Ecological Applications, 15(4), 2005, pp. 1317±1330 q 2005 by the Ecological Society of America IMPACTS OF LARGE-SCALE ATMOSPHERIC±OCEAN VARIABILITY ON ALASKAN FIRE SEASON SEVERITY PAUL A. DUFFY,1,5 JOHN E. WALSH,2 JONATHAN M. GRAHAM,3 DANIEL H. MANN,4 AND T. S COTT RUPP1 1Ecological Dynamics Modeling Group, Department of Forest Sciences, University of Alaska, Fairbanks, Alaska 99775 USA 2International Arctic Research Center, Fairbanks, Alaska 99775 USA 3Department of Mathematical Sciences, University of Montana, Missoula, Montana 59812 USA 4Institute of Arctic Biology, University of Alaska, Fairbanks, Alaska 99775 USA Abstract. Fire is the keystone disturbance in the Alaskan boreal forest and is highly in¯uenced by summer weather patterns. Records from the last 53 years reveal high vari- ability in the annual area burned in Alaska and corresponding high variability in weather occurring at multiple spatial and temporal scales. Here we use multiple linear regression (MLR) to systematically explore the relationships between weather variables and the annual area burned in Alaska. Variation in the seasonality of the atmospheric circulation±®re linkage is addressed through an evaluation of both the East Paci®c teleconnection ®eld and a Paci®c Decadal Oscillation index keyed to an annual ®re index. In the MLR, seven explanatory variables and an interaction term collectively explain 79% of the variability in the natural logarithm of the number of hectares burned annually by lightning-caused ®res in Alaska from 1950 to 2003. Average June temperature alone explains one-third of the variability in the logarithm of annual area burned. The results of this work suggest that the Paci®c Decadal Oscillation and the East Paci®c teleconnection indices can be useful in determining a priori an estimate of the number of hectares that will burn in an upcoming season. This information also provides insight into the link between ocean±atmosphere interactions and the ®re disturbance regime in Alaska. Key words: Alaska boreal forest; East Paci®c teleconnection; ecological disturbance regimes; ®re regimes; multiple linear regression; Paci®c Decadal Oscillation; teleconnections. INTRODUCTION tivities and developing a better understanding of how forecast climate change might impact the dominant dis- The boreal forest covers 12 3 106 km2 of the northern turbance mechanism. hemisphere and contains roughly 40% of the world's Within the North American boreal forest, Interior reactive soil carbon, an amount similar to that held in Alaska (i.e., the region between the Alaska and Brooks the atmosphere (Melillo et al. 1993, McGuire et al. Ranges) contains 56 3 106 burnable hectares and in- 1995, IPCC 2001). The biophysical phenomena af- cludes the largest national parks and wildlife refuges fecting carbon storage and high-latitude albedo make in the United States. Most of this huge area is roadless. the boreal forest an integral component of the global climate system (IPCC 2001). Fire-initiated succession For the period 1950±2003, wildland ®res burned an underlies the biophysical factors, and there is a pressing average of roughly 270 000 ha in Interior Alaska each need to characterize sensitivities and potential respons- year and they routinely threaten the lives, property, and es of the boreal forest disturbance regime to climatic timber resources of the sparse but growing population change (Schimel et al. 1997, Gower et al. 2001, Chapin (see Plate 1). Wildland ®res can threaten human values, et al. 2003). The impact of forecast climatic warming yet they play a crucial role in the maintenance of In- on ®re regimes in North America varies from a pre- terior Alaskan ecosystems. Despite the pervasive eco- diction for increased burning for Alaska and Canada nomic and ecological impacts, fundamental aspects of (Flannigan et al. 2000, 2001) to reduced ®re frequency the ®re regime in Interior Alaska are poorly understood. in eastern Canada (Carcaillet et al. 2001). Quanti®ca- Fire regimes consist of many components including tion of the links between climate and ®re in Alaska is frequency, duration, intensity, severity, seasonality, ex- not only essential for understanding the dominant land- tent, and spatial distribution. When complicating fac- scape-scale disturbance processes in Alaska, but it is tors such as interactions with other components of the also a valuable tool for planning ®re management ac- ecosystem (e.g., human impacts, weather, vegetation) and the importance of spatial and temporal scales are taken into account, the characterization of a ®re regime Manuscript received 12 May 2004; revised 4 August 2004; accepted 13 September 2004; ®nal version received 6 December requires a tremendous amount of data and appropriate 2004. Corresponding Editor: M. Friedl. analysis. One of the most basic aspects of a ®re regime 5 E-mail: [email protected] is the ®re cycle (i.e., the ®re recurrence interval for an 1317 1318 PAUL A. DUFFY ET AL. Ecological Applications Vol. 15, No. 4 PLATE 1. The Long Creek ®re of 2002, near Ruby, Alaska. Fire is the dominant landscape-scale disturbance mechanism in Interior Alaska. Photo credit: T. S. Rupp. area equivalent to the study area). There are only a few Alaska reveal that shifts in vegetation (i.e., increased ®eld studies from Interior Alaska that utilize ®re-scar dominance of Picea mariana) around 2400 yr BP are and/or tree age distributions to infer ®re cycle (Yarie associated with a corresponding shift in the ®re regime 1981, DeVolder 1999, Mann and Plug 1999). These (Lynch et al. 2003). The implication is that climatic studies were scattered over a region the size of Montana change occurring at decadal to centennial timescales and show that ®re cycles in Alaska are probably .250 in¯uenced the ®re regime through shifts in dominant years in the relatively moist, southern parts of the state vegetation. This yields the counterintuitive result that (D. H. Mann, personal communication), 80±100 years cooler and moister climate results in higher ®re fre- near Fairbanks in the central Interior (Mann et al. quency due to the increased dominance of the relatively 1995), and ,80 years for the Porcupine River valley more ¯ammable Picea mariana. This response appears in the northeastern portion of the state (Yarie 1981). to extend outside Interior Alaska to forests south of the These estimates are consistent with the results of Kas- Alaska Range as well (Lynch et al. 2004). These studies ischke et al. (2002), who used ®re perimeter data (from provide evidence that climatically induced shifts in aerial photography and remotely sensed data) from the dominant vegetation within the boreal forest over a past ®ve decades to estimate ®re cycles for different period of decades to centuries can potentially modify ecoregions of the interior. The uncertainty associated the ®re regime. Outside of Alaska in the Canadian bo- with these ®re cycle estimates is unknown. real forest, there is evidence that, on timescales of hun- At longer temporal scales but somewhat more coarse dreds to thousands of years, climate has a more direct resolution, charcoal and pollen analyses from varved in¯uence on ®re regime (Carcaillet and Richard 2000, lake sediments reveal critical information about the ®re Carcaillet et al. 2001). Hence, climate differentially frequency and interactions between ®re, climate, and exerts in¯uences on both vegetation composition and vegetation. Due to the limited dispersal of charcoal ®re regime, depending on the location within the boreal particles that are used in these analyses, the results forest as well as the resolution of the timescale of in- apply to limited spatial scales. Within Interior Alaska, terest. there have only been a few studies that utilize sediment As a mechanism that modi®es atmospheric circula- cores to gain insight about the ®re regime. Pollen and tion patterns at large spatial scales, atmospheric tele- charcoal data from several sediment cores in Interior connections affect weather throughout the northern August 2005 FIRE AND CLIMATE LINKAGE IN ALASKA 1319 hemisphere (Hurrell et al. 2003). Teleconnections are correlated anomalies of geopotential height (Wallace and Gutzler 1981, Barnston and Livezey 1987) that impact regional weather through recurring and persis- tent shifts in pressure and circulation across large spa- tial scales. Links between disturbance and weather that are mediated by teleconnections include; droughts and ®re in Canada (Bonsal et al. 1993, Bonsal and Lawford 1999, Skinner et al. 2002, Girardin et al. 2004), ®res in the Paci®c Northwestern United States (Hessl et al. 2004) and ®res in the Southwestern United States (Swetnam and Betancourt 1990). In Alaska, deviations from synoptic weather patterns have been correlated with the Paci®c Decadal Oscillation (Papineau 2001, Hartmann and Wendler 2003) as well as the El NinÄo/ FIG. 1. Map of Alaska, USA, identifying the seven cli- Southern Oscillation and Paci®c/North America pat- mate stations used in the statistical analyses. The enclosed terns (Hess et al. 2001). Speci®cally, the occurrence of areas with darker shading represent individual ®re perimeters large ®re years has been correlated with the presence from 1950 to 2003. of strong to moderate El NinÄo conditions (Hess et al. 2001). Our work moves a step further and quanti®es Cleve and Viereck 1983, Payette 1992, Mann and Plug the impact of these signals on the annual area burned 1999). in Alaska through the development of a statistical re- gression model. METHODS Experience and common sense dictate that ®re re- Fire data sponds to local weather conditions, but modeling re- sults indicate that the link between weather and ®re The Alaska Fire Service (AFS) maintains a database does not easily translate to the landscape scale (Flan- of ®res for the state of Alaska dating back to 1950. nigan and Harrington 1988, Hely et al. 2001, Wester- These data are commonly referred to as the Large Fire ling et al.