Interannual to decadal changes in area burned in Canada from 1781 to 1982 and the relationship to Northern Hemisphere land temperatures

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1 Global Ecology and Biogeography, (Global Ecol. Biogeogr.) (2007) 16, Blackwell Publishing Ltd RESEARCH PAPER Interannual to decadal changes in area burned in Canada from 1781 to 1982 and the relationship to Northern Hemisphere land temperatures Martin P. Girardin* Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 10380, Stn. Sainte-Foy, Quebec, QC, G1V 4C7, Canada ABSTRACT Aim Temporal variability of annual area burned in Canada (AAB-Can) from AD 1781 to 1982 is inferred from tree-ring width data. Next, correlation analysis is applied between the AAB-Can estimates and Northern Hemisphere (NH) warm season land temperatures to link recent interannual to decadal changes in area burned with large-scale climate variations. The rationale in this use of tree rings is that annual radial increments produced by trees can approximate area burned through sensing climate variations that promote fire activity. Location The statistical reconstruction of area burned is at the scale of Canada. Methods The data base of total area burned per year in Canada is used as the predictand. A set of 53 multicentury tree-ring width chronologies distributed across Canada is used as predictors. A linear model relating the predictand to the tree-ring predictors is fitted over the period The regression coefficients estimated for the calibration period are applied to the tree-ring predictors for as far back as 1781 to produce a series of AAB-Can estimates. Results The AAB-Can estimates account for 44.1% of the variance in the observed data recorded from 1920 to 1982 (92.2% after decadal smoothing) and were verified using a split sample calibration-verification scheme. The statistical reconstruction indicates that the positive trend in AAB-Can from c was preceded by three decades during which area burned was at its lowest during the past 180 years. Correlation analysis with NH warm season land temperatures from the late 18th century to the present revealed a positive statistical association with these estimates. *Correspondence: Martin P. Girardin, Natural Resources Canada, Canadian Forest Service, Laurentian Forestry Centre, 1055 du P.E.P.S., P.O. Box 10380, Stn. Sainte-Foy, Quebec, QC, G1V 4C7, Canada. martin.girardin@rncan.gc.ca Main conclusions As with previous studies, it is demonstrated that the upward trend in AAB-Can is unlikely to be an artefact from changing fire reporting practices and may have been driven by large-scale climate variations. Keywords Area burned, Canada, climate change, dendrochronology, dendroclimatological reconstruction, fire history, forest fires, Northern Hemisphere warming. INTRODUCTION In boreal Canada, wildfire is a primary natural process that organizes the physical and biological attributes of the forest, shapes landscape diversity and influences biogeochemical cycles (Weber & Flannigan, 1997; Bourgeau-Chavez et al., 2000). The mosaics of different vegetation types are to a large extent an expression of their respective fire regimes, and many boreal tree species have adapted to fire. Wildfire responds rapidly to changes in weather and climate in comparison to vegetation the rate and magnitude of fire-regime-induced changes to the boreal forest landscape can greatly exceed anything expected owing to climate change alone (Weber & Flannigan, 1997). In addition, watershed wildfires have profound effects on water quality in boreal surface waters. Trees take up considerable amounts of water through the process of transpiration and their removal can lead to enhanced erosion, leaching and particle and nutrient transport from soils to streams after heavy rainfall (Prepas et al., DOI: /j x Journal compilation 2007 Blackwell Publishing Ltd 557

2 M. P. Girardin 2003). Changes in nutrient fluxes after wildfires can lead to a complex series of biological changes that can affect the structure and function of aquatic ecosystems. Wildfires have also been recognized as significant sources of greenhouse gas emissions into the atmosphere (Amiro et al., 2001), which can potentially affect the climate. Most of this is in the form of carbon dioxide (CO 2 ), but quantities of carbon monoxide, methane, long-chain hydrocarbons and carbon particulate matter are also emitted. It is now widely accepted that our climate has been warming during the past century, in part as a result of human-induced increases in greenhouse gas emissions in the atmosphere (Intergovernmental Panel on Climate Change, 2001; Karoly & Wu, 2005). Area burned in Canada has also been steadily increasing since 1970, and it was suggested that the trend reflects a detectable influence of human-induced climate warming (Gillett et al., 2004). Much of the trend in area burned is due to increases in the frequency of large fire years (Kasischke & Turetsky, 2006). However, area burned varies more with temperatures on interdecadal time-scales than it does on interannual time-scales (Gillett et al., 2004). Hence, the current fire record (1920 to the present) is too short to verify whether large-scale temperature variations are indeed linked to changes in country-wide area burned. Fire records of area burned have also been criticized because of changing fire reporting and suppression practices over the past century (Van Wagner, 1988; Bourgeau-Chavez et al., 2000; Podur et al., 2002). It is thus inherently difficult to relate trends in climate, increased weather variability or some other factor, to the trend in area burned. The aim of this paper is to address temporal changes in annual area burned in Canada (AAB-Can) and their correlation with large-scale climate variability. First, temporal variability of AAB- Can from AD 1781 to 1982 is inferred from tree-ring width data. The rationale in this use of tree rings is that trees in temperate regions produce annual radial increments, where changes in ring width from one year to the next reflect changes in precipitation, temperature and drought, as well as other factors (Fritts, 2001). Trees can also sense climate variations that promote fire activity (Larsen, 1996). Spatial and temporal patterns of annual radial increments, as measured across extensive networks, can be used to infer variability in area burned and additionally extend area burned records at times during which there was no fire reporting. The methodology is commonly employed in climate reconstructions (Cook & Kairiukstis, 1990; Cook et al., 1994; D Arrigo et al., 2001, 2003) and has already been applied in the western United States and on the Boreal Shield to infer past fire activity (Westerling & Swetnam, 2003; Girardin et al., 2006a,b). Second, correlation analysis is applied between AAB-Can estimates and Northern Hemisphere (NH) warm season land temperature (Smith & Reynolds, 2005) to link interannual to decadal changes in AAB-Can with large-scale climate variations. METHODS Description of the area burned data The data base of total annual area burned in Canada (AAB-Can) with corrections applied to account for missing data (Van Wagner, 1988; Gillett et al., 2004) was used as the predictand (i.e. the variable to reconstruct). This data base covers the period from 1920 to Area burned data are provided by agencies responsible for the land base and are based on aerial photography, perimeter surveys or satellite imagery. Corrections were applied to the data because fire statistics are biased due to inconsistent and expanding detection systems (Murphy et al., 2000). The size of fire management areas is an increasing function of time (Podur et al., 2002). Hence, recent area burned records tend to be more accurate (Amiro et al., 2001). A logarithmic transformation (LOG) was applied to the AAB- Can record for statistical analyses. A Shapiro Wilk test indicated right-skewness (Zar, 1999) in the annual area burned frequency distributions (P < 0.001) and the LOG transformation was found to provide an adequate data adjustment to meet the normality assumption (P = over the period ). Description of the area burned predictors A set of site tree-ring width chronologies that demonstrated a satisfactory correlation with the LOG-transformed AAB-Can record was identified and subsequently used for the statistical reconstruction. Site tree-ring width chronologies are defined as averages of annual ring width measurements for one to several cores per tree and for several trees growing on similar ecological sites. Spearman correlation analysis was used to screen the AAB- Can record against 147 candidate tree-ring width chronologies covering the minimum interval 1781 to 1982 and distributed mostly across Canadian boreal regions (Contributors to the International Tree-Ring Data Bank, 2004; Girardin et al., 2006c). A few site tree-ring width chronologies located outside Canada were also included in the analysis, since weather systems associated with extreme fire years can cover vast land areas (Skinner et al., 1999). The period from 1781 to 1982 was chosen to maximize the length of the period of analysis, the distribution of chronologies across Canada and sub-signal strength (used to define a portion of a given chronology with a strong common signal). It is difficult to know a priori which site tree-ring width chronologies are potentially useful records of climate associated with variability in area burned. Therefore, some sort of variable screening is desirable to eliminate useless data (Cook et al., 2002). Note that, prior to screening, all tree-ring width measurements were processed to remove unwanted age/size-related trends using cubic smoothing splines of 66% of the ring width series length with 50% frequency response (Cook & Kairiukstis, 1990). Bi-weight robust means of these standardized measurement series were computed to create the site tree-ring width chronologies. All site tree-ring width chronologies were constructed using the ARSTAN program (Cook & Holmes, 1986). The correlation screening revealed a subset of 53 chronologies (Fig. 1; see Appendix S1 in Supplementary Material) out of the original 147 that correlated at P < 0.15 (Spearman r > 0.18 ) with the AAB-Can record over the common period Next, the 53 site tree-ring width chronologies (Fig. 1), held together on the common interval from 1781 to 1982, were 558 Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd

3 Area burned in Canada Figure 1 Distribution of the 61 tree-ring width chronologies used as annual area burned in Canada (AAB-Can) predictors. Symbols: correlation between predictors and estimated AAB-Can over ; symbols marked with a dot denote the eight chronologies used in a sensitivity analysis (for these the correlation is over ). Numbers refer to ecozones: 4, Taiga Plains; 5, Taiga Shield; 6, Boreal Shield; 9, Boreal Plains; 10, Prairies; 11, Taiga Cordillera; 12, Pacific Maritime; 13, Montane Cordillera; 14, Boreal Cordillera; 15, Hudson Plains. transformed into principal components (PCs) (SYSTAT, 2004) to remove multicollinearity. Fire-conducive weather across Canada exhibits strong regional similarities, largely due to the common influence of large-scale features of oceanic and atmospheric circulation (Skinner et al., 1999, 2006; Girardin et al., 2006c). Therefore, it is appropriate to summarize the dominant information contained within the tree-ring data set into a new set of uncorrelated variables. (The approach is very similar to that employed in reconstruction of large-scale sea surface temperature and sea-level pressure fields; e.g. D Arrigo et al., 2001, 2003; Cook et al., 2002.) A correlation matrix was used so that all site tree-ring width chronologies could contribute equally to the clustering of objects, independently of the variance exhibited by each one. Five PCs were retained for subsequent analyses (eigenvalues > 2.5, accounting for 35.9% of the total variance). Statistical reconstruction of the area burned Statistical reconstruction of past AAB-Can was conducted as follows. A stepwise multiple regression (SYSTAT, 2004) employing a backward selection was used to fit a linear model relating the LOG-transformed AAB-Can record (period ) to the PCs: y = α + β log x + β x + β x + ε j j j m mj j where ylog j is the LOG-transformed AAB-Can, x j the PCs (total of five candidate predictors), β the regression coefficients and ε j the error. Once the regression coefficients were estimated for the calibration period, they were applied to the PCs for as far back as 1781 to produce a series of LOG-transformed AAB-Can estimates. The stability of the regression model was tested using a split sample calibration-verification scheme (Table 1). Two (1) Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd 559

4 M. P. Girardin Table 1 Calibration and verification statistics of the statistical reconstruction of annual area burned in Canada (AAB-Can). subcalibrations of the periods and were conducted using the selected PCs in eqn 1 and the estimated regression coefficients were applied to the PCs over the verification periods and , respectively. The strength of the relationship between the reconstruction and observations over the verification periods was measured by the reduction of error (RE), coefficient of efficiency (CE), sign test and the product means test (PM) (Cook et al., 1994). Sensitivity analyses of biases Calibration period R Std. error First-order autocorrelation of residuals Durbin Watson statistic Verification period Correlation a RE b CE b PM c Sign test d 27/4 24/8 a Significant at P < 0.05 if r > b Both reduction of error (RE) and coefficient of efficiency (CE) are measures of shared variance between the observed and modelled series, but are usually lower than the R-square (R 2 ). The RE uses the calibration period mean as the standard of reference for calculating the total sum of instrumental squared deviations, allowing it to detect changes in the mean of the reconstructed values from the calibration period mean. The CE uses the verification period mean to calculate the total sum of instrumental squared deviations. A positive value for either statistic (RE and CE > 0) signifies that the regression model has some skill. c Considered significant at P < 0.05 if product means test (PM) > A significant PM test result indicates that the magnitude and the direction of year-to-year changes are statistically significant. d Agreement/disagreement: P < 0.05 if sign test 24/9. A significant sign test result indicates good fidelity in the direction of year-to-year changes in the real and reconstructed data. Regions with greater fire incidence and tree-ring data replication are likely to be weighted accordingly in the reconstruction (Girardin et al., 2006a). Few site tree-ring width chronologies were located across the major fire corridor (Fig. 1), i.e. from the Northwest Territories to north-western Ontario (Stocks et al., 2003), and therefore the selected data undersample the geographical areas of Canada in which most of the fires occur. The implications of these sampling biases were evaluated as follows. First, AAB-Can estimates and observations were correlated with seasonal averages of May to August 500-hPa geopotential heights over a 2.5 latitude by 2.5 longitude grid of the Northern and Southern Hemispheres (NCEP/NCAR grid; Kalnay et al., 1996). The grid has a temporal coverage from 1948 to the present. Geopotential height approximates the actual height above sea level (in metres) of a given pressure surface. By definition, a warm layer of air is less dense and thicker than a cool one; a region of warm air thus appears as a region of high upper atmospheric pressure. Large forest fires in Canada (size greater than 200 ha) are generally associated with prolonged blocking high-pressure systems in the upper atmosphere that cause obstruction, on a large scale, of the normal west-to-east progress of migratory storms (Skinner et al., 1999). To verify that AAB- Can estimates and observations were sensing similar year-to-year geopotential height variabilities, the obtained correlation values at each grid point were used to create 500-hPa geopotential height spatial correlation maps. The May to August period was chosen for correlation as it covers the fire season in Canada (Stocks et al., 2003). Spatial correlation maps were created using the Royal Netherlands Meteorological Institute (KNMI) Climate Explorer ( Spearman correlation coefficients were computed between the reconstruction of AAB-Can and observed area burned per year in 10 ecozones (out of a total of 15). Five ecozones with a small area burned were dropped from the analysis (namely Arctic Cordillera, Northern Arctic, Southern Arctic, Atlantic Maritime and Mixedwood Plain). For this analysis, annual fire data for each of the 10 ecozones were obtained from the Large Fire Database (LFDB; Stocks et al., 2003). The LFDB is a compilation of forest fire data (namely location, start date, detected date, cause and size) for the period covering and includes only fires greater than 200 ha in final size, which represents only a small percentage of all fires but accounts for most of the area burned (usually more than 97%; Stocks et al., 2003). A sampling bias is considered critical if ecozones with high area burned do not correlate with the AAB-Can estimates. RESULTS AND DISCUSSION Interannual to decadal changes in area burned The stepwise regression model indicated that 44.1% of the variance in the LOG-transformed AAB-Can can be accounted for by the first and second principal components (see Appendix S2 in Supplementary Material) of the 53 tree-ring width chronologies (adjusted R 2 = 42.3%, P < 0.001). The amount of common variance between AAB-Can estimates and observations is 92.2% if data are smoothed with least squares polynomial fittings across a moving 10-year window (P = 0.002, assuming n = 6 decades). Verification statistics of the strength of the subcalibration models (Table 1) indicate significant predictive skills of the statistical reconstruction of AAB-Can (Fig. 2). Positive RE and CE and significant correlation coefficients indicate tendencies for the reconstruction of AAB-Can to reproduce with confidence both high-frequency and relatively low-frequency variations in observed data. The PM and sign tests both indicate significant 560 Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd

5 Area burned in Canada Figure 2 Reconstruction of annual area burned in Canada (AAB-Can: logarithmic transformed, LOG) from The dashed black line shows observed data from 1920 to The shaded area represents standard error. Yellow curve, average May to August Northern Hemisphere (NH) land temperatures expressed as departures from the base period (Smith & Reynolds, 2005). Solid red line, regime shift detection with correction for serial correlation (AR(1)). Regime shift detection allows verification that changes in the mean from one period to another are not just a manifestation of a red noise process (probability σ = 0.10, cut-off length = 15 years; parameters AR(1) were estimated using the IP4 method: AAB-Can AR(1) = 0.53, NH temperatures AR(1) = 0.24; see Rodionov, 2006). predictive skills of the reconstruction to reproduce the magnitude and direction of year-to-year changes in the most recent period; lower skills were noted for the period prior to As is often the case with tree-ring reconstructions, the AAB-Can estimates exhibit lower year-to-year variability than the observed data and hence the reconstruction tends to underestimate extreme fire years (Fig. 2). Spatial correlation maps (Fig. 3a,b) indicate similar patterns between the AAB-Can estimates, AAB-Can observations and 500-hPa geopotential heights over Canada. Specifically, year-toyear AAB-Can changes, for both the estimates and observations, were positively correlated with 500-hPa height variability over north-western Canada (Fig. 4). Discrepancies are nevertheless noted with the centres of correlation, slightly displaced northwestward for AAB-Can estimates (Fig. 3b). Spatial correlation analysis with fire records from 10 ecozones (Table 2) also reveals similar patterns between the AAB-Can estimates and observations. Despite low sampling replication in north-western Canada, AAB-Can estimates appear to be representative of variability in fire activity (albeit more weakly than observed data) in the majority of fire-prone ecozones (Table 2). Biases appear to arise from the lower magnitude of year-to-year variability in estimated data, greater serial correlation and from weaknesses in estimating extreme fire years (Figs 2 & 4). Sampling biases were further evaluated as follows. Eight additional tree-ring width chronologies from the Boreal Shield (west), Boreal Plains and Taiga Plains ecozones (Fig. 1), covering the minimum interval , and correlating at P < 0.15 with the observed AAB-Can record, were added to the matrix of predictors (for a total of 61 tree-ring width chronologies). This new statistical reconstruction was intended to verify whether or not a better fit between AAB-Can estimates and observations could be obtained. Calibration (R 2 = 45.0%; period ) and verification (r = 0.68 and 0.53; RE = 0.40 and 0.33; CE = 0.36 and 0.17; PM = 3.11 and 3.77) statistics did not show an improvement over the original model. Estimates from this new reconstruction (see Appendix S3 in Supplementary Material) yielded a strong correlation with estimates from the original reconstruction over the common interval (r = 0.91). A sensitivity analysis in which the P parameter (used in the screening of site tree-ring width chronologies) varied from 0.10 to 1.00 further confirmed the robustness of the results. The CE and RE statistics are sensitive measurements of trends in data (Cook & Kairiukstis, 1990). The fact that their magnitude Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd 561

6 M. P. Girardin Figure 3 (a) (c) Correlation between the annual area burned in Canada (AAB-Can) and seasonal averages of May to August 500-hPa geopotential heights. The period of analysis is indicated above each map. In (d), estimated AAB-Can data (e.g ) were extended to 2005 using observed data; the variance of observed data was adjusted to correspond to that of the estimates. All data were ranked prior to analysis. Trend lines in area burned records and 500-hPa height data were removed prior to analyses of (a) and (b). is over the satisfactory level is likely to be an indication that low-frequency variations in observed AAB-Can from 1920 to 1982 were not the result of changing fire reporting practices. The estimated area burned shows striking changes in the mean over the past 220 years. Detection of regime shift (Rodionov, 2006) shows periods of low mean AAB-Can from , and (Fig. 2). The mean of the period was the second lowest on record. Periods of high mean AAB- Can were detected from , , , and, most recently, (or later). Can these variations in area burned be linked to large-scale climate variations? Connection with large-scale climate variability The statistical association between large-scale climate variations and interannual to decadal changes in AAB-Can was analysed over the instrumental period using average May to August NH land temperature data (Smith & Reynolds, 2005). The use of NH data was justified by a significant correlation between AAB-Can and geopotential height variations around the NH, particularly over the tropics (Fig. 3c,d) (correlation between average 500-hPa heights equatorward of 30 latitude and AAB- Can observations after detrending both data is r = 0.33 with 95% bootstrap CI [0.10; 0.53], period ). A previous study has already indicated that a trend in warm season weighted Canadian temperatures explained much of the trend in Canadian area burned from 1920 to 1999 (37% of explained variance in non-overlapping 5-year periods, n = 16; Gillett et al., 2004). The present analysis will place this trend in much broader temporal and climate scales. The NH land temperature data (Fig. 2) show a steady period of warming from about 1910 through the 1940s, a gradual cooling from the mid-1940s through the 1960s, and a steady temperature increase thereafter (Smith & Reynolds, 2005). As suggested in Fig. 2, these variations in NH land temperatures are correlated with estimated area burned data (Table 3). The statistical significance of these correlations against the red noise background (i.e. serial correlation) was tested as significant at the 95% level (Table 3) using a technique that re-samples blocks of data pairs (Mudelsee, 2003) (see also Appendices S4 & S5 in Supplementary Material). Based on this evidence, it can be affirmed that there is a statistical association between AAB-Can and large-scale climate variations: 562 Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd

7 Area burned in Canada Table 2 Spearman correlation analysis between estimates of annual area burned in Canada (AAB-Can) and area burned annually by large forest fires (size > 200 ha) in 10 Canadian ecozones during the period The correlation to the observed AAB-Can is also presented. Ecozones 5 and 6 were divided into west and east of 90 W sectors due to spatial extent. Correlations that tested significant at the 95% (σ 1 ) probability level after pre-whitening appear in bold. The percentage (%) of area burned per year in Canada accounted for by each ecozone is also shown. Ecozones % area burned over Correlation with area burned in Canada Estimates Observations Figure 4 Seasonal averages of May to August 500-hPa geopotential heights over north-western Canada from 1948 to 2005 and observed and estimated annual area burned in Canada (AAB-Can; logarithmic transformed, LOG). Dotted lines are least squares polynomial fitting across a moving window of 10 years. For height data the latitude range used is N and the longitude range used is W. Correlation between 500-hPa height data and estimated AAB-Can for is significant: r = 0.45 with 95% bootstrap confidence interval (95% CI) [0.18; 0.67] (the hypothesis of no correlation cannot be rejected at the 95% level when the confidence interval contains zero). Correlation between 500-hPa height data and observed AAB-Can data ( ): r = 0.46 with 95% CI [0.25; 0.63]. Correlation between AAB-Can estimates and observations is r = 0.61 with 95% CI [0.41; 0.76]. Data were detrended prior to correlation analyses. increases in NH land temperatures over the past 126 years match increases in AAB-Can. These results suggest that further increases in NH land temperatures may lead to increases in area burned, although such predictions fall outside the domain of the current historical data. Research has suggested that the frequency and persistence of blocking high-pressure systems in the upper atmosphere will increase in an enhanced CO 2 climate, especially over western North America (Meehl & Tebaldi, 2004). Given that area burned is closely tied to the occurrence and persistence of these systems (Figs 3 & 4; Skinner et al., 1999), one can expect greater AAB-Can under a warmer climate. Projections of future area burned in Canadian ecozones based on historical fire/climate relationships transferred to outputs from climate model simulations did suggest significant increases in area burned under a warmer climate (Flannigan et al., 2005). However, large regional variations in fire activity were predicted and some ecozones may show little change in area burned. In fact, according to analyses of forest age class distributions (expressed in percentage of the study area per age class), fire activity has decreased since c in eastern and north-western Canadian boreal regions (Larsen, 1997; Bergeron et al., 2004; Girardin et al., 2006c), owing to reductions in the rate of occurrence of extreme fire years and increases in precipitation that compensated for the warming. The transition from an unsynchronized 4. Taiga Plains 19.6% Taiga Shield 13.8% (west of 90 W) 12.3% (east of 90 W) 1.6% Boreal Shield 34.8% (west of 90 W) 26.0% (east of 90 W) 8.8% Boreal Plains 17.9% Prairies 0.5% Taiga Cordillera 1.5% Pacific Maritime 6.9% Montane Cordillera 0.2% Boreal Cordillera 2.1% Hudson Plains 1.0% Table 3 Pearson correlation analysis between the statistical reconstruction of annual area burned in Canada (AAB-Can) and May to August Northern Hemisphere (NH) land temperatures. Correlations (r) to the observed AAB-Can record and extended AAB-Can estimates are also presented. The coefficients were calculated with 95% confidence intervals (95% CI) using a nonparametric stationary bootstrap (Mudelsee, 2003). This technique re-samples blocks of data pairs to account for the presence of serial correlation in the time series. When the confidence interval contains zero, the hypothesis of no correlation cannot be rejected at the 95% level. Data were ranked and detrended prior to correlation analyses. Area burned data Period r 95% CI Area burned estimates [0.13; 0.48] Area burned observations [0.09; 0.49] Area burned estimates after extending to 2005 with observed data [0.14; 0.46] relationship between past warming and area burned (before AD 1870) and synchronicity between the two in the present and future (Larsen, 1997) has still not been elucidated. Concluding remarks The results show that networks of tree-ring width chronologies can provide well-replicated, statistically verified and long-duration Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd 563

8 M. P. Girardin estimates of AAB-Can. This application of tree-ring data is supported by the fact that most of the area burned in the boreal forest is attributed to large persistent blocking high-atmospheric pressure systems that cause dry fuel conditions and winds over large land areas (Skinner et al., 1999). Though over the past decades or so humans have been an important source of fire ignition (Stocks et al., 2003), dry forest fuels and wind are major contributors to large stand-destroying fires. It has been shown that blocking high-atmospheric pressure systems can also lead to water deficits in trees, and hence to reductions in tree radial growth (Garfin, 1998; Girardin & Tardif, 2005). In addition, previous studies provided evidence of statistical association between radial growth of a number of the boreal tree species used for this study and the Canadian Drought Code (CDC), which is used daily across Canada to monitor fire danger (Van Wagner, 1987). The CDC is an indicator of seasonal drought effects on the amount of moisture in the deep forest floor, and shows significant correlation with variability in the upper atmospheric circulation (Girardin et al., 2006c). The statistical reconstruction of area burned presented in this paper, nevertheless, should be interpreted with caution. Variance in the period before 1820 may have been affected by changes in the sample size for tree-ring measurement (Cook & Kairiukstis, 1990; Girardin et al., 2006c). Also, regions with greater fire incidence and tree-ring data replication are likely to be weighted accordingly in the reconstruction of AAB-Can (see Table 2). The greatest area burned occurred from the Northwest Territories to north-western Ontario as a result of fire-prone ecosystems, extreme fire weather, frequent lightning activity and reduced levels of protection (Table 2; Stocks et al., 2003). Still, only a few multicentury-long tree-ring width chronologies were available within these regions. Furthermore, the reconstruction of AAB- Can does not explicitly account for changes in vegetation and fuel characteristics, which may alter disturbance regimes. The effects of land-use history on forest structure, fuel accumulation and fire behaviour could also be confounding factors (Westerling et al., 2006); extrapolation of the findings to the recent area burned records (1982 to the present) should be done cautiously. Finally, because tree-ring width samples were detrended to remove growth trends, the statistical reconstruction of area burned is unlikely to contain information relative to centurylong climate changes. Follow-up studies should consider the application of more conservative detrending to tree-ring data and the inclusion of new ring-width data as well as other measured parameters to increase data replication in remote regions (tree-ring minimum and maximum density, earlywood and latewood width and density, etc.; see Drobyshev & Niklasson, 2004). Statistical reconstructions of regional variability in area burned could also be pursued in order to increase the spatial resolution, and the resolutions of year-to-year and extreme events and to investigate regional modes of variability (Westerling & Swetnam, 2003; Girardin et al., 2006a,b). The contribution of statistical reconstructions of countrywide area burned to our understanding of historical fire variability can be significant. Fire is recognized as driving much of the boreal forest carbon balance in North America and the potential forest sink is recognized in the Kyoto Protocol (Amiro et al., 2001; Apps et al., 2006). However, several years with greater numbers of fires could result in a situation where the forest would be a net carbon source (Kurz & Apps, 1999; Apps et al., 2006). Proxy records of area burned may provide information on the recurrence of such episodes and, additionally, could increase our understanding of their underlying causes. Furthermore, modelling of the impacts of climate change on future area burned in Canada (Bergeron et al., 2004; Flannigan et al., 2005) could benefit from the proxy records: simulations of area burned response to past solar, volcanic and greenhouse gas forcing and comparison with reconstructions of area burned could be undertaken in order to determine sensitivities of area burned to the forcing and a range of confidence in area burned forecasts. Statistical reconstructions of area burned could also be compared with proxy-based reconstructions of large-scale oceanic and atmospheric variations (Briffa et al., 2001; D Arrigo et al., 2001, 2003; Esper et al., 2005) for analysis of long-term stability of fire climate relationships. Large-scale sea surface temperature patterns can exert significant influence on fire weather across Canada (Skinner et al., 2006) and could have partly contributed to long-term changes in area burned in southeastern boreal Canada (Girardin et al., 2004, 2006c). Finally, statistical reconstructions of area burned may make valuable contributions to our understanding of the relationship between climate variability (average versus extremes) and post-fire forest age class distributions. A recent comparison between a regional-level statistical reconstruction of area burned and a forest age class distribution derived from a regional map of time since last fire has successfully been made over eastern Canada (Girardin et al., 2006a). A similar comparison was also made in northwestern Canada using increments in tree radial growth (Larsen, 1996). It is conceivable that statistical reconstructions of area burned could be developed across Canada and compared with fire-scar/age class distributions. Such analyses could provide quantitative means for measuring the influence of climate on fire activity (Larsen, 1996; Westerling & Swetnam, 2003; Girardin et al., 2006a). ACKNOWLEDGEMENTS I thank Mike D. Flannigan, Pierre Y. Bernier, Pamela Cheers, Isabelle Lamarre, David Currie, Matt McGlone and three anonymous referees for comments and the reviewing of the manuscript. I thank Kim Logan for providing the area burned data. This work was financed with Canadian Forest Service funds. REFERENCES Amiro, B.D., Todd, J.B., Wotton, B.M., Logan, K.A., Flannigan, M.D., Stocks, B.J., Mason, J.A., Martell, D.L. & Hirsch, K.G. (2001) Direct carbon emissions from Canadian forest fires, Canadian Journal of Forest Research, 31, Apps, M.J., Bernier, P.Y. & Bhatti, J.S. (2006) Forests in the global carbon cycle: implications of climate change. Climate change and managed ecosystems (ed. by J.S. Bhatti, R. Lal, M.J. Apps and M.A. Price), pp CRC Press, Boca Raton, FL. 564 Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd

9 Area burned in Canada Bergeron, Y., Flannigan, M., Gauthier, S., Leduc, A. & Lefort, P. (2004) Past, current and future fire frequency in the Canadian boreal forest: implications for sustainable forest management. Ambio, 33, Bourgeau-Chavez, L.L., Alexander, M.E., Stocks, B.J. & Kasischke, E.S. (2000) Distribution of forest ecosystems and the role of fire in the North American boreal region. Fire, climate change, and carbon cycling in the boreal forest (ed. by E.S. Kasischke and B.J. Stocks), pp Springer-Verlag, New York. Briffa, K.R., Osborn, T.J., Schweingruber, F.H., Harris, I.C., Jones, P.D., Shiyatov, S.G. & Vaganov, E.A. (2001) Lowfrequency temperature variations from a northern tree ring density network. Journal of Geophysical Research, 106 D3, Contributors to the International Tree-Ring Data Bank (2004) IGBP PAGES/World Data Center for Paleoclimatology, NOAA/NCDC Paleoclimatology Program, Boulder, CO. Cook, E.R. & Holmes, R. (1986) Guide for computer program ARSTAN. Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ. Cook, E.R. & Kairiukstis, L.A. (1990) Methods of dendrochronology: applications in the environmental sciences. Kluwer Academic Publishers, Boston, MA. Cook, E.R., Briffa, K.R. & Jones, P.D. (1994) Spatial regression methods in dendroclimatology: a review and comparison of two techniques. International Journal of Climatology, 14, Cook, E.R., D Arrigo, R.D. & Mann, M.E. (2002) A well-verified, multiproxy reconstruction of the winter North Atlantic Oscillation index since AD Journal of Climate, 15, D Arrigo, R., Villalba, R. & Wiles, G. (2001) Tree-ring estimates of Pacific decadal climate variability. Climate Dynamics, 18, D Arrigo, R.D., Cook, E.R., Mann, M.E. & Jacoby, G.C. (2003). Tree-ring reconstructions of temperature and sea-level pressure variability associated with the warm-season Arctic Oscillation since AD Geophysical Research Letters, 30, Drobyshev, I. & Niklasson, M. (2004) Linking tree rings, summer aridity, and regional fire data: an example from the boreal forests of the Komi Republic, East European Russia. Canadian Journal of Forest Research, 34, Esper, J., Wilson, R.J.S., Frank, D.C., Moberg, A., Wanner, H. & Luterbacher, J. (2005) Climate: past ranges and future changes. Quaternary Science Reviews, 24, Flannigan, M.D., Logan, K.A., Amiro, B.D., Skinner, W.R. & Stocks, B.J. (2005) Future area burned in Canada. Climatic Change, 72, Fritts, H.C. (2001) Tree rings and climate. Blackburn Press, Caldwell, NJ. Garfin, G.M. (1998) Relationships between winter atmospheric circulation patterns and extreme tree growth anomalies in the Sierra Nevada. International Journal of Climatology, 18, Gillett, N.P., Weaver, A.J., Zwiers, F.W. & Flannigan, M.D. (2004) Detecting the effect of climate change on Canadian forest fires. Geophysical Research Letters, 31, L Girardin, M.P. & Tardif, J. (2005) Sensitivity of tree growth to the atmospheric vertical profile in the Boreal Plains of Manitoba, Canada. Canadian Journal of Forest Research, 35, Girardin, M.P., Tardif, J., Flannigan, M.D. & Bergeron, Y. (2004) Multicentury reconstruction of the Canadian Drought Code from eastern Canada and its relationship with paleoclimatic indices of atmospheric circulation. Climate Dynamics, 23, Girardin, M.P., Bergeron, Y., Tardif, J.C., Gauthier, S., Flannigan, M.D. & Mudelsee, M. (2006a) A 229-year dendroclimatic-inferred record of forest fire activity for the Boreal Shield of Canada. International Journal of Wildland Fire, 15, Girardin M.P., Tardif, J. & Flannigan, M.D. (2006b) Temporal variability in area burned for the province of Ontario, Canada, during the past 200 years inferred from tree rings. Journal of Geophysical Research, 111, D Girardin, M.P., Tardif, J.C., Flannigan, M.D. & Bergeron, Y. (2006c) Synoptic-scale atmospheric circulation and boreal Canada summer drought variability of the past three centuries. Journal of Climate, 19, Intergovernmental Panel on Climate Change (IPCC) (2001) Climate change 2001 the scientific basis. Cambridge University Press, Cambridge. Kalnay, E., Kanamitsu, M., Kistler, R., Collins, W., Deaven, D., Gandin, L., Iredell, M., Saha, S., White, G., Woollen, J. Zhu, Y., Chelliah, M., Ebisuzaki, W., Higgins, W., Janowiak, J., Mo, K.C., Ropelewski, C., Wang, J., Leetmaa, A., Reynolds, R., Jenne, R. & Joseph, D. (1996) The NCEP/NCAR 40-year reanalysis project. Bulletin of the American Meteorological Society, 77, Karoly, D.J. & Wu, Q. (2005) Detection of regional surface temperature trends. Journal of Climate, 18, Kasischke, E.S. & Turetsky, M.R. (2006) Recent changes in the fire regime across the North American boreal region spatial and temporal patterns of burning across Canada and Alaska. Geophysical Research Letters, 39, L Kurz, W.A. & Apps, M.J. (1999) A 70-year retrospective analysis of carbon fluxes in the Canadian forest sector. Ecological Applications, 9, Larsen, C.P.S. (1996) Fire and climate dynamics in the boreal forest of northern Alberta, Canada, from AD 1850 to The Holocene, 6, Larsen, C.P.S. (1997) Spatial and temporal variations in boreal forest fire frequency in northern Alberta. Journal of Biogeography, 24, Meehl, G.A. & Tebaldi, C. (2004) More intense, more frequent, and longer lasting heat waves in the 21st century. Science, 305, Mudelsee, M. (2003) Estimating Pearson s correlation coefficient with bootstrap confidence interval from serially dependent time series. Mathematical Geology, 35, Murphy, P.J., Mudd, J.P., Stocks, B.J., Kasischke, E.S., Barry, D., Alexander, M.E. & French, N.H.F. (2000) Historical fire Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd 565

10 M. P. Girardin records in the North American boreal forest. Fire, climate change, and carbon cycling in the boreal forest (ed. by E.S. Kasischke and B.J. Stocks), pp Springer-Verlag, New York. Podur, J., Martell, D.L. & Knight, K. (2002) Statistical quality control analysis of forest fire activity in Canada. Canadian Journal of Forest Research, 32, Prepas, E.E., Pineal-Alloul, B., Steedman, R.J., Planas, D. & Charette, T. (2003) Impacts of forest disturbance on boreal surface waters in Canada. Towards sustainable management of the boreal forest (ed. by P.J. Burton, C. Messier, D.W. Smith and W.L. Adamowicz), pp NRC Research Press, Ottawa. Rodionov, S.N. (2006) Use of prewhitening in climate regime shift detection. Geophysical Research Letters, 33, L Skinner, W.R., Stocks, B.J., Martell, D.L., Bonsal, B. & Shabbar, A. (1999) The association between circulation anomalies in the mid-troposphere and area burned by wildland fire in Canada. Theoretical and Applied Climatology, 63, Skinner, W.R., Shabbar, A., Flannigan, M.D. & Logan K. (2006) Large forest fires in Canada and the relationship to global sea surface temperatures. Journal of Geophysical Research, 111, D Smith, T.M. & Reynolds, R.W. (2005) A global merged landair-sea surface temperature reconstruction based on historical observations ( ). Journal of Climate, 18, Stocks, B.J., Mason, J.A., Todd, J.B., Bosch, E.M., Wotton, B.M., Amiro, B.D., Flannigan, M.D., Hirsch, K.G., Logan, K.A., Martell, D.L. & Skinner, W.R. (2003) Large forest fires in Canada, Journal of Geophysical Research, 108, SYSTAT (2004) SYSTAT Version 11.0 software. SPSS Inc., Chicago. Van Wagner, C.E. (1987) Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35. Canadian Forestry Service, Ottawa. Van Wagner, C.E. (1988) The historical pattern of annual burned area in Canada. Forestry Chronicle, 64, Weber, M.G. & Flannigan, M.D. (1997) Canadian boreal forest ecosystem structure and function in a changing climate: impact on fire regimes. Environmental Reviews, 5, Westerling, A.L., & Swetnam, T.W. (2003) Interannual to decadal drought and wildfire in the western United States. Eos, Transactions, American Geophysical Union, 84, Westerling, A.L., Hidalgo, H.G., Cayan, D.R. & Swetnam, T. W. (2006) Warming and earlier spring increase western U.S. forest wildfire activity. Science Express, 313, Zar, J.H. (1999) Biostatistical analysis. 4th edn. Prentice Hall, NJ. BIOSKETCH Martin P. Girardin is a biologist and research scientist at the Canadian Forest Service of Natural Resources Canada. His research interests include reconstruction of the historical variability of climate and forest fires using palaeoclimatic data and time series of radial growth rings in order to better understand climate change impacts on forest productivity and to further explore adaptation strategies. Editor: Matt McGlone SUPPLEMENTARY MATERIAL The following supplementary material is available for this article: Appendix S1 Sources of the 53 tree-ring width chronologies. Appendix S2 Eigenvectors and principal components. Appendix S3 Sensitivity models. Appendix S4 Correlation with temperature (CRUTEM3v) grid boxes. Appendix S5 Wavelet coherency analyses. This material is available as part of the online article from: j x (This link will take you to the article abstract). Please note: Blackwell Publishing is not responsible for the content or functionality of any supplementary materials supplied by the authors. Any queries (other than missing material) should be directed to the corresponding author for the article. 566 Global Ecology and Biogeography, 16, , Journal compilation 2007 Blackwell Publishing Ltd