortality of any life history stage. This book provides a comprehensive exploration of the seedling stage of the plant life cycle. It considers the importance of seedlings in plant communities; environmental factors with special impact on seedlings; the morphological and physiological diversity of seedlings including mycorrhizae; the relationship of the seedling with other life stages; seedling evolution; and seedlings in human altered ecosystems, including deserts, tropical rainforests, and habitat restoration projects. The diversity of seedlings is portrayed by including specialised groups like orchids, bromeliads, and parasitic and carnivorous plants. Discussions of physiology, morphology, evolution and ecology are brought together to focus on how and why seedlings are successful. This important text sets the stage for future research and is valuable to graduate students and researchers in plant ecology, botany, agriculture and conservation.
Forest types within Glacier display markedly disparate structures; those within Yosemite possess similar structures. Two of the 3 Glacier forest types are characterized by pronounced compositional differences small and large size classes. In Yosemite, each forest type has a similar composition in both the small and large size classes. These geographic differences in forest structure and compositional trend are attributed to distinctive fire patterns in each region. In Yosemite, the strength and consistency of summer drought stress favor a surface fire regime in all forest types sampled, promoting uniformity of structure and compositional dynamics among Yosemite forests. In Glacier, site microclimates mitigate or exacerbate occasional dry summers, resulting in a broader spectrum of disturbance regimes among forest types, ranging from frequent surface fires on dry, open flats, through crown fires on intermediate settings, to infrequent fires on mesic, sheltered slopes. The diversity of disturbance regimes promotes heterogeneity of structure and complex patterns of compositional dynamics among Glacier forests. -from Author
A flatwoods longleaf pine wiregrass ecosystem, which regenerated naturally following wildfire in 1942, on the Coastal Plain of southern Georgia was treated over a period of four decades with prescribed fire at annual, biennial and triennial intervals during the winter dormant season. Burning caused substantial changes in the understory plant community, with significant reductions in the foliar cover of Ilex glabra in the shrub layer resulting in corresponding increases in the cover of Vaccinium myrsinites, Sporobolus curtissii, Aristida stricta and Andropogon spp. Understory plant species richness, diversity and evenness also increased as a result of periodic fire. Dormant-season burning decreased the cover of litter on the forest floor and significantly increased the standing biomass of A. stricta, S. curtissii, Andropogon spp., all other grasses and all forbs. Recurrent fire also prevented the development of a vigorous midstory, that impedes understory growth and poses a serious fire hazard to the stand. Overstory trees were largely unaffected by burning. Historical light grazing on the site produced no measurable effects on the plant community. Findings suggest that the biennial burning interval results in declines of I. glabra in the shrub layer and litter cover on the forest floor, leading to the largest increases in understory plant species richness and diversity and the biomass productivity of grasses and forbs. Although flatwoods plant communities evolved in environments characterized by growing-season fires of variable frequency, long-term application of dormant-season fire is also recommended as a useful option for sustaining resource values in this and similar longleaf pine wiregrass ecosystems.