Three centuries of annual area burned variability in northwestern North America inferred from tree rings

Annual area burned (AAB) variability in northwestern North America was inferred from 38 tree-ring width chronologies widely distributed across boreal regions and spanning the past 300 years and the minimum 1833-1998 interval. AAB estimates accounted for up to 61% of the variance in AAB observed from 1959 to 1998, and were verified using a split sample calibration-verification scheme. Spatial correlation maps of gridded temperature and precipitation data provided an indication of the reliability of the reconstruction to approximate fire-conducive climate variability beyond the period of calibration. Singular spectrum analysis and analysis of variance suggested that AAB has significantly changed during the course of the past 150 years toward increasing variance. Recent 1959-1998 decadal changes in AAB of northwestern North America fitted well within an oscillatory mode centred on 26.7 years and accounting for 21.1% of the variance in the reconstruction. As in previous studies, the current findings suggest that AAB is correlated to seasonal land/ocean temperature variability and that future warming could lead to greater AAB. However, the study has the weakness of not accounting for complex interactions between climate and ecosystem processes and thus its results should be interpreted with caution. We suggest that a calibration model conducted on multiple types of fire proxies (tree rings, charcoal data, fire scars and stand-establishment records) could be relevant for addressing these weaknesses.