Article

The National Fire and Fire Surrogate Study: effects of fuel reduction methods on forest vegetation structure and fuels

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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.

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... 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). ...
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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.
... For example, increasing native plant diversity is one of the most cited goals for ecological restoration [16], but presently, there is a dearth of research on how treatments affect understory plant communities over the long term [17][18][19][20][21][22]. In a meta-analysis, Schwilk et al. [23] found no consistent pattern in understory community diversity and abundance in response to forest restoration treatments. However, Abella and Springer [20] found that understory abundance consistently increased in studies that monitored changes for five years or longer. ...
... Numerous studies have illustrated significant shifts in forest structure resulting from forest restoration treatments in dry forest types [9,23,[43][44][45][46][47]. Researchers have found that the most significant effects on forest structure (i.e., stand density, tree canopy cover, and CBH) come from treatments that include some type of harvest combined with prescribed fire [23,44,[48][49][50][51]. Restoration and fuel reduction treatments are designed to change numerous forest stands and ecosystem properties, such as forest species composition, diameter distributions, and tree vigor [49]. ...
... Numerous studies have illustrated significant shifts in forest structure resulting from forest restoration treatments in dry forest types [9,23,[43][44][45][46][47]. Researchers have found that the most significant effects on forest structure (i.e., stand density, tree canopy cover, and CBH) come from treatments that include some type of harvest combined with prescribed fire [23,44,[48][49][50][51]. Restoration and fuel reduction treatments are designed to change numerous forest stands and ecosystem properties, such as forest species composition, diameter distributions, and tree vigor [49]. ...
Article
Exclusion of natural surface fires in warm/dry mixed-conifer forests of the western U.S. has increased potential for stand-replacing crown fires and reduced resilience of these systems to other disturbances, such as drought and insect attack. Tree thinning and the application of prescribed fire are commonly used to restore more resilient ecological conditions, but currently, there is a lack of long-term data with which to evaluate restoration treatment effectiveness in forest types where resprouting shrubs dominate understory communities. At a mixed-conifer site in southwestern Colorado, we compared forest structure and understory vegetation responses to three restoration treatments (thin/burn, burn, and control) over 10 years in a completely randomized and replicated experiment. Forest density, canopy cover, and crown fuel loads were consistently lower, and crown base height was higher, in thin/burn than burn or controls, but the effects diminished over time. Ten years following treatment, >99% of all plant species within both treatments and the control were native in origin. There were no differences between treatments in understory richness, diversity, cover, or surface fuels, but graminoid cover more than doubled in all treatments over the 15-year monitoring period. Similarly, there was more than a 250% increase post-treatment in shrub density, with the greatest increases in the thin/burn treatment. In addition, we saw an increase in the average shrub height for both treatments and the control, with shrub stems >80 cm becoming the dominant size class in the thin/burn treatment. Conifer seedling density was significantly lower in thin/burn compared with burn and control treatments after 10 years. Taken together, these conditions create challenges for managers aiming to reestablish natural fire patterns and sustain mixed-conifer forests. To limit the dominance of resprouting shrubs and facilitate conifer regeneration after overstory thinning and prescribed fire, managers may need to consider new or more intensive approaches to forest restoration, particularly given current and projected climate change.
... We believe the lack of burning effect is primarily due to treatment severity and resultant competitive conditions. The prescribed burns in the FFS study had little impact on mature (dbh > 10 cm) stem density (Table 1; [35,77]) and only resulted in minimal perceptible change to residual overstory trees. Although burning improves nutrient availability [78,79], water limitation or competition may be inhibiting full utilization of higher nutrient loads. ...
... Although burning improves nutrient availability [78,79], water limitation or competition may be inhibiting full utilization of higher nutrient loads. Low-severity burns are actually common when reintroducing fire (as in [77,80]) for fear of widespread overstory mortality or runaway crown fire that threatens nearby natural resources, structures, or lives. This study provides evidence that single-entry low-severity burning is largely ineffective at changing individual overstory tree growth trends and easy-to-measure tree attributes that confer resistance. ...
... It should be noted that a number of studies have shown that thinning alone insufficiently reduces crown fire potential, though reduction of crown fire potential is the primary objective of fuel reduction treatments (e.g., [24,77,81,82]). This is because thinning alone does not treat dead surface fuels and can even increase loading, may not treat mid-story ladder fuels, does not increase crown base heights as well as burning, and increases in-stand wind speeds, depending on burn prescription. ...
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Research Highlights: This study provides much needed insight into the development of resistance to disturbance and growth dynamics of overstory trees in response to restoration-based fuel reduction, and will be useful to scientists and managers attempting to better grasp the relative merits of restoration treatment types. Background and Objectives: Restoration-based fuel reduction treatments are common in dry, fire-prone forests of the western United States. The primary objective of such treatments is to immediately reduce a stand’s crown fire hazard. However, the impact of these treatments on residual trees is relevant to assess their longevity and resistance to future disturbances. In this study, we evaluate the effects of restoration on retained overstory ponderosa pine (Pinus ponderosa Lawson & C. Lawson) and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees in western Montana, where treatments were experimentally implemented 13 years prior as part of the national Fire and Fire Surrogate study. Materials and Methods: We examined tree attributes in response to the following replicated treatments: thin-only, burn-only, thin + burn, and a no-action control. We analyzed three different tree attributes that confer resistance to common disturbances: height-to-diameter ratio (resistance to wind), bark thickness (resistance to surface fire), and growth efficiency (resistance to bark beetles). Results: Our models suggest that thinning (with or without burning) alters tree attributes relative to the control in a manner that may increase tree resistance to wind and snow breakage, surface fire, and biotic agents such as bark beetles. Further analysis of annual growth of ponderosa pine and Douglas-fir varied by treatment type: thinning-based restoration (thin-only and thin + burn) increased diameter growth for both species, crown length and width in ponderosa pine, and crown length in Douglas-fir relative to unthinned treatments. Burning (burn-only and thin + burn) did not significantly affect tree growth relative to unburned treatments. Conclusions: While low-severity prescribed burning treatments are often used for restoration and have various ecosystem benefits, this study demonstrates that thinning (alone or in addition to burning) produces more measureable, beneficial results to overstory tree disturbance resistance metrics and growth.
... 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). ...
Article
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.
... 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). ...
Article
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]. ...
Article
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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.
... Potentially hazardous wildland fuels are an international concern (Flannigan et al., 2013;Keane, 2013;Williams, 2013;Schwilk et al., 2009). Recent wildfire events in the Amazon, Australia, and the United States have highlighted some significant dynamics, such as larger fire events generating higher suppression costs occurring during extended fire seasons in areas more heavily influenced by human development (Abatzoglou and Williams, 2016). ...
... Active forest management, which includes implementation of intensive silvicultural practices, may alter the amount and composition of potentially hazardous wildland fuels (Vander Yacht et al., 2019;Waldrop et al., 2016;Schwilk et al., 2009). Dominant vegetative types change over time through ecological succession in the presence or absence of natural and anthropogenic disturbances (Gilliam and Platt, 1999). ...
Article
Globally, prescribed fire, harvesting, and understory mastication, alone and in combination, are common forest management practices. Timber commodities, wildlife habitat, wildfire fuel reduction, soil conservation, and water quality are frequently targeted and assessed as these practices are utilized. In the 1960s, a study of paired, first-order watersheds was established in coastal South Carolina, USA, to evaluate the long-term impacts of forest management (i.e. prescribed fire, thinning, mastication of understory vegetation) on water quantity and quality. Following Hurricane Hugo in 1989, this included salvage logging on one watershed, but not the other. In 2015, these watersheds were comprehensively evaluated to determine differences in forest species composition, fuels, and soil chemistry. Softwood basal area was greater in the managed watershed than in the unmanaged watershed and hardwood basal area was greater in the unmanaged watershed than in the managed watershed. Total fuel mass did not differ between the two watersheds, but 1-hr and 1000-hr rotten fuel mass were greater on the unmanaged watershed. Ten-hr fuel mass was greater on the managed watershed. Calcium, nitrogen, magnesium, phosphorus, potassium, and pH differed between the litter (Oi horizon) and duff (Oe + Oa horizons) of both watersheds, but carbon only differed in the duff. Mineral soil (Ultisols, 0–10 and 10–20 cm depths) calcium and phosphorus differed between the watersheds, but pH and the other chemicals did not. Collectively, these results indicated that: (1) forest management and natural disturbance on these watersheds altered long-term forest structure; (2) different species compositions and the inclusion or exclusion of salvage logging after Hurricane Hugo produced different fuel compositions that may potentially impact potential wildfire hazard and fire behavior; (3) organisms as a primary soil-forming factor were impacted by long-term management, therefore, some soil chemical properties were affected. Collectively, these analyses highlighted the broad, long-term impacts to ecosystem properties and processes that might directly and indirectly result from active forest management and natural disturbance and the scale of site-specific assessment that might be considered when landowner objectives are targeted in forest management plans and practices.
... 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 . ...
Article
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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. ...
Article
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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.
... Regarding growth form, herbs, and grasses usually recover their cover soon, while shrubs usually need more time, after fuel reduction treatments (Schwilk et al., 2009;Abella and Springer, 2015). Although we did not detect significant differences in perennial group abundance among fuel reduction treatments, all of them showed a general trend to delay reaching UNTREATED values in those treatments involving fire, especially for native perennial species. ...
Article
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Reaching an equilibrium between timber production and biodiversity conservation is one of the aims increasingly pursued in forest plantations, since biodiversity favors plantation stability while contributing to the maintenance of native community structure and functions. This equilibrium is relevant for the Patagonian steppe, which has great diversity but low representation in protected areas. Residual slash management following pruning and thinning is necessary since the traditional practice of leaving slash on the floor may limit vegetation development and increase wildfire hazard. We assessed initial vegetation response to fuel reduction treatments in five exotic conifer plantations in the Patagonian forest-steppe ecotone. We implemented mastication, prescribed fire, and mastication plus prescribed fire fuel reduction treatments and compared initial vegetation richness, abundance, and composition among treatments and with that of an untreated control, having residues on the forest floor. After 3 years, we compared vegetation among fuel treatments and with the adjacent steppe at each site. We also compared residue cover among fuel treatments. Vegetation richness reached similar values in most fuel treatments a year after their implementation, whereas abundance gradually increased in all fuel reduction treatments; in general, the greatest abundance increase was in mastication. After 3 years, richness reached steppe values in most treatments (about 9 species/4 m2), whereas abundance did not (about 40% cover in the steppe and 5–25% in fuel treatments), and species composition similarity was low among treatments and steppe in all sites (Bray–Curtis index 0.1–0.6). Plant community composition was positively correlated with annual precipitation and initial residual biomass. In sites with less precipitation and initial residual biomass, native species predominated, and mastication had the lowest negative impact on initial vegetation recovery, whereas in sites with high proportion of exotic, vegetation recovery was led by exotics in all fuel reduction treatments; in a site having scarce vegetation within plantation, we did not detect any substantial change. Woody residues only decreased when their initial cover was >20%, and litter debris cover reloaded by the second year after fuel reduction. Our results suggest that mastication had the lowest negative short-term impact on native vegetation.
... Yet, like the body of work on historical fire regimes in the West, most of the existing research on fuel treatment effects on forest stand structure and fuel loadings in the region is similarly focused on forest types outside the PJ cover-type (i.e. Covington et al., 1997;Mast et al., 1999;Schwilk et al., 2009;Stephens et al., 2009;Safford et al., 2012;Ryan et al., 2013). While many sites have been treated with prescribed fire to reduce the risk of future wildfire, most have yet to experience wildfire following treatment. ...
Article
Piñon-juniper (PJ) woodlands comprise an important and extensive dry-site forest cover-type that has historically experienced high spatiotemporal variation in fire frequency and extent. We re-measured a network of permanent monitoring plots in a fire-suppressed PJ site the Davis Mountains of West Texas after prescribed fire (Rx) treatments and three subsequent wildfires to 1) quantify wildfire effects on PJ woodland stand structure and fuel loadings, and 2) evaluate the effects of Rx fires followed by wildfire. Although fire weather was extreme during the wildfire years, all three wildfires burned as low-severity fire events. Simultaneous autoregressive modeling revealed that total tree density and basal area declined significantly (P < 0.05) over the time-series, while surface fuel loadings increased significantly (P < 0.05), likely in response to fuel inputs from fire-induced tree mortality. However, tree mortality largely occurred in smaller tree size-classes (i.e. < 25 cm diameter at breast height). Neither fire severity, nor changes in stand structure varied significantly over the time-series among recent fire history types (i.e. no fire, Rx fire only, Rx fire + wildfire, or wildfire only) indicating that Rx fires had little effect on subsequent wildfire effects on forest structure or species composition. The low-severity nature of these recent wildfires under severe fire weather and in the wake of almost a century of fire suppression suggests that just because fires have been absent from a site for decades contemporary wildfires will not always result in fire regime characteristics that differ from historical fires in PJ-dominated areas.
... 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. ...
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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.
... Conservation and management practices influence non-target systems in diverse and complex ways. Fire and fire-surrogate management practices are critical for fuel reduction, creating snags, and overall maintaining balanced ecosystems (Schwilk et al. 2009). However, they seem to have mixed effects on biota. ...
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Synopsis Conservation and management activities are geared toward the achievement of particular goals for a specific species, or groups of species, at the population level or higher. Conversely, organismal or functional research is typically organized by hypothesis tests or descriptive work that examines a broader theory studying individual organismal traits. Here, we outline how integrative organismal biologists might conduct mutually beneficial, and meaningful research to inform or assist conservation and management biologists. We argue that studies of non-target species are very useful to both groups because non-target species can meet the goals of managers and organismal biologists alike, while also informing the other. We highlight our work on a threatened lizard species’ thermal physiology, behavior, and color pattern - all of which are impacted by species management plans for sympatric, threatened, bird species. We show that management practices affect activity time, thermal adaptation, and substrate use, while also altering predation rates, crypsis, ectoparasite load, and sexual coloration in the study species. These case studies exemplify the challenges of conservation and management efforts for threatened or endangered species in that non-target species can be both positively and negatively affected by those efforts. Yet, the collaboration of organismal biologists with conservation and management efforts provides a productive system for mutually informative research.
... The majority of individual-tree observations are derived from peer-reviewed studies [6][7][8]15,17,18,27, . Data were contributed by corresponding authors, or came from archived datasets from completed projects 28,40,[75][76][77][78] . Twelve additional datasets were not peer reviewed, but were summarized in professional reports or theses [79][80][81][82] or represent ongoing research or monitoring by land management professionals 26,[83][84][85][86][87][88][89] . ...
Article
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Wildland fires have a multitude of ecological effects in forests, woodlands, and savannas across the globe. A major focus of past research has been on tree mortality from fire, as trees provide a vast range of biological services. We assembled a database of individual-tree records from prescribed fires and wildfires in the United States. The Fire and Tree Mortality (FTM) database includes records from 164,293 individual trees with records of fire injury (crown scorch, bole char, etc.), tree diameter, and either mortality or top-kill up to ten years post-fire. Data span 142 species and 62 genera, from 409 fires occurring from 1981-2016. Additional variables such as insect attack are included when available. The FTM database can be used to evaluate individual fire-caused mortality models for pre-fire planning and post-fire decision support, to develop improved models, and to explore general patterns of individual fire-induced tree death. The database can also be used to identify knowledge gaps that could be addressed in future research.
... It is less clear if prescribed fire leads to negative or positive growth responses in residual trees. Fire often injures surviving trees, and prescribed fire is typically designed to only remove surface and ladder fuels (i.e., small trees), so that large reductions in stand basal area are uncommon (Schwilk et al., 2009). Managers have much greater control over treatment outcomes with thinning operations relative to prescribed fire, but both types of treatments can be designed to be more or less "aggressive, " with varying numbers of surviving trees. ...
Article
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Drought, coupled with rising temperatures, is an emerging threat to many forest types across the globe. At least to a degree, we expect management actions that reduce competition (e.g., thinning, prescribed fire, or both) to improve growth of residual trees during drought. The influences of management actions and drought on individual tree growth may be measured with high precision using tree-rings. Here, we summarize tree-ring-based assessments of the effectiveness of thinning and prescribed fire as drought adaptation tools, with special consideration for how these findings might apply to dry coniferous forests in the southwestern United States. The existing literature suggests that thinning treatments generally improve individual tree growth responses to drought, though the literature specific to southwestern coniferous forests is sparse. Assessments from studies beyond the southwestern United States indicate treatment effectiveness varies by thinning intensity, timing of the drought relative to treatments, and individualistic species responses. Several large-scale studies appear to conflict on specifics of how site aridity influences sensitivity to drought following thinning. Prescribed fire effects in the absence of thinning has received much less attention in terms of subsequent drought response. There are limitations for using tree-ring data to estimate drought responses (e.g., difficulties scaling up observations to stand- and landscape-levels). However, tree-rings describe an important dimension of drought effects for individual trees, and when coupled with additional information, such as stable isotopes, aid our understanding of key physiological mechanisms that underlie forest drought response.
... Harvesting operations inevitably generate some residual slash that affect surface fuel loads and fire behavior (Schwilk et al., 2009;Stephens et al., 2009;Prichard et al., 2010). When wholetree harvesting is utilized, thinning operations have negligible effect on surface fuels. ...
Article
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Fuels reduction treatments to mitigate fire behavior are common in ponderosa pine ecosystems of the western United States. While initial impacts of fuel treatments have been reported, less is known about treatment longevity as live and dead fuels change with time. We analyzed fuel dynamics in ponderosa pine–Douglas-fir forests 21–23 years following experimental fuel reduction designed as two independent studies of cutting combined with burning: one tested a commercial thinning strategy, while a second tested a retention shelterwood strategy to reduce fuels while also restoring ponderosa pine forests. Treated units were harvested in 1992 and half of the units were prescribed burned 1 to 2 years later. After 22 to 23 years post-treatment, few differences in fuel load persist and all treatments have increased ladder fuels in the form of live saplings and seedlings. Canopy fuel loads were lower in treated units compared to untreated control units; however, no other canopy fuel metric differed between treatments. The only persistent difference in surface fuels was in the retention shelterwood, where 1 h fuels were lower in the treated units compared to control units. Crown fire hazard varied greatly, but means were similar between treatments. The increased hazard was driven by increases in live surface fuels from seedlings and saplings in the retention shelterwood, which increased canopy bulk density and reduced canopy base height. The overstory was still dominated by ponderosa pine 22–23 years later for all treatments, but the smaller size classes were primarily Douglas-fir, suggesting that without future disturbance, dominance will shift from pine to Douglas-fir dominated forests. The exception to this was the cut+fall burn treatment in the commercial thinning, where ponderosa pine outnumbered Douglas-fir trees across all size classes. The treatments that included a broadcast prescribed burn killed many existing seedlings and saplings. Our findings support other studies showing fuel reduction and restoration treatments are most successful with a combination of cutting and burning strategies, but also show that fuel treatments in low-elevation dry forests will likely not remain effective for much longer than historical mean fire return intervals.
... It is beyond the scope of this work to fully investigate whether negative effects of prescribed fire on WNPP were due to increased mortality, slower growth by surviving trees due to fire-caused injuries, and/or reduced ingrowth of trees. Dore et al. (2016) cored trees in the same plots used for this study to investigate treatment effects on individual tree growth (up to 2009) and found a significant negative effect of burn-only, a significant positive effect of mechanical-only, and no significant effect of mechanical-burn treatments for the period 2003-2009. However, Collins et al. (2014 report increased mortality rates in burn-only and mechanical-burn stands for the years 2003-2009. ...
Article
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Forests are the largest terrestrial carbon stock, and disturbance regimes can have large effects on the structure and function of forests. Many dry temperate forests in the western United States are adapted to a regime of frequent, low‐to‐moderate severity fire. The disruption of this disturbance regime over the last century has shifted forest conditions, making them more susceptible to high‐severity fire. Fuel treatments have been shown to effectively reduce wildfire hazard, often with co‐benefits to ecological values. However, the effects of fuel treatments on forest carbon are complex, often characterized by direct costs (e.g., carbon emissions from prescribed fire) and wildfire‐contingent benefits (increased resistance of live tree carbon to wildfire). In this study, we employ risk‐sensitive carbon accounting and empirical data from a replicated field experiment to evaluate the stand‐scale carbon effects of four management regimes over a 14‐yr period in a historically frequent‐fire adapted forest. All three active treatment regimes immediately increased stable live tree carbon stocks over no‐treatment controls. In most contexts examined, mechanical‐only or no‐treatment controls will maximize expected total carbon stocks when incorporating wildfire risk and the carbon stability of live biomass, dead biomass, and offsite forest products, although we acknowledge our wildfire modeling may underestimate C losses, particularly in the control stands. Undoubtedly, many other ecosystem and social values besides carbon will be important factors that influence fuel and restoration treatments.
... 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 . ...
Article
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Almost half of the total organic carbon (C) in terrestrial ecosystems is stored in forest soils. By altering rates of input or release of C from soils, forest management activities can influence soil C stocks in forests. In this review, we synthesize current evidence regarding the influences of 13 common forest management practices on forest soil C stocks. Afforestation of former croplands generally increases soil C stocks, whereas on former grasslands and peatlands, soil C stocks are unchanged or even reduced following afforestation. The conversion of primary forests to secondary forests generally reduces soil C stocks, particularly if the land is converted to an agricultural land-use prior to reforestation. Harvesting, particularly clear-cut harvesting, generally results in a reduction in soil C stocks, particularly in the forest floor and upper mineral soil. Removal of residues by harvesting whole-trees and stumps negatively affects soil C stocks. Soil disturbance from site preparation decreases soil C stocks, particularly in the organic top soil, however improved growth of tree seedlings may outweigh soil C losses over a rotation. Nitrogen (N) addition has an overall positive effect on soil C stocks across a wide range of forest ecosystems. Likewise, higher stocks and faster accumulation of soil C occur under tree species with N-fixing associates. Stocks and accumulation rates of soil C also differ under different tree species, with coniferous species accumulating more C in the forest floor and broadleaved species tending to store more C in the mineral soil. There is some evidence that increased tree species diversity could positively affect soil C stocks in temperate and subtropical forests, but tree species identity, particularly N-fixing species, seems to have a stronger impact on soil C stocks than tree species diversity. Management of stand density and thinning have small effects on forest soil C stocks. In forests with high populations of ungulate herbivores, reduction in herbivory levels can increase soil C stocks. Removal of plant biomass for fodder and fuel is related to a reduction in the soil C stocks. Fire management practices such as prescribed burning reduce soil C stocks, but less so than wildfires which are more intense. For each practice, we identify existing gaps in knowledge and suggest research to address the gaps.
... 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. ...
Article
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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.
... 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). ...
... 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). ...
Technical Report
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... Mapping the impact of human activities in natural and semi-natural environments is crucial for tackling land degradation and desertification processes (Schwilk et al. 2009). ...
Article
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Land managers and decision makers need sound, evidence-based information about land degradation patterns and about the effectiveness of their management practices. Obtaining such information is particularly difficult in Mediterranean lands, where for long time a combination of anthropogenic pressure, high topographical and climatic variability, and frequent disturbances created a complicated and unstable environment. Likewise, the normalized difference vegetation index (NDVI) has gained particular recognition in the scientific community as proxy of vegetation and degradation – given the importance of the vegeta- tion cover in preventing land degradation in drylands. This study aims at designing a methodology to provide land managers in a representative site in Northwest of Tunisia with spatially explicit, up-to-date information on the state of their land, the pressures driving land degradation, and the effectiveness of their management efforts using remotely sensed NDVI data. To apply the NDVI index in the process of land degradation assessment, the variance of the annual average NDVI was analysed within different landscape units, which they were identified based on land cover, aspect, and slope steepness parameters. After calibrating and validating the land degradation mapping methodology using field observations, the obtained land degradation patterns were related with spatial information about overuse and monoculture, as well as fallow and agroforestry practices. Therefore, the main objective of this study is to create land degradation maps that show the state of semi-natural dry ecosys- tems and highlight man - made land degradation, then to assess the relation between patterns shown by the degradation map and the main pressure drivers affecting the study site. More specifically, three hypotheses derived from information provided by experts and stakeholders has to be proven: (1) Ecosystems are more sensitive to pressure in steep and south-facing terrain than in flat and north-facing areas. (2) Inappropriate management mainly the monoculture not led only to a physical soil deterioration but also the decline of soil fertility, particularly if they occur repeatedly in continuous way, especially in hilly south terrains, which are more exposed to solar radiation and erosion by wind and rain. (3) Third, we aimed to obtain evidence of the impact of specific land management practices that had been identified as promising.
... 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). ...
Article
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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.
... 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. ...
Article
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.
... 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). ...
Article
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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.
... 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]. ...
Article
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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.
... 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. ...
Article
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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. ...
Article
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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). ...
Article
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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.
... 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). ...
Article
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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.
... 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. ...
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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. ...
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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.
... 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). ...
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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.
... 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). ...
Article
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. This article is protected by copyright. All rights reserved.
... 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 . ...
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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.
... 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). ...
Chapter
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.
Article
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.
Article
Longleaf pine (Pinus palustris) plantations have been established across the southeastern United States as the first step in restoring the declining savanna ecosystem, yet their long-term stability and development to open canopied woodlands may depend on early density and vegetation management treatments. Commercial thinning (CT) was applied in 2012 (year 18) to a long-term study of longleaf pine community restoration at the Savannah River Site near Aiken, SC to increase abundance of herb species. Although precommercial thinning (PCT) and woody vegetation control increased herb species cover during the first five years of the study, measurements from years 17 to 22 indicated a continuing decline in their cover since year 9 despite application of CT in year 18. Herb species richness, however, increased from 21 to 43 species per 100 m² area during the last six years of the study, likely a result of disturbances associated with CT and an ice storm in year 20. Application of PCT in 1994, when the plantations were 8–11 years old, reduced their susceptibility to stem bending and breakage from the ice storm, which occurred 20 years later in 2014. Measured 5 months before the storm, pine height: dbh ratio (HD) averaged 78 and 90 for trees growing with and without PCT, respectively, and stem breakage (% of trees) from the ice storm increased linearly (r² = 0.53) with HD. Pine mortality from the combined effects of glaze ice and wind differed with (28%) versus without (45%) PCT, indicating that long-term stability of longleaf pine plantations depends on early density management.
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Fires shape the biogeochemistry and functioning of many ecosystems, and fire frequencies are changing across much of the globe. Frequent fires can change soil carbon (C) and nitrogen (N) storage by altering the quantity and chemistry of plant inputs through changes in plant biomass and composition as well as altering decomposition of soil organic matter. How decomposition rates change with shifting inputs remains uncertain because most studies focus on the effects of single fires, where transient changes may not reflect responses to decadal changes in burning frequencies. Here, we sampled seven sites exposed to different fire frequencies. In four of the sites, we intensively sampled both soils and plant communities across four ecosystems in North America and Africa spanning tropical savanna, temperate coniferous savanna, temperate broadleaf savanna, and temperate coniferous forest ecosystems. Each site contained multiple plots burned frequently for 33‐61 years and nearby plots that had remained unburned over the same period replicated at the landscape scale. Across all sites, repeatedly burned plots had 25‐185% lower bulk soil C and N concentrations but also 2‐10‐fold lower potential decomposition of organic matter compared to unburned sites. Soil C and N concentrations and extracellular enzyme activities declined with frequent fire because fire reduced both plant biomass inputs into soils and dampened the localized enrichment effect of tree canopies. Examination of soil extracellular enzyme activities revealed that fire decreased the potential turnover of organic matter in the forms of cellulose, starch, and chitin (p<0.0001) but not polyphenol and lignin (p=0.09), suggesting a shift in soil C and N cycling. Inclusion of δ13C data from three additional savanna sites (19‐60 years of altered fire frequencies) showed that soil C losses were largest in sites where estimated tree inputs into soils declined the most (r2=0.91, p<0.01). In conclusion, repeated burning reduced C and N storage, consistent with previous studies, but fire also reduced potential decomposition, likely contributing to slower C and N cycling. Trees were important in shaping soil carbon responses across sites, but the magnitude of tree effects differed and depended on how tree biomass inputs into soil responded to fire.
Technical Report
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A Natural England evidence review on the occurrence, causes, prevention and management of wildfires on open, semi-natural habitats in the UK, with a particular focus on heathlands and peatlands in England. This reflects Natural England’s role and interest in relation to maintaining and restoring the structure and function of semi-natural habitats, including supporting ecosystem services and related government environment objectives and policies. Supporting information and summary data are given in 12 appendices.
Chapter
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.
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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.
Article
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.
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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.
Article
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.
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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.
Article
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.
Article
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.
Technical Report
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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.
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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.
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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.
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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.
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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,
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
Chapter
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.
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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.
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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
Article
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.
Article
Longleaf pine (Pinus palustris) ecosystems are native to nine states of the southern region of the United States. Longleaf pine can grow on a variety of site types including wet flatwoods and savannas along the Atlantic and Gulf coastal plain, higher droughty sand deposits from the fall line sandhills to the central ridge of Florida (Stout and Marion 1993), and the montane slopes and ridges of Alabama and northwest Georgia up to 600 m elevation (Boyer 1990b). This region has a humid subtropical climate (Bailey 1995). Maximum July temperatures average 29°C to >35°C while minimums during January range from 0°C to 13°C. The mean annual precipitation is 1040–1750 mm and is well distributed through the year. Growing season is comparatively long, ranging from 300+ days in Florida to 220 days at the northern limit of longleaf pine’s range. During the late summer and fall, hurricanes can develop over the Atlantic Ocean, move westward, and impact coastal plain forests. Such tropical storms are a principal large-scale disturbance agent for longleaf pine forests near the seacoast.