Long-term Fire Regimes, Forest Composition and Climate in Pacific Canada

Abstract:

A multi-proxy paleoenvironmental study including contiguous macroscopic charcoal analysis was conducted on sediments, spanning the last 14,000 years, from a low elevation lake on the south coast of British Columbia, Canada. The overall goals were to determine long-term changes in fire frequency and magnitude, and to assess the relationship between fire regimes and changes in forest composition and climate. Chironomid-inferred temperatures suggest a cool late-glacial interval with mean July air temperatures (MJAT) ~2°C cooler than modern. Forests were open, dominated by lodgepole pine, and characterized by low-severity fires that occurred every 175 years, on average. An interval of warmer, drier summers, with MJAT 2-3°C warmer than modern, occurred between 10,500 and 8000 cal yr BP. This period was marked by open Douglas-fir forests with abundant ferns, grasses and other herbs. Fires were less frequent, occurring about every 275 years, but increased in magnitude, relative to those during the preceding pine forests. Early Holocene establishment of oak savannah and a late Holocene period of oak expansion both coincide with brief increases in fire frequency. Inferred temperatures were generally cooler in the mid- and late Holocene, and forests were dominated by Douglas-fir with other conifer species as secondary constituents, reflecting an increasingly closed forest canopy and a higher fuel load. Fire frequency decreased slightly in the mid-Holocene to a mean fire return interval of about 300 years. However, in the late Holocene, despite generally cool conditions, fire regimes shifted to more frequent, low-severity fires, a pattern that has been observed elsewhere in western North America. This increase in fire activity may be linked to enhanced interannual climate variability, with periods of frequent El Niño/Southern Oscillation events leading to drier summers and/or lengthening of the fire season. Increasing Indigenous populations and their use of fire may also explain, at least in part, the late Holocene increase in fire frequency.