ABSTRACT: Holocene fire-climate relationships in eastern British Columbia.

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Abstract

A century-long trend of increasing summer temperature in the Pacific Northwest, along with increasing loads of forest fuels, has raised concerns that forest fires are already becoming more frequent, larger, and more difficult to control. Clearly, how climatic change affects forest fire frequency is of interest to many stakeholders. The effect of long-term climate change on fire frequency, though, is masked by several important local conditions of forest fire: fuel loads, topography, a source of ignition, and the specific weather after ignition. Summarizing fire over increasingly larger areas reduces the importance of these local “controls” relative to regional climate, but it is not clear at what spatial scale regional climate imposes itself over local controls. One way to examine fire controls is to compare long-term records of fire events recorded in lake sediments: climatic control should result in some degree of synchroneity of fire events among sites. We compare fire-event records from five sites spanning a range of modern forest cover in eastern British Columbia. The site closest to the ecotone between dense and more open forest types shows the largest changes in fire occurrence over the Holocene, consistent with changes between a high and low-severity fire regime. An oxygen isotope record and a lake-level record indicate that the period of low-severity fires (6.5 to 4 ka) was also a period of low effective moisture. In contrast, at the remaining four sites there are only subtle changes in fire occurrence, with fire intervals ranging between 150 to 300 years throughout the Holocene. These fire intervals are too long to be related to endogenous controls such as fuel accumulation, and pollen data suggest that the landscape always supported continuous forest. Attributing subtle changes in fire intervals to causal climatic factors first requires determining whether such changes are consistent among sites. To this end, we compared two 5000-year fire records from sites only 11-km apart but separated by numerous natural firebreaks. We evaluated two independent properties of fire history: 1) fire-interval distribution, a measure of the overall incidence of fire, and 2) fire synchroneity, a measure of the co-occurrence of fire. Median fire intervals differed between the sites prior to, but not after, 2500 years before present. We also found no statistical synchrony between fire-episode dates between the two sites at any temporal scale, but for the last 2500 years marginal levels of synchrony occurred at centennial scales. The two records composited, however, show a strong match with regional climate reconstructions and with fire history from coastal British Columbia. This suggests that composite fire histories, rather than single sites, are most likely to be correlated with climate. Overall, these results indicate 1) that sites with similar modern conditions may have experienced different fire intervals and asynchronous patterns in fire episodes, likely because single site have insufficient power to detect subtle changes in fire history because local controls outweigh the synchronizing effect of climate, and 2) that the influence of climate on fire occurrence has varied with changing climatic variability over time. Further use of these quantitative methods for sediment-based fire history should continue to reveal the spatial hierarchy of fire controls.

Bibliography of Canadian Geomorphology