Climate Change Impacts On Bark Beetle Outbreaks And The .
31SOUTHWESTFIRE SCIENCECONSORTIUMEcological Restoration InstituteWorking Paper No. 31Climate Change Impacts onBark Beetle Outbreaks andthe Impact of Outbreaks onSubsequent FiresJune 2014
Intermountain West Frequent-Fire Forest RestorationEcological restoration is a practice that seeks to heal degraded ecosystems by reestablishingnative species, structural characteristics, and ecological processes. The Society for EcologicalRestoration International defines ecological restoration as “an intentional activity thatinitiates or accelerates the recovery of an ecosystem with respect to its health, integrityand sustainability .Restoration attempts to return an ecosystem to its historic trajectory”(Society for Ecological Restoration International Science and Policy Working Group 2004).Most frequent-fire forests throughout the Intermountain West have been degraded duringthe last 150 years. Many of these forests are now dominated by unnaturally dense thicketsof small trees, and lack their once diverse understory of grasses, sedges, and forbs. Forestsin this condition are highly susceptible to damaging, stand-replacing fires and increasedinsect and disease epidemics. Restoration of these forests centers on reintroducing frequent,low-severity surface fires—often after thinning dense stands—and reestablishing productiveunderstory plant communities.The Ecological Restoration Institute at Northern Arizona University is a pioneer inresearching, implementing, and monitoring ecological restoration of frequent-fire forests ofthe Intermountain West. By allowing natural processes, such as low-severity fire, to resumeself-sustaining patterns, we hope to reestablish healthy forests that provide ecosystemservices, wildlife habitat, and recreational opportunities.The Southwest Fire Science Consortium (SWFSC) is a way for managers, scientists, andpolicy makers to interact and share science. SWFSC’s goal is to see the best available scienceused to make management decisions and scientists working on the questions managersneed answered. The SWFSC tries to bring together localized efforts to develop scientificinformation and to disseminate that to practitioners on the ground through an inclusive andopen process.This publication would not have been possible without funding from the USDA ForestService and the Southwest Fire Science Consortium. The views and conclusions containedin this document are those of the author(s) and should not be interpreted as representing theopinions or policies of the United States Government. Mention of trade names or commercialproducts does not constitute their endorsement by the United States Government or the ERI.Author: Monica GaylordReviewers: Barbara Benz, Chris Fettig, David Huffman, Andrea Thode, and Barb Satink Wolfson, Amy WaltzSeries Editor: Tayloe DubayPlease use the following citation when referring to this Working Paper:Gaylord, M.L. 2014. Climate Change Impacts on Bark Beetle Outbreaks and the Impact of Outbreaks onSubsequent Fires. ERI Working Paper No. 31. Ecological Restoration Institute and Southwest Fire ScienceConsortium, Northern Arizona University: Flagstaff, AZ. 7 p.
Table of ContentsIntroduction.1The Insect.1Ecological Role and Impacts.1Climate Change and Bark Beetle Outbreaks.1Direct Impacts of Temperature on Bark Beetles .2Impact of Temperature Changes on Range Expansion orContraction of Bark Beetles.2Indirect Effects on Bark Beetles: Impact of Climate Change on The Host.2Conclusions.3Management Implications.3Impact of Beetle Outbreaks on Future Fires.3Changes in Fuels Post-Outbreaks.4Changes in Fire Behavior Post-Outbreaks.4Attacked Stands Versus Unattacked Stands.4Research from Southwestern Forests.5Management for Post-Beetle Outbreaks.5Conclusions.5Management Implications.6References.6
IntroductionThe InsectBark beetles are small insects that can have profoundimpacts on forests. While some species of bark beetlesprimarily attack trees that are recently dead or dying, othersattack live, vigorous trees and can cause tree mortalityacross extensive areas. The Southwest is home to multiplespecies of bark beetles (Gaylord et al. 2006, Williams et al.2008). In southwestern ponderosa pine forests the mostnotable bark beetle species belong to the genus Ips orDendroctonus.1Ecological Role and ImpactsBark beetles and fire share similar roles in southwesternponderosa pine forests. Similar to fire, bark beetlesare natural disturbance agents and help with nutrientredistribution. Tree mortality from bark beetles helpswith snag formation, providing vital habitat for cavitynesting birds and bats, among other wildlife. Barkbeetles are a food source for other insects and birds. Atlow bark beetle populations, tree mortality ranges fromindividual trees to small groups of trees leading to gapformation and increasing forest heterogeneity. Epidemics,or outbreaks, are also part of the natural cycle. In someecosystems, such as mountain pine beetle in lodgepolepine, the forests are homogenized after these outbreaks,helping to perpetuate lodgepole pine stands (Brown 1975).Historically, epidemics would subside due to excessivelycold temperatures or because suitable host material (treespecies or preferred diameter range) becomes depleted.Current bark beetle outbreaks in some forest systemsare considered unprecedented (Raffa et al. 2008, Bentzet al. 2009). Recent outbreaks appear to be driven bywarmer/drier climate, an abundance of overstockedand homogeneous stands caused by past managementdecisions, or a combination of these and other factors(Raffa et al. 2008, Bentz et al. 2010, Fettig et al. 2013). Inessence, large-scale outbreaks are a result of many of thesame factors driving catastrophic fires and, consequently,both disturbance agents are having impacts on ecosystemsthat are often outside of their historic range of variability.Key Information about Bark Beetles Native to pine ecosystems in the SouthwestNatural disturbance agentsRange in size from 0.008-0.33 inches (2.0-8.3mm)Adults are black to reddish brown in colorKill trees by using a mass attack strategy thatcan overwhelm the tree’s defensive capabilitiesLifecycle depends on species and location; canhave multiple generations per year or only onegeneration every two yearsActive when ambient temperatures begin toexceed 59 F (15 C), although this varies byspeciesMajority of the lifecycle occurs under the barkof the treeBark beetle success is measured by its ability toreproduce, for most beetle species tree death isa prerequisite for beetle successEvidence of attack Resin is a tree’s primary defense against beetles Pitch tubes will form on the tree bole (Fig. 1) Woodpeckers often flake off the outer portionof bark Sawdust-like material (frass) will collect inbark crevices (Fig. 1) Needles often do not turn yellow until manymonths after attack Beetles create “galleries” on the surface of thewood and in the bark (Fig. 2)temperature will likely have a direct impact on the beetleitself whereas water stress (exacerbated by increasedtemperatures and periods of drought) will indirectlyimpact the beetle via impacts on the host trees (Bentz et al.2010).Climate Change and Bark Beetle OutbreaksHistorically, tree mortality from bark beetles insouthwestern forests is often correlated with drought events(Kleinman et al. 2012, USDA 2013). Climate models forthe southwestern U.S. forecast more frequent and severedrought events (Seager et al. 2007). Given past associationsbetween drought events and bark beetle outbreaks itappears likely that bark beetle-driven tree mortality couldalso increase. In addition to water deficits, most models forthe Southwest indicate this area will experience increasedtemperatures (Seager et al. 2007). During the most recentdrought event in the Southwest (2002), temperatureswere warmer than normal (Breshears et al. 2005). Bothof these factors (higher temperatures and increased waterstress) could impact bark beetle populations. Changes inFigure 1: Pitch tube and frass in bark crevices on a ponderosa pine tree.CLIMATE CHANGE IMPACTS ON BARK BEETLE OUTBREAKS AND THE IMPACT OF OUTBREAKS ONSUBSEQUENT FIRESEcological Restoration Institute
already occur in their respective ranges, they may not be aswell defended against the new beetle species. Furthermore,recent work in Canada suggests that host trees growing inregions that historically have been climatically inhospitableto bark beetles, may be less resistant to beetle attacks thantrees of the same species growing in areas where outbreakshave historically occurred (Cudmore et al. 2010). Thus,with climate change beetles may move to new geographicareas where hosts may be less well-defended, leading toincreased beetle populations. Conversely, if suitable hostsare restricted to a smaller range, or are not able to surviveunder changing climate conditions, the beetles may perishdue to the loss of their food source (Williams and Liebhold2002).Figure 2: Bark beetle galleries.Direct Impacts of Temperature on Bark BeetlesBecause temperature is a major driver of their physiologicalprocesses, all bark beetle species will be affected in someway by climate change. Depending on the species and theirgeographic location, the generation time for beetles canrange from multiple generations per year to one generationevery one to two years. Increased temperatures generallyaccelerate the rate of bark beetle development. Accelerateddevelopment may have positive or negative impactson beetle populations depending on the circumstances(reviewed in Bentz et al. 2010). For instance, quickerbeetle development and warmer temperatures earlierin the spring and/or later in the fall could allow beetlesto increase the number of generations completed. Thiscould allow for faster buildups of populations and agreater likelihood of beetle epidemics. On the other hand,accelerated development could cause increased beetlemortality. Because some stages of beetle development aremuch cold hardier than others (Bentz and Mullins 1999,Tran et al. 2007), accelerated development may cause thebeetles to enter winter in a developmental stage that is notas resistant to freezing temperatures. This could lead to adecline in beetle populations due to higher mortality ratesduring winter. Similarly, beetles with only one generation ayear are dependent on a relatively synchronous emergencein order to mass attack trees. Loss of synchrony couldoccur due to changing temperatures and this could bedetrimental to these beetle species (Logan and Bentz 1999,Bentz et al. 2010).Impact of Temperature Changes on Range Expansion orContraction of Bark BeetlesIncreased temperatures could also allow beetles to spreadfarther north or to higher elevations than their historicrange (Logan and Bentz 1999, Williams and Liebhold 2002,Tran et al. 2007, Sambaraju et al. 2011). Beetles enteringnew geographic regions will encounter different beetle andpredator complexes and it is unknown how these novelinteractions will be resolved. In addition, beetles may beencountering new hosts. While these hosts likely have welldeveloped defenses against those bark beetle species thatEcological Restoration InstituteIndirect Effects on Bark Beetles: Impact of Climate Change onthe HostAn additional indirect impact of climate change on beetlepopulations is the impact warming temperatures anddroughts could have on the host tree. Trees that are waterstressed may be more susceptible to bark beetle attacks.Drought has well-established physiological impacts on trees(McDowell et al. 2008, McDowell 2011). In both ponderosaand piñon pine trees, water stress has been shown to reduceor even shut down photosynthesis, this leads to less carbonassimilation and eventual carbon limitation (Gaylord et al.2007, Plaut et al. 2012). Resin is the first line of defense thatbark beetles encounter when attacking a tree. Since resin iscarbon based, lower amounts of carbon assimilation by thetree is hypothesized to lead to less resin production (Hermsand Mattson 1992, McDowell et al. 2008). Lethal barkbeetle attacks also can occur during drought due to reasonsother than lower resin production. For instance, volatile andauditory emissions from trees during drought may attractbeetles (Mattson and Haack 1987, Kelsey et al. 2014). Infact, a recent study in piñon pine in New Mexico found thatplots subjected to an experimental drought treatment hada much higher percentage of tree mortality than ambientand irrigated plots. The majority of piñons that died in thisstudy had evidence of piñon ips attacks, despite the factthat resin volume did not decline (Gaylord et al. 2013).After beetle populations have built up in stressed trees,some beetle populations, such as mountain pine beetle andspruce beetle begin to attack and kill even vigorous trees,while other outbreaks, such as piñon ips, subside once thetrees are no longer stressed.Past management actions may increase the impacts ofclimate change. For example, in ponderosa pine forests ofthe Southwest, years of fire suppression have led to forestswith higher stand densities; these increased densities alsolead to increased competition among trees, particularlyfor limited resources such as water (Kolb et al. 1998).Increased temperatures could increase the level of waterstress the trees experience (i.e., increased evaporation andtranspiration with increased temperatures), that couldaccelerate the rate of tree mortality (Williams et al. 2013).CLIMATE CHANGE IMPACTS ON BARK BEETLE OUTBREAKS AND THE IMPACT OF OUTBREAKS ONSUBSEQUENT FIRES2
3Conclusions Climate change will most likely increase the frequencyand severity of bark beetle outbreaks in the Southwest. Increased beetle populations could occur due to theshortened generation times and longer developmentalperiods caused by increasing temperatures. Howevernot all species will be able to successfully adapt to theincreased temperatures (Bentz et al. 2014). Drought and increased temperatures will likely lead tomore stressed trees and thus more susceptible hosts. Due to warming temperatures, beetles may be ableto expand their range and encounter trees with lowerresistance to bark beetle attacks. Beetles distribution is dependent on the range of suitablehost trees. If the suitable host-tree range contracts due toclimate change, the bark beetle range will also contract.Conversely, if the host-tree range expands, the beetlerange may also expand. In addition, while we understand some of theinteractions and feedbacks that might occur betweenbark beetles and their host, it is impossible to model allof the interactions that occur in an ecosystem, such asthe impact of climate change on bark beetle predators orcompeting herbivores. Furthermore, when bark beetlesenter the trees they bring along specific fungi as well asother organisms including mites and nematodes. Manyof these other organisms are thought to play a role inbeetle success. How these other organisms respond tochanging temperatures could also influence the impact ofclimate change on beetle success (Six et al. 2011).Management Implications Thinning appears to reduce water stress in southwesternforests, thus thinning treatments may help to mitigatesome of the impacts of climate change (reviewed in Fettiget al. 2007). Managers should attempt to maintain or improve forestresilience through restoration, such as mechanicalthinning and prescribed burning, or other managementactivities. These preventative treatments are the bestoption. After bark beetle populations build up, beetlesare much more difficult to control. Options includesanitation, salvage, and the use of insecticides andsemiochemicals. These methods are generally only usedon a small scale for individual, high-value trees or areasand are generally impractical at the landscape level(Gaylord 2014).Impact of Beetle Outbreaks on Future FiresLandscape-level tree mortality from bark beetle epidemicsis a concern for land managers in terms of future firerisk and behavior. The widely held assumption has beenthat bark beetle associated tree mortality increases firerisk and severity. Intuitively this makes sense. Hightree mortality leads to an increase in dry fuels on thelandscapes. However, several recent papers (Romme etal. 2006, Kulakowski and Veblen 2007, Jenkins et al. 2008,Bond et al. 2009, DeRose and Long 2009, Klutsch et al.2011, Simard et al. 2011, Jenkins et al. 2012) as well ascomprehensive literature reviews (Hicke et al. 2012, Jenkinset al. 2014) have suggested that the relationship is not asstraightforward as first thought. What is evident is thattree mortality from bark beetles is likely to impact fire riskand behavior for many years after the actual outbreak hasended.After a beetle outbreak, affected stands will undergochanges in tree species, fuel structure, fuel type, and canopyand surface fuel loads. All of these factors can impactfuture fire risk and behavior. Many different researchershave attempted to model the impact of the tree mortalityand stand regrowth on future fires. Although there issome disagreement among the different models, there isgeneral consensus that stand fire susceptibility does notincrease or decrease in a strictly linear fashion post-barkbeetle outbreaks (reviewed in Hicke et al. 2012). Timesince outbreak appears to be an important variable indetermining stand susceptibility to fire post-outbreak;however, because different stand types will change atdifferent rates post-outbreak (rate of needle drying, snagfall, and understory regrowth are all variable and depend onmultiple factors), most studies will address what “phase” astand is in post-outbreak in addition to, or rather than just,“years” post-fire (Box 1).Green phaseHealthy, uninfested treesRed phaseNeedles still on the tree.Low foliar moisture content*Note: Needle color depends on treespecies, may be greenishGray phaseAfter needle dropTrees appear grayFine twigs still remain on trees early inthis phase, but will drop in the latter partOlder or OldphaseSnags fallUnderstory vegetation increasesSeedlings establishBox 1. Stands are categorized in “phases” in relation to bark beetleoutbreaks (from Hicke et al. 2012).There are also some acknowledged shortcomingsof modeling potential fire in stands post-bark beetleepidemics (Jenkins et al. 20
well defended against the new beetle species. Furthermore, recent work in Canada suggests that host trees growing in regions that historically have been climatically inhospitable to bark beetles, may be less resistant to beetle attacks than trees of the same species growing in areas where outbreaks have historically occurred (Cudmore et al. 2010).
2.2 Impacts of sea-level rise and remote climate 9 2.3 Impacts of extreme weather events 11 3. Climate impacts on food security 15 3.1 Climate change and food production 15 3.2 Climate change impacts on food access and livelihoods 16 3.3 Climate change impacts on nutrition and utilis
1 Key Climate Change Impacts and Adaptation—Fossil Fuel Extraction and Transport 10 2 Key Climate Change Impacts and Adaptation—Thermal Power 14 3 Temperature Effect on Nuclear Power Efficiency 15 4 Key Climate Change Impacts and Adaptation—Nuclear Power 18 5 Key Climate Change Impacts and Adaptation—Hydropower 24 .
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