Elsevier

Forest Ecology and Management

Volume 262, Issue 9, 1 November 2011, Pages 1686-1696
Forest Ecology and Management

The influence of mountain pine beetle outbreaks and drought on severe wildfires in northwestern Colorado and southern Wyoming: A look at the past century

https://doi.org/10.1016/j.foreco.2011.07.016Get rights and content

Abstract

Outbreaks of bark beetles and drought both lead to concerns about increased fire risk, but the relative importance of these two factors is the subject of much debate. We examined how mountain pine beetle (MPB) outbreaks and drought have contributed to the fire regime of lodgepole pine forests in northwestern Colorado and adjacent areas of southern Wyoming over the past century. We used dendroecological methods to reconstruct the pre-fire history of MPB outbreaks in twenty lodgepole pine stands that had burned between 1939 and 2006 and in 20 nearby lodgepole pine stands that were otherwise similar but that had not burned. Our data represent c. 80% of all large fires that had occurred in lodgepole pine forests in this study area over the past century. We also compared Palmer Drought Severity Index (PDSI) and actual evapotranspiration (AET) values between fire years and non-fire years. Burned stands were no more likely to have been affected by outbreak prior to fires than were nearby unburned stands. However, PDSI and AET values were both lower during fire years than during non-fire years. This work indicates that climate has been more important than outbreaks to the fire regime of lodgepole pine forests in this region over the past century. Indeed, we found no detectable increase in the occurrence of high-severity fires following MPB outbreaks. Dry conditions, rather than changes in fuels associated with outbreaks, appear to be most limiting to the occurrence of severe fires in these forests.

Highlights

► We compare pre-fire history of outbreaks in burned and unburned lodgepole pine stands. ► Over the past century, burned stands were no more likely to be affected by outbreaks. ► PDSI and AET values were both lower during fire years than during non-fire years. ► Drought has been more important than outbreaks to the fire regime in these forests.

Introduction

Across western North America ongoing outbreaks of bark beetles (Dendroctonus spp.) have killed trees over hundreds of thousands of square km, especially in forests dominated by lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia Engelm) (Raffa et al., 2008). These outbreaks have high economic costs and have led to widespread concern among the public, managers, and policy makers about the increased risk of forest fires. However, there is ongoing debate about how important outbreaks actually are to fire risk in contrast to the potentially overriding influence of climate and fire weather on the fire regime of these forests.

Bark beetles (Dendroctonus spp.) are native to the forests of western North America. Under endemic conditions they normally infest and kill only the largest and most stressed trees that are scattered throughout a forest (Safranyik, 2004). However, when climate warms and forest conditions are appropriate, populations can to epidemic size. Under such outbreak conditions beetles mount pheromone-mediated mass attacks that overwhelm the natural defenses of healthy trees and can kill lodgepole pine over many thousands of hectares (Raffa, 2001; Raffa and Berryman, 1983; Safranyik, 2004).

Despite the long-standing expectation that bark beetle outbreaks increase the risk of wildfires, recent research has shed doubt on this expectation for some forest types. In considering this topic, it is important to distinguish “fuel hazard”, which normally refers to the fuel complex (type, volume, arrangement) that determines the probability of ignition and resistance to control regardless of the fuel type’s weather-influenced moisture content (Hardy, 2005) versus “fire risk”, which refers to the chance that a fire might start based on all causative agents (i.e. fuel hazard, ignition source and weather). Thus, we use “risk” to refer to the probability of a fire event actually occurring. Several studies in Colorado have generally concluded that outbreaks of spruce beetle (Dendroctonus rufipennis Kirby. Coleoptera: Scolytidae) in forests dominated by Engelmann spruce (Picea engelmannii Parry ex Engelm) and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) do little or nothing to increase the risk of wildfires. For example, after a 1940s spruce beetle outbreak that killed trees over thousands of hectares of subalpine forests in Colorado, there was no increase in the numbers of fires compared to unaffected spruce-fir forests (Bebi et al., 2003). Furthermore, beetle-affected stands more susceptible to a low-severity fire that spread through adjacent were not forest several years after the outbreak subsided (Kulakowski et al., 2003). During the severe drought of 2002, large fires affected extensive areas of Colorado, including some spruce-fir stands that were previously affected by the 1940s outbreak of spruce beetle, but the outbreak had only a minor influence on the behavior of this fire (Bigler et al., 2005). Likewise, outbreaks of spruce beetle that were ongoing at the time had no detectable effect on the extent or severity of fires that occurred in 2002 in Colorado (Kulakowski and Veblen, 2007). These empirical findings are consistent with modeling studies that predict reductions in the probability of active crown fire after spruce beetle outbreaks due to reductions in canopy bulk density (Derose and Long, 2009, Jenkins et al., 2008, Page and Jenkins, 2007b). However, modeling studies should be reviewed with caution since many crown fire models greatly underestimate fire potential (Cruz and Alexander, 2010).

Instead of outbreaks being the most important determinants of fires in spruce-fir forests, climatic conditions appear to have an overriding effect on fire regimes in these ecosystems. It is well established that in spruce-fir forests, extensive fires are highly dependent on infrequent, severe droughts (Buechling and Baker, 2004, Schoennagel et al., 2007, Sibold et al., 2006, Sibold and Veblen, 2006). Under such extreme drought conditions, increased dead fuels from bark beetle outbreaks appear to play only a minor role, if any, in increasing fire risk.

Although the evidence is compelling that outbreaks of spruce beetles do not substantially increase the risk of wildfires in spruce-fir forests, it is less clear what effect outbreaks of mountain pine beetle (MPB; Dendroctonus ponderosae Hopk. Coleoptera: Scolytidae) have on the fire regime of lodgepole pine forest. In contrast to spruce-fir forests, mature lodgepole pine forests often tend to be less dense and located in more xeric settings, and thus the fire risk in these forests may be more sensitive to changes in fuels such as those brought about by outbreaks. Outbreaks of mountain pine beetle have been shown to clearly alter fuel structures (Klutsch et al., 2009, Page and Jenkins, 2007a, Simard et al., 2011), but the actual effects of these changes in fuels on subsequent fire risk and behavior have not yet been fully elucidated.

Lodgepole pine stands affected by MPB outbreaks in 1972–1975 had an 11% higher probability of burning in the 1988 Yellowstone fires but stands that were affected by outbreaks in 1980–1983 were not more likely to have burned in the same fire (Lynch et al., 2006), which is consistent with modeling studies of predicted fire behavior following outbreaks (Page and Jenkins, 2007a, Page and Jenkins, 2007b, Jenkins et al., 2008). Kulakowski and Veblen (2007) found that outbreaks of MPB that were ongoing at the time did not affect the extent or severity of a wildfire in 2002 in Colorado and suggested that changes in fuels brought about by outbreaks may be overridden by climatic conditions. Likewise, following outbreaks in Yellowstone National Park (YNP) canopy moisture content and canopy bulk density were both reduced (Simard et al., 2011). Although the canopy was drier immediately after outbreaks in YNP, simulation modeling found no increase in fire risk because of the more important effect of reductions in canopy bulk density (Simard et al., 2011). In Colorado, West (2010) examined the actual occurrence of fires following outbreak and found that MPB-caused mortality of lodgepole pine in the 1980s has not contributed to an increase in fire frequency over the subsequent 25 years.

In contrast to the effect of outbreaks, there is strong evidence linking severe forest fires in lodgepole pine forests in Colorado to drought conditions (Schoennagel et al., 2007, Sibold et al., 2006, Sibold and Veblen, 2006) and in other ecosystems real-time fire weather has been shown to be of greater importance than fuels to fire occurrence (Bessie and Johnson, 1995). The fire regime of lodgepole pine forests in the Southern Rocky Mountains has primarily been characterized by large, infrequent, and severe (i.e. primarily stand-replacing) fires that have been associated with drought. In southern Wyoming, a fire rotation of 182 years and a mean fire interval of 31 years were reported (Kipfmueller and Baker, 2000). Similarly, in lodgepole pine forests in Rocky Mountain National Park, a fire rotation of 162–216 years and a mean fire interval of 26–41 years were reported (Sibold et al., 2006). Sibold et al. (2006) also found that the percent of patches that had burned and that were <100 ha in extent accounted for 19–33% of all patches and that the median size of large (>100 ha) fires varied from 282 to 836 ha, depending on location. Although low-severity surface fires do sometimes occur in subalpine forests, they affect a very small portion (e.g. <3%) of the landscape (Sibold et al., 2006). The large severe fires that have shaped lodgepole pine forests of Colorado over the past several centuries have occurred during short-term periods of significant drought (negative PDSI), which have been associated with extreme cool (negative) phases of El Nino Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO) and positive departures from mean Atlantic Multi-decadal Oscillation (AMO) values (Schoennagel et al., 2007, Sibold and Veblen, 2006).

Emerging research does not support the hypothesis that outbreaks of MPB increase the risk of wildfires in lodgepole pine forests but does suggests that fire risk is strongly associated with climatic conditions. However, our current understanding of how outbreaks influence the risk and spread of fire is based primarily on modeling studies and empirical studies that have only examined individual fire events or changes in fuels (but see West, 2010). There is a paucity of studies that have examined the degree to which outbreaks have been associated with actual wildfires in lodgepole pine forest over a long period of time. The central question of the present study was: did stands of lodgepole pine that burned in stand-replacing fires over the past century have a different history of pre-fire MPB outbreaks than nearby stands that did not burn?

Section snippets

Study area

The twenty study sites are located in a c. 150 km by 150 km area of western Colorado and adjacent areas of southern Wyoming (Fig. 1). Sites are dominated by lodgepole pine and range in elevation from 2420 m to 3017 m. High severity fires occurred at these sites between 1939 and 2006 and varied in size from 8 to 4094 ha (USDA Forest Service unpublished data). Median fire size was 263 ha. Over the past centuries forests dominated by lodgepole pine in this region have been characterized by infrequent

Site selection

USDA Forest Service maps and documents identified a total of 25 fires >8 ha that had occurred in lodgepole pine forests in Colorado and southern Wyoming over the past century (USDA Forest Service unpublished data). We set out to sample all 25 of these burned stands, but during the course of field work we were unable to locate evidence of several of the smaller fires and thus collected data at only 20 sites, each of which included pairs of burned and unburned stands (Fig. 1). Nevertheless, it is

Reconstruction of disturbance history

Data on all sampled stands are listed in Table 1 and data from select stands are displayed in Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, which illustrate how dendroecological data were used to reconstruct disturbance history. Dendroecological data from all stands are not displayed in figures due to space constraints. Based on coincident establishment dates, stands originated between the 1620s and the 1900s (Table 1, and Fig. 2, Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7). Although the goal of

Effect of outbreaks and drought on fire occurrence

This study suggests that over the past century drought has been more important than MPB outbreaks in determining the occurrence of severe fires in lodgepole pine forests of northwestern Colorado and adjacent areas of southern Wyoming. Stands that were burned in high-severity fires were no more likely to have been affected by outbreaks than nearby stands that did not burn. However, years during which fires occurred were significantly drier (based both on PDSI and AET) than years during which

Acknowledgements

This work was supported by the National Science Foundation under Awards DEB 0743351 and 0743498. For research assistance we thank C. Griffin, M. Trivette, C. Kriso, J.M. Smith, and H. Westermann. For helpful comments on an earlier draft of this paper we thank T. Veblen and D. West.

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