Tree response and mountain pine beetle attack preference,reproduction and emergence timing in mixed whitebark and lodgepole pine stands

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Thinning Jeffrey pine stands to reduce susceptibility to bark beetle infestations in California,U.S.A.

• 1 Bark beetles (Coleoptera: Curculionidae, Scolytinae) are commonly recognized as important tree mortality agents in coniferous forests of the western U.S.A.
• 2 High stand density is consistently associated with bark beetle infestations in western coniferous forests, and therefore thinning has long been advocated as a preventive measure to alleviate or reduce the amount of bark beetle‐caused tree mortality.
• 3 The present study aimed to determine the effectiveness of thinning to reduce stand susceptibility to bark beetle infestations over a 10‐year period in Pinus jeffreyi forests on the Tahoe National Forest, California, U.S.A. Four treatments were replicated three times within 1‐ha square experimental plots. Treatments included thinning from below (i.e. initiating in the smallest diameter classes) to a residual target basal area (cross‐sectional area of trees at 1.37 m in height) of: (i) 18.4 m²/ha (low density thin); (ii) 27.6 m²/ha (medium density thin); (iii) 41.3 m²/ha (high density thin); and (iv) no stand manipulation (untreated control).
• 4 Throughout the present study, 107 trees died as a result of bark beetle attacks. Of these, 71% (75 trees) were Abies concolor killed by Scolytus ventralis; 20.6% (22 trees) were Pinus ponderosa killed by Dendroctonus ponderosae; 4.7% (five trees) were P. jeffreyi killed by Dendroctonus jeffreyi; 1.8% (two trees) were P. jeffreyi killed by Ips pini; 0.9% (one tree) were P. jeffreyi killed by Orthotomicus (= Ips) latidens; 0.9% (one tree) were P. ponderosa killed by both Dendroctonus brevicomis and D. ponderosae; and 0.9% (one tree) were P. jeffreyi killed by unknown causes.
• 5 In the low density thin, no pines were killed by bark beetles during the 10‐year period. Significantly fewer trees (per ha/year) were killed in the low density thin than the high density thin or untreated control. No significant treatment effect was observed for the percentage of trees (per year) killed by bark beetles.

     

Facilitation in bark beetles: endemic mountain pine beetle gets a helping hand

• 1 Endemic populations of the bark beetle Dendroctonus ponderosae attack weakened lodgepole pine (Pinus contorta var. latifolia) trees that are often previously infested by other bark beetle species, such as Pseudips mexicanus.
• 2 The effect of interactions on D. ponderosae was assessed by examining host selection and productivity of D. ponderosae in trees containing P. mexicanus and trees infested solely by D. ponderosae.
• 3 The findings obtained show that D. ponderosae attacked hosts previously occupied by P. mexicanus at greater densities, and offspring emerged earlier compared with hosts infested by D. ponderosae alone. Additionally, D. ponderosae larvae in P. mexicanus‐infested trees were found to require a significantly lower amount of resource to complete development with no loss in size.
• 4 The presence of P. mexicanus may affect host condition, improving the subcortical environment for endemic D. ponderosae, ultimately aiding in population maintenance at low levels. Hosts in this state should be preferentially attacked by D. ponderosae.

     

Effects of fire and spruce beetle outbreak legacies on the disturbance regime of a subalpine forest in Colorado

Aim There is increasing research attention being given to the role of interactions among natural disturbances in ecosystem processes. We studied the interactions between fire and spruce beetle (Dendroctonus rufipennis Kirkby) disturbances in a Colorado subalpine forest. The central questions of this research were: (1) How does fire history influence stand susceptibility to beetle outbreak? And conversely, (2) How does prior occurrence of a beetle outbreak influence stand susceptibility to subsequent fire? Methods We reconstructed the spatial disturbance history in a c. 4600 ha area by first identifying distinct patches in the landscape on aerial photographs. Then, in the field we determined the disturbance history of each patch by dating stand origin, fire scars, dates of mortality of dead trees, and releases on remnant trees. A geographical information system (GIS) was used to overlay disturbance by fire and spruce beetle. Results and main conclusions The majority of stands in the study area arose following large, infrequent, severe fires occurring in c. 1700, 1796 and 1880. The study area was also affected by a severe spruce beetle outbreak in the 1940s and a subsequent low‐severity fire. Stands that originated following stand‐replacing fire in the late nineteenth century were less affected by the beetle outbreak than older stands. Following the beetle outbreak, stands less affected by the outbreak were more affected by low‐severity fire than stands more severely affected by the outbreak. The reduced susceptibility to low‐severity fire possibly resulted from increased moisture on the forest floor following beetle outbreak. The landscape mosaic of this subalpine forest was strongly influenced by the interactions between fire and insect disturbances.     

Intrinsic and extrinsic determinants of mountain pine beetle population growth

1 Mountain pine beetle Dendroctonus ponderosae populations have large, economically significant outbreaks. Density dependence and environmental variability are expected to have important effects on their dynamics. We analysed time series data from an outbreak in the 1930s to determine the relative importance of population density and environmental variability on local population growth rates.
2 Resource depletion occurred rapidly at the scale of 0.4 ha and population growth rates were strongly density dependent. Annual environmental changes did not have detectable effects on population growth rates, leading to the conclusion that intrinsic processes influenced local population density more than extrinsic factors during this outbreak.
3 Our calculated value of r _max (1.16) does not suggest intrinsically cyclic population dynamics. Our estimate of r _max and density dependence will be useful in developing applied models of mountain pine beetle outbreaks, and the subsequent evaluation of management strategies.     

Evaluation of genetic variation of attack and resistance in lodgepole pine in the early stages of a mountain pine beetle outbreak

We examined variation of attack by mountain pine beetle (MPB) (Dendroctonus ponderosae) (Coleoptera: Scolytidae) in a 20-year-old lodgepole pine open-pollinated (OP) family trial composed of 165 OP parent-trees originating from local and nonlocal provenances. Trees were assessed in the summer of 2005 for traits relating to attack, survival, gallery formation, and infection from the fungi associated with MPB, Ophiostoma sp. Successful initial attack was determined by the presence of dead (i.e., red or gray foliage) crowns and the presence of entrance holes from MPB. Eighty-seven percent of the trees still had green crowns in the fall of 2005 (GC05), with family mean differences ranging from 46 to 100%. The mean number of pitch tubes per tree (PT#05) (in a sampling area of 225 cm² at breast height) was 1.5 with a range from 0 to 14 per tree. However, for pitch tubes indicating presence or absence (PTP05) of MPB on each tree, 63% of the trees had pitch tubes and family means ranged from 7 to 100%. Significant levels of genetic variation were found for GC05 and PTP05 with narrow-sense heritabilities of 0.20 (SE = 0.11) and 0.26 (SE = 0.07), respectively. Provenance differences were also significant, indicating that some population structure is present for these “resistance” attributes. The family breeding value correlations between 10-year height growth and PTP05 was 0.22, indicating that at the population level faster growing families may be slightly more subject to attack. An intensive survey of a subsample of 442 trees in the test (selected from 50 families with a range of attack levels) was conducted, and trees were reassessed for the presence of attack, presence of blue stain in the wood, and presence of galleries and egg chambers/eggs in the gallery. Family means ranged from 0 to 57% for blue stain, 11 to 63% for galleries, and 0 to 57% for egg chambers/eggs; however, due to the relatively small sample sizes and the nature of the binary data, family differences were not highly statistically significant. Further research is underway to improve the quantitative genetic parameters and determine the actual mechanisms at work; however, it is clear the MPB does react differently to host genotypes in terms of initial attack.     

Influences of the biophysical environment on blister rust and mountain pine beetle,and their interactions,in whitebark pine forests

Aim To understand how the biophysical environment influences patterns of infection by non‐native blister rust (caused by Cronartium ribicola) and mortality caused by native mountain pine beetles (Dendroctonus ponderosae) in whitebark pine (Pinus albicaulis) communities, to determine how these disturbances interact, and to gain insight into how climate change may influence these patterns in the future. Location High‐elevation forests in south‐west Montana, central Idaho, eastern and western Oregon, USA. Methods Stand inventory and dendroecological methods were used to assess stand structure and composition and to reconstruct forest history at sixty 0.1‐ha plots. Patterns of blister rust infection and mountain pine beetle‐caused mortality in whitebark pine trees were examined using nonparametric Kruskal–Wallis ANOVA, Mann–Whitney U‐tests, and Kolmogorov–Smirnov two‐sample tests. Stepwise regression was used to build models of blister rust infection and mountain pine beetle‐related mortality rates based on a suite of biophysical site variables. Results Occurrence of blister rust infections was significantly different among the mountain ranges, with a general gradient of decreasing blister rust occurrence from east to west. Evidence of mountain pine beetle‐caused mortality was identified on 83% of all dead whitebark pine trees and was relatively homogenous across the study area. Blister rust infected trees of all ages and sizes uniformly, while mountain pine beetles infested older, larger trees at all sites. Stepwise regressions explained 64% and 58% of the variance in blister rust infection and beetle‐caused mortality, respectively, indicating that these processes are strongly influenced by the biophysical environment. More open stand structures produced by beetle outbreaks may increase the exposure of surviving whitebark pine trees to blister rust infection. Main conclusions Variability in the patterns of blister rust infection and mountain pine beetle‐caused mortality elucidated the fundamental dynamics of these disturbance agents and suggests that the effects of climate change will be complex in whitebark pine communities and vary across the species’ range. Interactions between blister rust and beetle outbreaks may accelerate declines or facilitate the rise of rust resistance in whitebark pine depending on forest conditions at the time of the outbreak.     

Observations and modeling of aboveground tree carbon stocks and fluxes following a bark beetle outbreak in the western United States

Bark beetle epidemics result in tree mortality across millions of hectares in North America. However, few studies have quantified impacts on carbon (C) cycling. In this study, we quantified the immediate response and subsequent trajectories of stand‐level aboveground tree C stocks and fluxes using field measurements and modeling for a location in central Idaho, USA that experienced an outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins). We measured tree characteristics in lodgepole pine (Pinus contorta) plots spanning a range of structure and mortality conditions. We then initialized the forest vegetation simulator, an individual tree‐based model, with these measurements and simulated the response of aboveground production of C fluxes as well as trajectories of C stocks and fluxes in the coming decades. Mountain pine beetles killed up to 52% of the trees within plots, with more larger trees killed. C stocks in lodgepole pine were reduced by 31–83% following the outbreak, and plot‐level C fluxes decreased 28–73%. Modeled C stocks increased nearly continuously following the infestation, recovering to preoutbreak levels in 25 years or less. Simulated aboveground tree C fluxes increased following the immediate postoutbreak decrease, then subsequently declined. Substantial variability of C stocks and fluxes among plots resulted from the number and size of killed and surviving trees. Our study illustrates that bark beetle epidemics alter forest C cycling unlike stand‐replacement wildfires or clear‐cut harvests, due in part to incomplete mortality coupled with the preference by beetles for larger trees. The dependency of postoutbreak C stocks and fluxes on stand structure suggests that C budget models and studies in areas experiencing mountain pine beetle disturbances need to include size distribution of trees for the most accurate results.     

Sub‐lethal effects of monoterpenes on reproduction by mountain pine beetles

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Breach of the northern Rocky Mountain geoclimatic barrier: initiation of range expansion by the mountain pine beetle

Aim Our aim is to examine the historical breach of the geoclimatic barrier of the Rocky Mountains by the mountain pine beetle (Dendroctonus ponderosae Hopkins). This recent range expansion from west of the North American continental divide into the eastern boreal forest threatens to provide a conduit to naïve pine hosts in eastern North America. We examine the initial expansion events and determine potential mechanism(s) of spread by comparing spread patterns in consecutive years to various dispersal hypotheses such as: (1) meso‐scale atmospheric dispersal of insects from source populations south‐west of the Rocky Mountains in British Columbia (i.e. their historical range), (2) anthropogenic transport of infested plant material, and (3) spread of insect populations across adjacent stands via corridors of suitable habitat. Location British Columbia, Canada. Methods We explore potential mechanism(s) of invasion of the mountain pine beetle using spatial point process models for the initial 3 years of landscape‐level data collection, 2004–2006. Specifically, we examine observed patterns of infestation relative to covariates reflecting various dispersal hypotheses. We select the most parsimonious models for each of the initial 3 years of invasion using information criteria statistics. Results The initial range expansion and invasion of the beetle was characterized by aerial deposition along a strong north‐west to south‐east gradient, with additional aerial deposition and localized dispersal from persisting populations in following years. Main conclusions Following deposition of a wave front of mountain pine beetles parallel to the Rocky Mountains via meso‐scale atmospheric dispersal, the areas of highest intensity of infestations advanced up to 25 km north‐east towards jack pine (Pinus banksiana) habitat in a single year. There appeared to be no association between putative anthropogenic movement of infested materials and initial range expansion of the mountain pine beetle across the continental divide.     

Defence syndromes in lodgepole – whitebark pine ecosystems relate to degree of historical exposure to mountain pine beetles

Warming climate is allowing tree‐killing bark beetles to expand their ranges and access naïve and semi‐naïve conifers. Conifers respond to attack using complex mixtures of chemical defences that can impede beetle success, but beetles exploit some compounds for host location and communication. Outcomes of changing relationships will depend on concentrations and compositions of multiple host compounds, which are largely unknown. We analysed constitutive and induced chemistries of Dendroctonus ponderosae's primary historical host, Pinus contorta, and Pinus albicaulis, a high‐elevation species whose encounters with this beetle are transitioning from intermittent to continuous. We quantified multiple classes of terpenes, phenolics, carbohydrates and minerals. Pinus contorta had higher constitutive allocation to, and generally stronger inducibility of, compounds that resist these beetle–fungal complexes. Pinus albicaulis contained higher proportions of specific monoterpenes that enhance pheromone communication, and lower induction of pheromone inhibitors. Induced P. contorta increased insecticidal and fungicidal compounds simultaneously, whereas P. albicaulis responses against these agents were inverse. Induced terpene accumulation was accompanied by decreased non‐structural carbohydrates, primarily sugars, in P. contorta, but not P. albicaulis, which contained primarily starches. These results show some host species with continuous exposure to bark beetles have more thoroughly integrated defence syndromes than less‐continuously exposed host species.     

Ability of stress‐related volatiles to attract and induce attacks by Xylosandrus germanus and other ambrosia beetles

• 1 Xylosandrus germanus typically colonizes physiologically‐stressed deciduous hosts but it is increasingly being recognized as a key pest of ornamental nursery stock. We tested the attractiveness of common plant stress‐related volatiles to ambrosia beetles occupying the nursery agroecosystem, as well as their ability to induce attacks on selected trees. Experiments were conducted in Ohio, U.S.A.
• 2 Stress volatile attractiveness was first assessed by positioning traps baited with acetaldehyde, acetone, ethanol and methanol in ornamental nurseries. Cumulative trap counts confirmed that ethanol was the most attractive stress‐related volatile to X. germanus. Methanol‐baited traps were slightly attractive to X. germanus, whereas traps baited with acetaldehyde and acetone were not attractive to any ambrosia beetle.
• 3 A series of tree injection experiments were also conducted to determine the ability of these volatiles to induce attacks by ambrosia beetles under field conditions. Injection of ethanol into Magnolia virginiana induced the largest number of attacks, whereas injection of acetaldehyde induced more attacks than methanol or acetone. Xylosandrus germanus was the most predominant species emerging from M. virginiana injected with each of the stress‐related volatiles. No attacks by wood‐boring beetles were observed on water injected or uninjected control trees.
• 4 Solid‐phase microextraction–gas chromatography–mass spectrometry confirmed the emission of acetaldehyde, acetone, ethanol and methanol after their injection into M. virginiana.
• 5 Xylosandrus germanus has an efficient olfactory‐based mechanism for differentiating among host volatile cues. Injecting select trees with stress‐related volatiles, particularly ethanol, shows promise as a trap tree strategy for X. germanus and other ambrosia beetles.

     

Distribution of early‐arriving saproxylic beetles on standing dead Scots pine trees

• 1 The community composition of early‐arriving saproxylic beetles on 80 standing and recently deceased Scots pine trees (Pinus sylvestris L.) was examined.
• 2 In total, 34 species of saproxylic beetles were found, comprising at least four well‐defined groups of species identified by correspondence analysis.
• 3 Bark thickness, trunk/branch diameter and height above ground significantly affected community composition, with bark thickness being the most important factor.
• 4 Overall, 13.7% of the variance in species composition was explained by section of the tree that was sampled, a variable that encompasses the three aforementioned parameters.

     

Area‐wide application of verbenone‐releasing flakes reduces mortality of whitebark pine Pinus albicaulis caused by the mountain pine beetle Dendroctonus ponderosae

• 1 DISRUPT Micro‐Flake Verbenone Bark Beetle Anti‐Aggregant flakes (Hercon Environmental, Inc., Emigsville, Pennsylvania) were applied in two large‐scale tests to assess their efficacy for protecting whitebark pine Pinus albicaulis Engelm. from attack by mountain pine beetle Dendroctonus ponderosae Hopkins (Coleoptera: Scolytinae) (MPB). At two locations, five plots of equivalent size and stand structure served as untreated controls. All plots had early‐ to mid‐outbreak beetle populations (i.e. 7.1–29.2 attacked trees/ha). Verbenone was applied at 370 g/ha in both studies. Intercept traps baited with MPB aggregation pheromone were placed near the corners of each plot after the treatment in order to monitor beetle flight within the plots. Trap catches were collected at 7‐ to 14‐day intervals, and assessments were made at the end of the season of stand structure, stand composition and MPB attack rate for the current and previous years.
• 2 Applications of verbenone flakes significantly reduced the numbers of beetles trapped in treated plots compared with controls at both sites by approximately 50% at the first collection date.
• 3 The applications also significantly reduced the proportion of trees attacked in both Wyoming and Washington using the proportion of trees attacked the previous year as a covariate in the model for analysis of current year attack rates; in both sites, the reduction was ≥ 50%.
• 4 The flake formulation of verbenone appears to have promise for area‐wide treatment by aerial application when aiming to control the mountain pine beetle in whitebark pine forests.

     

Spatial–temporal analysis of species range expansion: the case of the mountain pine beetle, Dendroctonus ponderosae

Aim The spatial extent of western Canada’s current epidemic of mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae, Scolytinae), is increasing. The roles of the various dispersal processes acting as drivers of range expansion are poorly understood for most species. The aim of this paper is to characterize the movement patterns of the mountain pine beetle in areas where range expansion is occurring, in order to describe the fine‐scale spatial dynamics of processes associated with mountain pine beetle range expansion. Location Three regions of Canada’s Rocky Mountains: Kicking Horse Pass, Yellowhead Pass and Pine Pass. Methods Data on locations of mountain pine beetle‐attacked trees of predominantly lodgepole pine (Pinus contorta var. latifolia) were obtained from annual fixed‐wing aircraft surveys of forest health and helicopter‐based GPS surveys of mountain pine beetle‐damaged areas in British Columbia and Alberta. The annual (1999–2005) spatial extents of outbreak ranges were delineated from these data. Spatial analysis was conducted using the spatial–temporal analysis of moving polygons (STAMP), a recently developed pattern‐based approach. Results We found that distant dispersal patterns (spot infestations) were most often associated with marginal increases in the areal size of mountain pine beetle range polygons. When the mountain pine beetle range size increased rapidly relative to the years examined, local dispersal patterns (adjacent infestation) were more common. In Pine Pass, long‐range dispersal (> 2 km) markedly extended the north‐east border of the mountain pine beetle range. In Yellowhead Pass and Kicking Horse Pass, the extension of the range occurred incrementally via ground‐based spread. Main conclusions Dispersal of mountain pine beetle varies with geography as well as with host and beetle population dynamics. Although colonization is mediated by habitat connectivity, during periods of low overall habitat expansion, dispersal to new distant locations is common, whereas during periods of rapid invasion, locally connected spread is the dominant mode of dispersal. The propensity for long‐range transport to establish new beetle populations, and thus to be considered a driver of range expansion, is likely to be determined by regional weather patterns, and influenced by local topography. We conclude that STAMP appears to be a useful approach for examining changes in biogeograpical ranges, with the potential to reveal both fine‐ and large‐scale patterns.     

Tree‐mediated interactions between the jack pine budworm and a mountain pine beetle fungal associate

1. Coniferous trees deploy a combination of constitutive (pre‐existing) and induced (post‐invasion), structural and biochemical defences against invaders. Induced responses can also alter host suitability for other organisms sharing the same host, which may result in indirect, plant‐mediated interactions between different species of attacking organisms. 2. Current range and host expansion of the mountain pine beetle (Dendroctonus ponderosae Hopkins; MPB) from lodgepole pine‐dominated forests to the jack pine‐dominated boreal forests provides a unique opportunity to investigate whether the colonisation of jack pine (Pinus banksiana Lamb.) by MPB will be affected by induced responses of jack pine to a native herbaceous insect species: the jack pine budworm (Choristoneura pinus pinus Freeman; JPBW). 3. We simulated MPB attacks with one of its fungal associates, Grosmannia clavigera Robinson‐Jeffrey & Davidson, and tested induction of either herbivory by JPBW or inoculation with the fungus followed by a challenge treatment with the other organism on jack pine seedlings and measured and compared monoterpene responses in needles. 4. There was clear evidence of an increase in jack pine resistance to G. clavigera with previous herbivory, indicated by smaller lesions in response to fungal inoculations. In contrast, although needle monoterpenes greatly increased after G. clavigera inoculation and continued to increase during the herbivory challenge, JPBW growth was not affected, but JPBW increased the feeding rate to possibly compensate for altered host quality. 5. Jack pine responses varied greatly and depended on whether seedlings were treated with single or multiple organisms, and their order of damage.     

Soil-surface carbon dioxide efflux and microbial biomass in relation to tree density 13 years after a stand replacing fire in a lodgepole pine ecosystem

The effects of fire on soil‐surface carbon dioxide (CO₂) efflux, F_S, and microbial biomass carbon, C_mic, were studied in a wildland setting by examining 13‐year‐old postfire stands of lodgepole pine differing in tree density (< 500 to > 500 000 trees ha^?1) in Yellowstone National Park (YNP). In addition, young stands were compared to mature lodgepole pine stands (～110‐year‐old) in order to estimate ecosystem recovery 13 years after a stand replacing fire. Growing season F_S increased with tree density in young stands (1.0 µmol CO₂ m^?2 s^?1 in low‐density stands, 1.8 µmol CO₂ m^?2 s^?1 in moderate‐density stands and 2.1 µmol CO₂ m^?2 s^?1 in high‐density stands) and with stand age (2.7 µmol CO₂ m^?2 s^?1 in mature stands). Microbial biomass carbon in young stands did not differ with tree density and ranged from 0.2 to 0.5 mg C g^?1 dry soil over the growing season; C_mic was significantly greater in mature stands (0.5–0.8 mg C g^?1 dry soil). Soil‐surface CO₂ efflux in young stands was correlated with biotic variables (above‐ground, below‐ground and microbial biomass), but not with abiotic variables (litter and mineral soil C and N content, bulk density and soil texture). Microbial biomass carbon was correlated with below‐ground plant biomass and not with soil carbon and nitrogen, indicating that plant activity controls not only root respiration, but C_mic pools and overall F_S rates as well. These findings support recent studies that have demonstrated the prevailing importance of plants in controlling rates of F_S and suggest that decomposition of older, recalcitrant soil C pools in this ecosystem is relatively unimportant 13 years after a stand replacing fire. Our results also indicate that realistic predictions and modeling of terrestrial C cycling must account for the variability in tree density and stand age that exists across the landscape as a result of natural disturbances.     

Performance of the tree‐killing bark beetles Ips typographus and Pityogenes chalcographus in non‐indigenous lodgepole pine and their historical host Norway spruce

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Relationships of subalpine forest fires in the Colorado Front Range with interannual and multidecadal‐scale climatic variation

Aim An understanding of past relationships between fire occurrence and climate variability will help to elucidate the implications of climate‐change scenarios for future patterns of wildfire. In the present study we investigate the relationships between subalpine‐zone fire occurrence and climate variability and broad‐scale climate patterns in the Pacific and Atlantic Oceans at both interannual and multidecadal time‐scales. Location The study area is the subalpine zone of Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa), and lodgepole pine (Pinus contorta) in the southern sector of the Rocky Mountain National Park, which straddles the continental divide of the northern Colorado Front Range. Methods We compared years of widespread fire from AD 1650 to 1978 for the subalpine zone of southern Rocky Mountain National Park, with climate variables such as measures of drought, and indices such as the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Atlantic Multidecadal Oscillation (AMO). Results Years of extensive subalpine‐zone fires are significantly related to climate variability, phases of ENSO, the PDO, and the AMO, as well as to phase combinations of ENSO, the PDO, and the AMO at both interannual and centennial time‐scales. Main conclusions Years of extensive fires are related to extreme drought conditions and are significantly related to the La Niña phase of ENSO, the negative (cool) phase of the PDO, and the positive (warm) phase of the AMO. The co‐occurrence of the phase combination of La Niña‐negative PDO‐positive AMO is more important to fire occurrence than the individual influences of the climate patterns. Low‐frequency trends in the occurrence of this combination of climate‐pattern phases, resulting from trends in the AMO, are the primary climate pattern associated with periods of high fire occurrence (1700–89 and 1851–1919) and a fire‐free period (1790–1850). The apparent controlling influence of the AMO on drought and years of large fires in the subalpine forests of the Colorado Front Range probably applies to an extensive area of western North America.