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Shaw DC, Beedlow PA, Lee EH, Woodruff DR, Meigs GW, Calkins SJ, Reilly MJ, Merschel AG, Cline SP, Comeleo RL. The complexity of biological disturbance agents, fuels heterogeneity, and fire in coniferous forests of the western United States. FOREST ECOLOGY AND MANAGEMENT 2022; 525:1-27. [PMID: 36968296 PMCID: PMC10031511 DOI: 10.1016/j.foreco.2022.120572] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/17/2023]
Abstract
Forest biological disturbance agents (BDAs) are insects, pathogens, and parasitic plants that affect tree decline, mortality, and forest ecosystems processes. BDAs are commonly thought to increase the likelihood and severity of fire by converting live standing trees to more flammable, dead and downed fuel. However, recent research indicates that BDAs do not necessarily increase, and can reduce, the likelihood or severity of fire. This has led to confusion regarding the role of BDAs in influencing fuels and fire in fire-prone western United States forests. Here, we review the existing literature on BDAs and their effects on fuels and fire in the western US and develop a conceptual framework to better understand the complex relationships between BDAs, fuels and fire. We ask: 1) What are the major BDA groups in western US forests that affect fuels? and 2) How do BDA-affected fuels influence fire risk and outcomes? The conceptual framework is rooted in the spatiotemporal aspects of BDA life histories, which drive forest impacts, fuel characteristics and if ignited, fire outcomes. Life histories vary among BDAs from episodic, landscape-scale outbreaks (bark beetles, defoliators), to chronic, localized disturbance effects (dwarf mistletoes, root rots). Generally, BDAs convert aboveground live biomass to dead biomass, decreasing canopy fuels and increasing surface fuels. However, the rate of conversion varies with time-since-event and among BDAs and forest types, resulting in a wide range of effects on the amount of dead fuels at any given time and place, which interacts with the structure and composition of the stand before and subsequent to BDA events. A major influence on fuels may be that BDAs have emerged as dominant agents of forest heterogeneity creation. Because BDAs play complex roles in fuels and fire heterogeneity across the western US which are further complicated by interactions with climate change, drought, and forest management (fire suppression), their impacts on fuels, fire and ecological consequences cannot be categorized simply as positive or negative but need to be evaluated within the context of BDA life histories and ecosystem dynamics.
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Affiliation(s)
- David C. Shaw
- Department of Forest Engineering, Resources, and Management, Oregon State University, 216 Peavy Forest Science Complex, 3100 SW Jefferson Way, Corvallis, OR 97331, USA
| | - Peter A. Beedlow
- U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA
| | - E. Henry Lee
- U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA
| | - David R. Woodruff
- USDA Forest Service, Pacific Northwest Research Station, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97333, USA
| | - Garrett W. Meigs
- Washington State Department of Natural Resources, 1111 Washington St. SE, Olympia, WA 98504, USA
- Department of Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, 3180 SW Jefferson Way, Corvallis, OR 97331, USA
| | - Stephen J. Calkins
- Department of Forest Engineering, Resources, and Management, Oregon State University, 216 Peavy Forest Science Complex, 3100 SW Jefferson Way, Corvallis, OR 97331, USA
| | - Matthew J. Reilly
- USDA Forest Service, Pacific Northwest Research Station, Forestry Sciences Laboratory, 3200 SW Jefferson Way, Corvallis, OR 97333, USA
| | - Andrew G. Merschel
- Department of Forest Ecosystems and Society, Oregon State University, 321 Richardson Hall, 3180 SW Jefferson Way, Corvallis, OR 97331, USA
| | - Steven P. Cline
- U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA
| | - Randy L. Comeleo
- U.S. Environmental Protection Agency, 200 SW 35th Street, Corvallis, OR 97333, USA
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Wayman RB, Safford HD. Recent bark beetle outbreaks influence wildfire severity in mixed-conifer forests of the Sierra Nevada, California, USA. ECOLOGICAL APPLICATIONS : A PUBLICATION OF THE ECOLOGICAL SOCIETY OF AMERICA 2021; 31:e02287. [PMID: 33426715 DOI: 10.1002/eap.2287] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/13/2020] [Revised: 09/11/2020] [Accepted: 10/06/2020] [Indexed: 06/12/2023]
Abstract
In temperate forests, elevated frequency of drought related disturbances will likely increase the incidence of interactions between disturbances such as bark beetle epidemics and wildfires. Our understanding of the influence of recent drought and insect-induced tree mortality on wildfire severity has largely lacked information from forests adapted to frequent fire. A recent unprecedented tree mortality event in California's Sierra Nevada provides an opportunity to examine this disturbance interaction in historically frequent-fire forests. Using field data collected within areas of recent tree mortality that subsequently burned in wildfire, we examined whether and under what conditions wildfire severity relates to severity of prefire tree mortality in Sierra Nevada mixed-conifer forests. We collected data on 180 plots within the 2015 Rough Fire and 2016 Cedar Fire footprints (California, USA). Our analyses identified prefire tree mortality as influential on all measures of wildfire severity (basal area killed by fire, RdNBR, and canopy torch) on the Cedar Fire, although it was less influential than fire weather (relative humidity). Prefire tree mortality was influential on two of three fire-severity measures on the Rough Fire, and was the most important predictor of basal area killed by fire; topographic position was influential on two metrics. On the Cedar Fire, the influence of prefire mortality on basal area killed by fire was greater under milder weather conditions. All measures of fire severity increased as prefire mortality increased up to prefire mortality levels of approximately 30-40%; further increases did not result in greater fire severity. The interacting disturbances shifted a pine-dominated system (Rough Fire) to a cedar-pine-fir system, while the pre-disturbance fir-cedar system (Cedar Fire) saw its dominant species unchanged. Managers of historically frequent-fire forests will benefit from utilizing this information when prioritizing fuels reduction treatments in areas of recent tree mortality, as it is the first empirical study to document a relationship between prefire mortality and subsequent wildfire severity in these systems. This study contributes to a growing body of evidence that the influence of prefire tree mortality on wildfire severity in temperate coniferous forests may depend on other conditions capable of driving extreme wildfire behavior, such as weather.
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Affiliation(s)
- Rebecca B Wayman
- Department of Environmental Science and Policy, University of California, Davis, 95616, USA
| | - Hugh D Safford
- Department of Environmental Science and Policy, University of California, Davis, 95616, USA
- Pacific Southwest Region, USDA Forest Service, Vallejo, California, 94592, USA
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Andrus RA, Hart SJ, Tutland N, Veblen TT. Future dominance by quaking aspen expected following short‐interval, compounded disturbance interaction. Ecosphere 2021. [DOI: 10.1002/ecs2.3345] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Affiliation(s)
- Robert A. Andrus
- Department of Geography University of Colorado Boulder Colorado USA
- Department of Forest and Wildlife Ecology University of Wisconsin – Madison Madison Wisconsin USA
| | - Sarah J. Hart
- Department of Forest and Wildlife Ecology University of Wisconsin – Madison Madison Wisconsin USA
| | - Niko Tutland
- Department of Forest and Wildlife Ecology University of Wisconsin – Madison Madison Wisconsin USA
| | - Thomas T. Veblen
- Department of Geography University of Colorado Boulder Colorado USA
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The Fire and Tree Mortality Database, for empirical modeling of individual tree mortality after fire. Sci Data 2020; 7:194. [PMID: 32572035 PMCID: PMC7308274 DOI: 10.1038/s41597-020-0522-7] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/15/2020] [Accepted: 05/14/2020] [Indexed: 11/18/2022] Open
Abstract
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. Measurement(s) | plant morphology trait • tree mortality • fire • tree fire injury • wildfire | Technology Type(s) | digital curation | Factor Type(s) | year of data collection • geographic location of fire • tree fire injury | Sample Characteristic - Organism | trees | Sample Characteristic - Environment | forest ecosystem | Sample Characteristic - Location | Cascades Region • Blue Mountains • Far Northern Rockies • Sierra Nevada • Piedmont Province • Region of Piedmont • Atlantic and Gulf Coastal Plain Floristic Province • Northern Rocky Mountains Provincial Park |
Machine-accessible metadata file describing the reported data: 10.6084/m9.figshare.12369293
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Andrus RA, Hart SJ, Veblen TT. Forest recovery following synchronous outbreaks of spruce and western balsam bark beetle is slowed by ungulate browsing. Ecology 2020; 101:e02998. [PMID: 32012254 DOI: 10.1002/ecy.2998] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/27/2019] [Revised: 12/17/2019] [Accepted: 01/03/2020] [Indexed: 11/08/2022]
Abstract
Understanding how severe disturbances and their interactions affect forests is key to projecting ecological change under a warming climate. Substantial increases in some biotic disturbances, such as bark beetle outbreaks, in temperate forest ecosystemsmay compromise recovery to a forest vegetation type (i.e., physiognomic recovery or resilience), especially if subsequent biotic disturbances (e.g., herbivory) alter recovery mechanisms. From 2005 to 2017, severe outbreaks (>90% mortality) of spruce bark beetles (SB, Dendroctonus rufipennis) affected Engelmann spruce (Picea engelmannii) across 325,000 ha of spruce and subalpine fir (Abies lasiocarpa) forest in the southern Rocky Mountains, USA. Concurrently, an outbreak of western balsam bark beetle (WBBB, Dryocoetes confuses) infested subalpine fir across at least 47,000 of these hectares. We explored the capacity of 105 stands affected by one or two bark beetle outbreaks and browsing of juvenile trees by ungulates to return to a forest vegetation type in the context of pre-outbreak forest conditions and topography. Nine initial forest trajectories (i.e., at least several decades) were identified from four pre-outbreak forest types affected by three biotic disturbances that occurred at different spatial scales and severities. Most stands (86%) contained surviving nonhost adult trees in the main canopy (fir and aspen [Populus tremuloides]) and many surviving juveniles of all species, implying that they are currently on a trajectory for physiognomic recovery. Stands composed exclusively of large-diameter spruce were affected by a severe SB outbreak and were most vulnerable to a transition to a low-density forest, below regional stocking levels (<370 trees/ha). Greater pre-outbreak stand structural complexity and species diversity were key traits of stands with a higher potential for physiognomic recovery. However, all multispecies stands shifted in relative composition of the main canopy to nonhost species, suggesting low potential for compositional recovery over the next several decades. Most post-outbreak stands (86%) exceeded regional stocking levels with trees taller than the browse zone (<2 m). As such, ungulate browsing on over half of all juveniles will primarily affect the rate of infilling of the forest canopy and preferential browsing of more palatable species will influence the composition of the future forest canopy.
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Affiliation(s)
- Robert A Andrus
- Department of Geography, University of Colorado, Boulder, Colorado, 80309, USA
| | - Sarah J Hart
- Department Forest and Wildlife Ecology, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA
| | - Thomas T Veblen
- Department of Geography, University of Colorado, Boulder, Colorado, 80309, USA
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Fire Severity Drives the Natural Regeneration of Cytisus scoparius L. (Link) and Salix atrocinerea Brot. Communities and the Germinative Behaviour of These Species. FORESTS 2020. [DOI: 10.3390/f11020124] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Abstract
Research Highlights: Data indicated that fire severity modulates natural regeneration of Cytisus scoparius and Salix atrocinerea communities and drives much stronger effects on the germination of the dominant species. Background and Objectives: Previous studies demonstrated that fire severity induces different behaviours in plant species. Mother plant age is an important feature that must also be considered in plans of forest restoration. The objectives were to determine, in field studies, the effect of fire severity on the natural regeneration of C. scoparius and S. atrocinerea communities, to know the role of mother plant age on the germination of seeds of C. scoparius and S. atrocinerea, and to quantify their germination response at different levels of fire severity, in laboratory settings. Material and Methods: We have analysed the role of fire severity on the natural regeneration of C. scoparius and S. atrocinerea communities considering cover and height. Forty 30 × 30 m plots were randomly located in C. scoparius and S. atrocinerea communities. Fire severity on the germination of dominant species was tested through different levels of smoke, charcoal, ash, and heat. Results: High severity reduced the vertical cover and growth in height of the two communities and favoured the increase of cover of woody species in the C. scoparius community and herbaceous species in the S. atrocinerea community. Mother plant age determined germination percentages of C. scoparius seeds. Germination of C. scoparius was increased by moderate heat, and heat and smoke; and fire severity greatly reduced germination of S. atrocinerea. Conclusions: The regeneration responses after fire were largely controlled by interactions between the fire severity and the individual species regeneration strategies. For restoration purposes, C. scoparius seeds should be treated with 80 °C and smoke for 10 min, in order to increase germination; however, Salix seeds should be used without treatment and immediately after dispersion.
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Harvey BJ, Andrus RA, Anderson SC. Incorporating biophysical gradients and uncertainty into burn severity maps in a temperate fire‐prone forested region. Ecosphere 2019. [DOI: 10.1002/ecs2.2600] [Citation(s) in RCA: 22] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/04/2023] Open
Affiliation(s)
- Brian J. Harvey
- School of Environmental & Forest Sciences University of Washington Campus Box 352100 Seattle Washington 98195 USA
| | - Robert A. Andrus
- Department of Geography University of Colorado Boulder GUGG 110, 260 UCB Colorado 80309 USA
| | - Sean C. Anderson
- Pacific Biological Station Fisheries and Oceans Canada 3190 Hammond Bay Rd. Nanaimo British Columbia V9T 6N7 Canada
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Hess KA, Cullen C, Cobian-Iñiguez J, Lenske V, Ramthun JS, Magness DR, Bolten JD, Foster AC, Spruce J. Satellite-Based Assessment of Grassland Conversion and Related Fire Disturbance in the Kenai Peninsula, Alaska. REMOTE SENSING 2019; 11. [PMID: 32021702 PMCID: PMC6999731 DOI: 10.3390/rs11030283] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Spruce beetle-induced (Dendroctonus rufipennis (Kirby)) mortality on the Kenai Peninsula has heightened local wildfire risk as canopy loss facilitates the conversion from bare to fire-prone grassland. We collected images from NASA satellite-based Earth observations to visualize land cover succession at roughly five-year intervals following a severe, mid-1990's beetle infestation to the present. We classified these data by vegetation cover type to quantify grassland encroachment patterns over time. Raster band math provided a change detection analysis on the land cover classifications. Results indicate the highest wildfire risk is linked to herbaceous and black spruce land cover types, The resulting land cover change image will give the Kenai National Wildlife Refuge (KENWR) ecologists a better understanding of where forests have converted to grassland since the 1990s. These classifications provided a foundation for us to integrate digital elevation models (DEMs), temperature, and historical fire data into a model using Python for assessing and mapping changes in wildfire risk. Spatial representations of this risk will contribute to a better understanding of ecological trajectories of beetle-affected landscapes, thereby informing management decisions at KENWR.
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Affiliation(s)
- Katherine A. Hess
- NASA DEVELOP National Program, NASA Langley Research Center MS 307, Hampton, VA 23681, USA
- Department of Geographical Sciences, University of Maryland, College Park, MD 20742, USA
- Correspondence: ; Tel.: +1-973-223-4266; ; Tel.: +1-301-614-6529; ; Tel.: +1-907-260-2814
| | - Cheila Cullen
- NASA DEVELOP National Program, NASA Langley Research Center MS 307, Hampton, VA 23681, USA
- NOAA-Crest Center, The City University of New York, Bronx, New York 10453, USA
| | - Jeanette Cobian-Iñiguez
- NASA DEVELOP National Program, NASA Langley Research Center MS 307, Hampton, VA 23681, USA
- Department of Mechanical Engineering, University of California, Riverside; Riverside, CA 92521, USA
| | - Victor Lenske
- NASA DEVELOP National Program, NASA Langley Research Center MS 307, Hampton, VA 23681, USA
- Science Systems and Applications, Inc., 10210 Greenbelt Rd, Lanham, MD 20706, USA
| | - Jacob S. Ramthun
- NASA DEVELOP National Program, NASA Langley Research Center MS 307, Hampton, VA 23681, USA
- Department of Geography, University of South Carolina, Columbia, SC 29208, USA
| | - Dawn R. Magness
- Kenai National Wildlife Refuge, U.S. Fish and Wildlife Service, Soldotna, AK 99669, USA
- Correspondence: ; Tel.: +1-973-223-4266; ; Tel.: +1-301-614-6529; ; Tel.: +1-907-260-2814
| | - John D. Bolten
- Hydrological Sciences Laboratory, NASA Goddard Space Flight Center, Mail Code 617.0, Greenbelt, MD 20771, USA
- Correspondence: ; Tel.: +1-973-223-4266; ; Tel.: +1-301-614-6529; ; Tel.: +1-907-260-2814
| | - Adrianna C. Foster
- Universities Space Research Association, 7178 Columbia Gateway Dr, Columbia, MD 21046, USA
| | - Joseph Spruce
- Science Systems and Applications, Inc., 10210 Greenbelt Rd, Lanham, MD 20706, USA
- NASA Stennis Space Center, Stennis Space Center, MS, United States
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Foster AC, Shuman JK, Shugart HH, Negron J. Modeling the interactive effects of spruce beetle infestation and climate on subalpine vegetation. Ecosphere 2018. [DOI: 10.1002/ecs2.2437] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/01/2023] Open
Affiliation(s)
- Adrianna C. Foster
- NASA Goddard Space Flight Center; 8800 Greenbelt Road Greenbelt Maryland 22071 USA
- Department of Environmental Sciences; University of Virginia; 376 Clark Hall, 291 McCormick Road Charlottesville Virginia 22904 USA
| | - Jacquelyn K. Shuman
- National Center for Atmospheric Research; 1850 Table Mesa Drive Boulder Colorado 80305 USA
| | - Herman H. Shugart
- Department of Environmental Sciences; University of Virginia; 376 Clark Hall, 291 McCormick Road Charlottesville Virginia 22904 USA
| | - Jose Negron
- USDA Forest Service; Rocky Mountain Research Station; 240 West Prospect Fort Collins Colorado 80526 USA
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Mapping Progression and Severity of a Southern Colorado Spruce Beetle Outbreak Using Calibrated Image Composites. FORESTS 2018. [DOI: 10.3390/f9060336] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
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Carlson AR, Sibold JS, Assal TJ, Negrón JF. Evidence of compounded disturbance effects on vegetation recovery following high-severity wildfire and spruce beetle outbreak. PLoS One 2017; 12:e0181778. [PMID: 28777802 PMCID: PMC5544207 DOI: 10.1371/journal.pone.0181778] [Citation(s) in RCA: 22] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2016] [Accepted: 07/06/2017] [Indexed: 11/26/2022] Open
Abstract
Spruce beetle (Dendroctonus rufipennis) outbreaks are rapidly spreading throughout subalpine forests of the Rocky Mountains, raising concerns that altered fuel structures may increase the ecological severity of wildfires. Although many recent studies have found no conclusive link between beetle outbreaks and increased fire size or canopy mortality, few studies have addressed whether these combined disturbances produce compounded effects on short-term vegetation recovery. We tested for an effect of spruce beetle outbreak severity on vegetation recovery in the West Fork Complex fire in southwestern Colorado, USA, where much of the burn area had been affected by severe spruce beetle outbreaks in the decade prior to the fire. Vegetation recovery was assessed using the Landsat-derived Normalized Difference Vegetation Index (NDVI) two years after the fire, which occurred in 2013. Beetle outbreak severity, defined as the basal area of beetle-killed trees within Landsat pixels, was estimated using vegetation index differences (dVIs) derived from pre-outbreak and post-outbreak Landsat images. Of the seven dVIs tested, the change in Normalized Difference Moisture Index (dNDMI) was most strongly correlated with field measurements of beetle-killed basal area (R2 = 0.66). dNDMI was included as an explanatory variable in sequential autoregressive (SAR) models of NDVI2015. Models also included pre-disturbance NDVI, topography, and weather conditions at the time of burning as covariates. SAR results showed a significant correlation between NDVI2015 and dNDMI, with more severe spruce beetle outbreaks corresponding to reduced post-fire vegetation cover. The correlation was stronger for models which were limited to locations in the red stage of outbreak (outbreak ≤ 5 years old at the time of fire) than for models of gray-stage locations (outbreak > 5 years old at the time of fire). These results indicate that vegetation recovery processes may be negatively impacted by severe spruce beetle outbreaks occurring within a decade of stand-replacing wildfire.
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Affiliation(s)
- Amanda R. Carlson
- Graduate Degree Program in Ecology, Colorado State University, Fort Collins, Colorado, United States of America
- Department of Anthropology, Colorado State University, Fort Collins, Colorado, United States of America
- * E-mail:
| | - Jason S. Sibold
- Department of Anthropology, Colorado State University, Fort Collins, Colorado, United States of America
| | - Timothy J. Assal
- U.S. Geological Survey, Fort Collins Science Center, Fort Collins, Colorado, United States of America
| | - Jose F. Negrón
- U.S. Forest Service, Rocky Mountain Research Station, Fort Collins, Colorado, United States of America
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Fire Severity Controlled Susceptibility to a 1940s Spruce Beetle Outbreak in Colorado, USA. PLoS One 2016; 11:e0158138. [PMID: 27438289 PMCID: PMC4954647 DOI: 10.1371/journal.pone.0158138] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2016] [Accepted: 06/10/2016] [Indexed: 11/20/2022] Open
Abstract
The frequency, magnitude, and size of forest disturbances are increasing globally. Much recent research has focused on how the occurrence of one disturbance may affect susceptibility to subsequent disturbances. While much has been learned about such linked disturbances, the strength of the interactions is likely to be contingent on the severity of disturbances as well as climatic conditions, both of which can affect disturbance intensity and tree resistance to disturbances. Subalpine forests in western Colorado were affected by extensive and severe wildfires in the late 19th century and an extensive and severe outbreak of spruce beetle (Dendroctonus rufipennis) in the 1940s. Previous research found that most, but not all, of the stands that burned and established following the late 19th century fires were not susceptible to the 1940s outbreak as beetles preferentially attack larger trees and stands in advanced stages of development. However, previous research also left open the possibility that some stands that burned and established following the 19th century fires may have been attacked during the 1940s outbreak. Understanding how strongly stand structure, as shaped by disturbances of varying severity, affected susceptibility to past outbreaks is important to provide a baseline for assessing the degree to which recent climate change may be relaxing the preferences of beetles for larger trees and for stands in latter stages of structural development and thereby changing the nature of linked disturbances. Here, dendroecological methods were used to study disturbance history and tree age of stands in the White River National Forest in Western Colorado that were identified in historical documents or remotely-sensed images as having burned in the 19th century and having been attacked by spruce beetle in the 1940s. Dendroecological reconstructions indicate that in young post-fire stands only old remnant trees that survived the otherwise stand-replacing fires were killed in the 1940s outbreak. No young post-fire trees (< ca. 128 years) were susceptible to the 1940s outbreak, implying that under the relatively cool and wet conditions of the mid-20th century, susceptibility to and spatial patterns of spruce beetle outbreak were most likely controlled by variations in severity of prior disturbance by fire. This study provides a baseline for comparing linked disturbances under the relatively warmer and drier conditions of recent (e.g. post-1990) outbreaks in order to assess how climate mitigates the degree to which pre-disturbance history and structure affect susceptibility to disturbances.
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