Historical Fire Regimes Research Articles

Boreal Caribou Can Coexist with Natural but Not Industrial Disturbances

ABSTRACTFor species at risk, it is important that demographic models be consistent with our most recent knowledge because alternate model versions can have differing predictions for wildlife and natural resource management. To establish and maintain this consistency, we can compare predicted model values to current or past observations and demographic knowledge. When novel predictor information becomes available, testing for consistency between modeled and observed values ensures the best models are used for robust, evidence‐based, wildlife management. We combine novel information on the extent of historical disturbance regimes (industrial and fire) to an existing demographic model and predict historical and projected demographics of woodland caribou (Rangifer tarandus caribou). Exploring 6 simulation experiments across 5 populations in Alberta, Canada, we identify the relative importance of industrial disturbance, fire, and population density to observed population size and growth rate. We confirm the onset of significant declines across all 5 populations began approximately 30 years ago, demonstrate these declines have been consistent, and conclude they are more likely due to industrial disturbance from the oil and gas sector within contemporary population ranges than historical fire regimes. These findings reinforce recent research on the cause of woodland caribou declines. Testing for consistency between observations and models prescribed for species recovery is paramount for assessing the cause of declines, projecting population trends, and refining recovery strategies for effective wildlife management. We provide a novel simulation method for conducting these tests. © 2020 The Authors. The Journal of Wildlife Management published by Wiley Periodicals LLC on behalf of The Wildlife Society.

Limited Effects of Long-Term Repeated Season and Interval of Prescribed Burning on Understory Vegetation Compositional Trajectories and Indicator Species in Ponderosa Pine Forests of Northeastern Oregon, USA

Fire exclusion has dramatically altered historically fire adapted forests across western North America. In response, forest managers reduce forest fuels with mechanical thinning and/or prescribed burning to alter fire behavior, with additional objectives of restoring forest composition, structure, and ecosystem processes. There has been extensive research on the effects of fuel reduction and restoration treatments on trees, fuels, regeneration, and fire behavior; but less is known about how these treatments influence understory vegetation, which contains the majority of vascular plant diversity in many dry conifer forests. Of particular interest is how understory vegetation may respond to the season and interval of prescribed burning. The season and interval of prescribed burning is often determined by operational constraints rather than historical fire regimes, potentially resulting in fire conditions and burn intervals to which native plants are poorly adapted. In this study, we examined how understory vegetation has responded to season and interval of prescribed burning in ponderosa pine (Pinus ponderosa) forests in the Blue Mountains of northeastern Oregon, USA. Using over a decade (2002–2015) of understory vegetation data collected in stands with different intervals (5 versus 15 year) and seasons (spring versus fall) of prescribed burning, we quantified how season and interval of prescribed burning has influenced understory vegetation compositional trajectories and indicator species over time. Season of prescribed burning resulted in different understory communities and distinct trajectories of understory composition over time, but interval of burning did not. Indicator species analysis suggests fall burning is facilitating early seral species, with native annual forbs displaying ephemeral responses to frequent burning, while invasive cheatgrass (Bromus tectorum) increased in abundance and frequency across all treatments over time. These findings indicate that understory vegetation in these ecosystems are sensitive to seasonality of burning, but the responses are subtle. Our findings suggest season and interval of prescribed burning used in this study do not result in large changes in understory vegetation community composition, a key consideration as land managers increase the pace and scale of prescribed fire in these forests.

Demographic trends in community functional tolerance reflect tree responses to climate and altered fire regimes.

Forests of the western United States are undergoing substantial stress from fire exclusion and increasing effects of climate change, altering ecosystem functions and processes. Changes in broad-scale drivers of forest community composition become apparent in their effect on survivorship and regeneration, driving demographic shifts. Here we take a community functional approach to forest demography, by investigating mean drought or shade functional tolerance in community assemblages. We created the Community Mean Tolerance Index (CMTI), a response metric utilizing drought/shade tolerance trade-offs to identify communities undergoing demographic change from a functional trait perspective. We applied the CMTI to Forest Inventory and Analysis data to investigate demographic trends in drought and shade tolerance across the southern Rocky Mountains. To find the major drivers of change in community tolerance within and across forest types, we compared index trends to climate and fire-exclusion-driven disturbance, and identified areas where demographic change was most pronounced. We predicted that greater shifts in drought tolerance would occur at lower forest type ecotones where climate stress is limiting and that shifts in shade tolerance would correspond to excursions from the historic fire regime leading to greater changes in forest types adapted to frequent, low-intensity fire. The CMTI was applied spatially to identify sites likely to transition to oak shrubfield, where disturbance history combined with a species-driven demographic shift toward drought tolerance. Within forest types, lower elevations are trending toward increased drought tolerance, while higher elevations are trending toward increased shade tolerance. Across forest types, CMTI difference peaked in mid-elevation ponderosa pine and mixed-conifer forests, where fire exclusion and autecology drive demographic changes. Peak CMTI difference was associated with fire exclusion in forest types adapted to frequent fire. At higher elevations, site-level stand dynamics appear to be influencing demographic tolerance trends more than broad climate drivers. Through a community demographic approach to functional traits, the CMTI highlights areas and forest types where ecosystem function is in the process of changing, before persistent vegetation type change occurs. Applied to regional plot networks, the CMTI provides an early warning of shifts in community functional processes as climate change pressures continue.

The effects of inter-fire interval on flora-fauna interactions in a sub-alpine landscape

Attributes of the historical fire regime can influence fauna dynamics by moderating vegetation composition and the availability of appropriate habitat. Despite this, the indirect impacts of specific aspects of the long-term fire regime are often overlooked. Here we conduct a field survey at 27 sites in sub-alpine forests of Namadgi National Park (Australia) to determine if one aspect of the historical fire regime, inter-fire interval, influences vertebrate fauna and fauna habitat. Sites were stratified between three inter-fire interval categories (20, 65–70 and 83 years between the previous two fires) in areas last burned 15-years prior to the field survey. All other environmental attributes thought to impact vegetation dynamics were controlled for in the study design. Fauna activity was monitored for four weeks using trail cameras and 15 habitat attributes were assessed using field and remote-sensing survey techniques. Most fauna and habitat variables did not differ among the inter-fire interval categories. However, mountain brush-tail possum detections increased with inter-fire interval length, with 260% more observations within the older treatment when compared with the younger treatment. Large tree density, maximum tree height, and tree canopy cover were 40%, 38% and 12% higher within the older than younger treatment, respectively. Conversely, small tree density was 78% lower within the older treatment. Our results show a limited response of fauna to historical fire-return intervals of >20 years within a fire-prone forest, however also highlight that species-/habitat-specific responses do manifest via indirect effects pathways at longer fire return intervals.

Are plant community responses to wildfire contingent upon historical disturbance regimes?

AbstractBackgroundEcological disturbance is a major driver of ecosystem structure and evolutionary selection, and theory predicts that the frequency and/or intensity of disturbance should determine its effects on communities. However, adaptations of species pools to different historical disturbance regimes are rarely considered in the search for generalizable community responses to disturbance. To explore how the severity of disturbance affects plant diversity patterns, we review studies of understorey plant community responses to wildfire in conifer forests of western North America across a gradient of departure from historical fire regimes.Review findingsWe find that post‐fire plant species richness may generally be maximized at disturbance severities that match the predominant historical disturbance regime in a given ecosystem. Studies that examined multiple spatial scales indicate that plant community responses to fire are likely to be scale dependent, suggesting that post‐disturbance monitoring should consider community responses at multiple scales.SynthesisOur review highlights that consideration of historical disturbance regimes might improve the ability to predict the effects of disturbance on communities. We discuss future research needs; quantitative studies that compare community responses to fire at multiple scales across different historical fire regimes would be particularly useful. Ultimately, consideration of disturbance as a multivariate problem is likely to lead to greater inference than traditional bivariate approaches.

Habitat selection by spotted owls after a megafire reflects their adaptation to historical frequent-fire regimes

Climate and land-use change have led to disturbance regimes in many ecosystems without a historical analog, leading to uncertainty about how species adapted to past conditions will respond to novel post-disturbance landscapes. We examined habitat selection by spotted owls in a post-fire landscape. We tested whether selection or avoidance of severely burned areas could be explained by patch size or configuration, and whether variation in selection among individuals could be explained by differences in habitat availability. We applied mixed-effects models to GPS data from 20 spotted owls in the Sierra Nevada, California, USA, with individual owls occupying home ranges spanning a broad range of post-fire conditions after the 2014 King Fire. Individual spotted owls whose home ranges experienced less severe fire ( 100-m into severe fire patches. Spotted owls avoided areas that had experienced post-fire salvage logging but the interpretation of this effect was nuanced. Owls also avoided areas that were classified as open and/or young forest prior to the fire. Our results support the hypothesis that spotted owls are adapted to historical fire regimes characterized by small severe fire patches in this region. Shifts in disturbance regimes that produce novel landscape patterns characterized by large, homogeneous patches of high-severity fire may negatively affect this species.

Current and historical fire regimes of the Pechora-ilych nature reserve and its surroundings

Current and historical fire regimes of the Pechora-ilych nature reserve and its surroundings

Fire regimes and structural changes in oak-pine forests of the Mogollon Highlands ecoregion: Implications for ecological restoration

Ponderosa pine (Pinus ponderosa) forests occur at their warmer, drier environmental limits in the Mogollon Highlands ecoregion (MHE) of the Southwestern United States, and are commonly found in stringers or discrete stands that form ecotones with interior chaparral. These “rear edge” forests are likely to be highly vulnerable to rapid changes in structure and composition with climate warming, drought, and wildfire. There is increasing interest in understanding historical conditions, ecosystem changes, and restoration needs for MHE forests. However, comprehensive reconstruction analysis of fire regimes and stand structure has not been done for these systems, which differ from many montane ponderosa pine forests by having an abundance of understory shrubs. In this study we used demographic data from field plots, fire scar samples, and dendroecology to reconstruct historical fire regimes and landscape structure at ponderosa pine-dominated sites that spanned a range of environmental conditions on the Prescott and Tonto National Forests. We found strong evidence of historical surface fire regimes with mean fire intervals ranging 1.3–15.6 years across the five MHE sites during the period 1700–1879. We found very little evidence of historical high-severity fire at any study site. Historical forest structure was open with tree densities ranging 84.7–136.4 trees ha−1 and stand basal area (BA) ranging 4.5–8.4 m2 ha−1. Historical composition showed codominance of ponderosa pine, Arizona white oak (Quercus arizonica), Emory oak (Q. emoryi), and Gambel oak (Q. gambelii). Thus, oak species and likely other hardwoods were important historical components of these ecosystems. Contemporary forests are greater in stand density and BA by 359–703% and 285–502%, respectively, compared to historical estimates. In addition, we observed contemporary shifts in species composition. Changes related to disruption of historical fire regimes have increased susceptibility of ponderosa pine forests in the MHE to rapid shifts in structure and composition that may come about with climate change and high-intensity wildfire. Meeting fuels reduction and ecological restoration goals will be challenging for land managers due to vigorous regeneration responses of shrubs to tree thinning, prescribed burning, or other management activities. Managers will be required to balance attention to historical reference conditions, conservation of biological diversity, and needs for fuels management.

Biogeography of fire regimes in western U.S. conifer forests: A trait‐based approach

AbstractAimFunctional traits are a crucial link between species distributions and the ecosystem processes that structure those species’ niches. Concurrent increases in the availability of functional trait data and our ability to model species distributions present an opportunity to develop functional trait biogeography (i.e., the mapping of functional traits across space). Functional trait biogeography can improve process‐based predictions about the resistance of certain species assemblages to changing environmental conditions across landscape scales. We illustrate this concept by developing the first trait‐based, quantitative ranking of fire resistance (adult tree survival) in North American conifer species and mapping that fire resistance across space.Location and time periodWestern continental USA, present day.Major taxa studiedTwenty‐nine common conifer tree species.MethodsWe compiled six traits for each species: three relating to tree morphology and three relating to litter flammability. We combined these traits into a single fire resistance score and used community‐weighted averaging to estimate the fire resistance scores of different forest communities, using interpolated species distribution and relative abundance data.ResultsSpecies associated historically with frequent fire have high fire resistance scores (e.g.,Pinus ponderosa), reflected by thick bark, tall crowns and flammable litter. Species associated with subalpine or arid conditions have low fire resistance scores (e.g.,Picea engelmanniiandPinus edulis), reflected by thin bark, short stature, poor self‐pruning and low litter flammability. A map of forest community fire resistance across the western USA reveals agreement with independent assessments of historical fire regimes, while also identifying areas where community‐wide species traits might be mismatched with historical fire regimes.Main conclusionsQuantifying the functional traits that confer resistance to tree‐killing fire provides a direct link between ecosystem disturbance and community resistance. Understanding this link is crucial to evaluation of the long‐term resilience of different forest types under dynamic fire regimes. Our work represents the first known spatial representation of fire resistance traits at a regional scale and, as such, provides a link between functional traits and biogeography relevant to a critical ecosystem process.

Consideration of anthropogenic factors in boreal forest fire regime changes during rapid socio-economic development: case study of forestry districts with increasing burnt area in the Sakha Republic, Russia

This paper presents an original approach to characterizing historical fire regimes for regions with limited fire data. Fire variables were derived from satellite datasets and regional fire occurrence statistics. They defined the integral elements of a fire regime such as historical trends, spatiotemporal evolution, fire seasonality and causes. Temporal evolution was investigated based on a regime shift detection method developed by Rodionov while changes in the fire regime were analyzed for statistical significance using the Mann–Kendall trend test and Sen’s slope estimator. A descriptive analysis was performed to assess fire seasonality, causes, and together formed the basis for this methodology. We validated the proposed approach by assessing historical fire activity in the Sakha Republic (Yakutia), which is one of the most fire-prone regions of Russia. The assessment was conducted with data from the period of 1996–2018. We detected increases in historical fire activity as well as thresholds of change in the fire regime. Changes during the analysis period included lengthening of fire season, increased burned area extent, and extension of peak fire period. Overall, significant changes in the fire regime were detected in the regions strongly affected by warming and increasing anthropogenic alteration.

Vegetation and fire variability in the central Cerrados (Brazil) during the Pleistocene-Holocene transition was influenced by oscillations in the SASM boundary belt

This study investigates historical fire regimes and arboreal cover variability in the Brazilian Cerrados, a large Neotropical savanna ecosystem, during the Pleistocene-Holocene transition and early Holocene, and then tests whether the observed variation is linked to South American Summer Monsoon (SASM) variability. We present high-resolution pollen, XRF and charcoal records from Lagoa Feia, located in central Cerrados, and assess how they compare with other pollen records from the surrounding region to investigate regional trends of vegetation and fire regime variability. Our results show that the Pleistocene-Holocene transition was marked by a wet episode, which included moist forest expansion and rising lake levels, that correlates well with increased monsoon activity in a large area of central South America during the same period. This wet episode chronologically coincides with the Younger Dryas cold episode in the northern hemisphere. Our data revealed moisture declines in central Cerrados after 11.2 kyr BP, along with centennial-scale fluctuations from dry to wet conditions throughout the early Holocene period until around 6 kyr BP. These dry/wet oscillations are associated with weakened SASM activity and repeated shifts of its belt position during this time range. Fire activity increased in central Cerrados just after the onset of drier conditions during the early Holocene, and likely contributed to decreased arboreal cover at that time. A trend of increasing moisture in the region was observed after 6 kyr BP. Our study reveals how centennial and millennial-scale changes in monsoon activity influenced arboreal cover, diversity and fire regimes in the central Cerrados.

Fire legacies, heterogeneity, and the importance of mixed-severity fire in ponderosa pine savannas

Globally, savanna ecosystems are shifting outside of “safe operating spaces” due to removal of their primary self-reinforcing feedback—fire—and subsequent erosion of disturbance legacies. Restoring savannas will require reinstating fire feedbacks. But knowledge gaps in the nature of historic fire regimes and how mechanisms such as time-since-fire and fire severity interact to produce disturbance legacies hinders development of ecologically relevant restoration targets. A theory-based approach for determining restoration targets is to compare structures produced by time-since-fire/fire severity interactions to structures that fostered animal communities that historically inhabited savannas. Here, we use a space-for-time substitution to quantify interactive effects of time-since-fire and fire severity on vegetation structures related to known animal community habitat preferences by surveying sites in 10-year-old and 27-year-old mixed-severity fires that occurred in an eastern ponderosa pine (Pinus ponderosa) savanna where fire was excluded since European settlement. Our specific objectives are to 1), quantify the relative strengths and interactive effects of time-since-fire and a full fire severity gradient on multivariate vegetation structure across landscape patches and 2), assess relationships between multivariate vegetation structures and time-since-fire/fire severity classes across landscape patches. We used a stratified random design to distribute 112 sampling plots by fire severity (unburned, low, moderate, and high) and time-since-fire (10 years and 27 years) and measured stand structure, tree cavity characteristics, and coarse woody debris. The interaction of time-since-fire and fire severity drove structure complexity for 27 years post-fire, but fire severity explained > 14 times the amount of variation in structure than time-since-fire alone. A full fire severity gradient (low-, moderate-, and high-severities) and structural changes within patches that experienced different fire severities generated sufficient patch-level heterogeneity to foster animal communities historically native to eastern ponderosa pine savannas. Structures generated by low fire severity alone, which is a common goal of fire management in easternmost ponderosa pine savannas, did not reflect sufficient structural complexity to support the diversity of endemic animal species. This indicates information legacies (i.e., represented by the distribution of species traits in a community) were shaped by mixed severity fire regimes, which provides further support for the scientific premise that management goals seeking to minimize variation in fire regimes (e.g., low intensity and low-severity fires only) is less able to support a full array of biodiversity. Rather, mixed-severity fire is an important driver of structural heterogeneity, fosters diverse information legacies, and enhances ponderosa pine savanna resilience to extreme fire.

Standing dead trees contribute significantly to carbon budgets in Australian savannas

Previous estimates of greenhouse gas emissions from Australian savanna fires have incorporated on-ground dead wood but ignored standing dead trees. However, research from eucalypt woodlands in southern Queensland has shown that the two pools of dead wood burn at similar rates. New field data from semiarid savannas across northern Australia confirmed that standing dead trees comprise about four times the mass of on-ground dead wood. Further, the proportion of total woody biomass comprising dead wood increases with decreasing fire frequency and a decreasing proportion of late dry season (August to December) fires. This gives scope for increasing the carbon stock in the dead wood pool with a reduced fire frequency. Following a previously published approach to quantify total dead wood loads in savannas, new and previously collected data on tree stand structures were used across the whole savanna zone to quantify dead wood loads in equilibrium with historic fire regimes. New parameters are presented for calculating dead wood dynamics including dead trees in Australia’s savannas.

Wildfire Risk and Hazardous Fuel Reduction Treatments Along the US-Mexico Border: A Review of the Science (1986-2019)

The ecosystems along the border between the United States and Mexico are at increasing risk to wildfire due to interactions among climate, land-use, and fuel loads. A wide range of fuel treatments have been implemented to mitigate wildfire and its threats to valued resources, yet we have little information about treatment effectiveness. To fill critical knowledge gaps, we reviewed wildfire risk and fuel treatment studies that were conducted near the US-Mexico border and published in the peer-reviewed literature between 1986 and 2019. The number of studies has grown during this time in warm desert to forest ecosystems on primarily federal lands. The most common study topics included fire effects on native species, the role of invasive species and woody encroachment on wildfire risk, historical fire regimes, and remote sensing and modeling to study wildfire risk across the landscape. A majority of fuel treatment studies focused on prescribed burns, and fuel treatments collectively had mixed effects on mitigating future wildfire risk and threats to ecosystems depending on vegetation and fire characteristics. The diversity of ecosystems and land ownership along the US-Mexico border present unique challenges for understanding and managing wildfire risk, and also create opportunities for collaboration and cross-site studies to promote knowledge across broad environmental gradients.

Fire history in Andean Araucaria–Nothofagus forests: coupled influences of past human land-use and climate on fire regimes in north-west Patagonia

Historical fire regimes are critical for understanding the potential effects of changing climate and human land-use on forest landscapes. Fire is a major disturbance process affecting the Andean Araucaria forest landscape in north-west Patagonia. The main goals of this study were to reconstruct the fire history of the Andean Araucaria–Nothofagus forests and to evaluate the coupled influences of climate and humans on fire regimes. Reconstructions of past fires indicated that the Araucaria forest landscape has been shaped by widespread, stand-replacing fires favoured by regional interannual climate variability related to major tropical and extratropical climate drivers in the southern hemisphere. Summer precipitation and streamflow reconstructions tended to be below average during fire years. Fire events were significantly related to positive phases of the Southern Annular Mode and to warm and dry summers following El Niño events. Although Euro-Chilean settlement (1883–1960) resulted in widespread burning, cattle ranching by Pehuenche Native Americans during the 18th and 19th centuries also appears to have changed the fire regime. In the context of climate change, two recent widespread wildfires (2002 and 2015) affecting Araucaria forests appear to be novel and an early indication of a climate change driven shift in fire regimes in north-west Patagonia.

Contemporary Fire Regimes Provide a Critical Perspective on Restoration Needs in the Mexico-United States Borderlands

The relationship between people and wildfire has always been paradoxical: fire is an essential ecological process and management tool, but can also be detrimental to life and property. Consequently, fire regimes have been modified throughout history through both intentional burning to promote benefits and active suppression to reduce risks. Reintroducing fire and its benefits back into the Sky Island mountains of the United States-Mexico borderlands has the potential to reduce adverse effects of altered fire regimes and build resilient ecosystems and human communities. To help guide regional fire restoration, we describe the frequency and severity of recent fires over a 32-year period (1985-2017) across a vast binational region in the United States-Mexico borderlands and assess variation in fire frequency and severity across climate gradients and in relation to vegetation and land tenure classes. We synthesize relevant literature on historical fire regimes within 9 major vegetation types and assess how observed contemporary fire characteristics vary from expectations based on historical patterns. Less than 28% of the study area burned during the observation period, excluding vegetation types in warmer climates that are not adapted to fire (eg, Desertscrub and Thornscrub). Average severity of recent fires was low despite some extreme outliers in cooler, wetter environments. Midway along regional temperature and precipitation gradients, approximately 64% of Pine-Oak Forests burned at least once, with fire frequencies that mainly corresponded to historical expectations on private lands in Mexico but less so on communal lands, suggesting the influence of land management. Fire frequency was higher than historical expectations in extremely cool and wet environments that support forest types such as Spruce-Fir, indicating threats to these systems possibly attributable to drought and other factors. In contrast, fires were absent or infrequent across large areas of Woodlands (~73% unburned) and Grasslands (~88% unburned) due possibly to overgrazing, which reduces abundance and continuity of fine fuels needed to carry fire. Our findings provide a new depiction of fire regimes in the Sky Islands that can help inform fire management, restoration, and regional conservation planning, fostered by local and traditional knowledge and collaboration among landowners and managers.

The 1840 Western Port journey and Aboriginal fire history in the grassy ecosystems of lowland, mesic south-eastern Australia

The historic influence of human fire and the role of ‘top-down’ vs ‘bottom-up’ drivers on ecosystems globally is highly contested, and our knowledge of regime diversity is poor. This paper uses an early European account as a case study to describe Aboriginal fire history in south-eastern Australia based on links between fire and: grasslands, native foods and culture. The route and observations detailed in Assistant Protector William Thomas’ 1840 account of a journey led by Aborigines to Western Port, Victoria, were overlayed with grass-tree boundaries compiled from historic plans. The narrative provides direct evidence of up to moderate-scale and intensity burns (with minimal fine-scale patchiness), undertaken in the height of the dry season, opportunistically linked to rainfall. The fires targeted open grassy ‘plains’ to maintain and access preferred hunter-gathering grounds. A synthesis of the earliest records supports high frequency anthropogenic burning maintaining alternative vegetation states with dynamic boundaries on elevated alluvial plains and, in places, adjoining swamps. The narrative represents an important primary source for studying traditional society, including the description of a local historic fire regime (‘koyuga burning’). Establishing such a fire regime ‘benchmark’ has the potential to stimulate new interdisciplinary research around the complex processes controlling grass-tree patterns, and build confidence that fire-stick farming was potentially instrumental in grassland formation, and integral to grassland maintenance throughout this region.

Identifying opportunities for the use of broadcast prescribed fire on Colorado’s Front Range

Increasing the pace and scale of fuel treatments to protect social and ecological values from severe wildfire is a major initiative of numerous land management agencies, organizations, and collaborative groups throughout the western United States, including the Colorado Front Range. Broadcast prescribed fire is a relatively low-cost, effective management tool for achieving fuels reduction at scale but is often challenging to implement due to complex socio-ecological factors. We present results of a multi-criteria suitability analysis intended to identify need, opportunities, and constraints for the use of broadcast prescribed fire on Colorado’s Front Range based on spatial factors including wildfire hazard, vegetation and fuel types, historical fire regimes, presence of existing fuel treatments, wildland-urban interface development, and predicted prescribed fire behavior. Within our 1.7 million-hectare (ha) analysis area, over 228,000 ha (approximately 13%) were classified as highly suitable for broadcast prescribed fire. Areas of high suitability were split roughly 50:50 between public federal lands (5.3%) and private lands (6.1%), emphasizing the importance of implementing prescribed fire across ownerships to meet management objectives at scale. Patch size analysis revealed opportunities for large-scale (>500 ha) prescribed fire projects spatially distributed throughout the Front Range. These areas may serve as anchors for developing projects focused on protecting values at risk, including wildland-urban interface communities and water resources. Results of this analysis can be used in collaborative settings to develop comprehensive fuels reduction and forest restoration strategies that incorporate the use of prescribed fire, including identifying where mechanical treatments could be applied on the landscape to facilitate the use of broadcast prescribed fire over large extents, as well as where prescribed fire may be a viable option for long-term maintenance of treatments. While our analysis focuses on the Colorado Front Range, it can be replicated in other fire-prone areas of the western United States based on the availability of local data.

High-severity wildfire limits available floral pollen quality and bumble bee nutrition compared to mixed-severity burns.

High-severity wildfires, which can homogenize floral communities, are becoming more common relative to historic mixed-severity fire regimes in the Northern Rockies of the U.S. High-severity wildfire could negatively affect bumble bees, which are typically diet generalists, if floral species of inadequate pollen quality dominate the landscape post-burn. High-severity wildfires often require more time to return to pre-burn vegetation composition, and thus, effects of high-severity burns may persist past initial impacts. We investigated how wildfire severity (mixed- vs. high-severity) and time-since-burn affected available floral pollen quality, corbicular pollen quality, and bumble bee nutrition using percent nitrogen as a proxy for pollen quality and bumble bee nutrition. We found that community-weighted mean floral pollen nitrogen, corbicular pollen nitrogen, and bumble bee nitrogen were greater on average by 0.82%N, 0.60%N, and 1.16%N, respectively, in mixed-severity burns. This pattern of enhanced floral pollen nitrogen in mixed-severity burns was likely driven by the floral community, as community-weighted mean floral pollen percent nitrogen explained 87.4% of deviance in floral community composition. Only bee percent nitrogen varied with time-since-burn, increasing by 0.33%N per year. If these patterns persist across systems, our findings suggest that although wildfire is an essential ecosystem process, there are negative early successional impacts of high-severity wildfires on bumble bees and potentially on other pollen-dependent organisms via reductions in available pollen quality and nutrition. This work examines a previously unexplored pathway for how disturbances can influence native bee success via altering the nutritional landscape of pollen.

Cambio postincendio en la estructura y composición del estrato arbóreo y carga de combustibles en un bosque de Pinus douglasiana de México

En los bosques de pino con un régimen histórico de incendios superficiales frecuentes de baja severidad, bajo condiciones extremas del estado del tiempo o a consecuencia de la supresión del fuego, la severidad de los incendios puede incrementar. La respuesta de la vegetación a eventos de incendios severos se estudió en una cronosecuencia postincendio de 8 años, 28 años y 60 años en un bosque dominado por Pinus douglasiana en México. La densidad de árboles con diámetro > 2.5 cm y su área basal alcanzaron valores de 1095 árboles ha-1 ± 609 árboles ha-1 y 10.0 m2 ha-1 ± 2.2 m2 ha-1 a los 8 años; a los 28 años el área basal (44.6 m2 ha-1 ± 3.0 m2 ha-1) fue cercana a la de los rodales de 60 años (49.0 m2 ha-1 ± 6.4 m2 ha-1). Los pinos mantuvieron el mayor valor de importancia con el tiempo postincendio, se observó la formación de un subdosel de latifoliadas tolerantes a la sombra y un aumento en la riqueza y diversidad de especies; también se documentó que periodos largos sin incendios incrementan la carga de combustibles superficiales ligeros (66.0 Mg ha-1 ± 5.5 Mg ha-1 en los sitios de 28 años y 61.5 Mg ha-1 ± 4.6 Mg ha-1 en los sitios de 60 años) y modifican el comportamiento del fuego (velocidad de propagación de 1.3 m s-1 a 4.1 m s-1 y la longitud de la llama de 0.7 m a 1.7 m entre los rodales de 8 años y los de 28 años y 60 años combinados). El manejo de los bosques de pino debe considerar el balance entre objetivos de conservación biológica y mitigación del peligro de incendios de alta severidad.