Holocene fire history and climate change in southern British Columbia, based on high-resolution analyses of sedimentary charcoal

Little is known about the role of fire in the cool and wet mountain hemlock rainforests located in the Coast and Cascade Mountains of southern British Columbia (BC). Macroscopic charcoal accumulation rates (CHAR) from radiocarbon-dated lake sediment cores were used along with 102 accelerator mass spectrometry (AMS) ages on soil charcoal to reconstruct long-term fire histories around two lakes. Fires were rare at the mountain hemlock sites, with intervals between fire events ranging from centuries to millennia. In the Frozen Lakes watershed, 18 of 40 fires (45%) in the last 9000 years are identified as local events, based on soil charcoal AMS ages. In the Mt. Barr Cirque Lake watershed, 11 of 46 fires (24%) in the last 7500 years are identified as local events. CHAR peaks lacking evidence of local fire likely represent extra-local fires. Overall fire frequency around both Frozen and Mt. Barr Cirque lakes has varied with climate throughout the Holocene, with 6--8 events/1000 years between 11,000 to 8800 cal years BP at Frozen Lake during the dry and warm early Holocene. Fires decrease to 3--5 events/1000 years from 8800 to 6400 cal years BP around both lakes as rainforest is established in the region. A variable fire period with a range of 2--8 events/1000 years occurs between from 6400 to 3500 cal years BP. A synchronous decrease to 3--5 events/1000 years from 3500 to 2400 cal years BP corresponds with Neoglacial advances in the region. A dramatic return to more frequent fire (5--8 events/1000 years) from 2400 to 1300 cal years BP suggests that prolonged summer drought occurred more often during this interval. Modern fire frequencies of 2--5 events/1000 years were established after 1300 cal years BP.; The timing of frequent fire episodes in mountain hemlock, western hemlock and montane spruce forests across southern BC correspond with century-scale solar activity maxima inferred from the atmospheric Delta14C record. Spectral analysis revealed significant periodicities at ∼218 years in the 11,000-year Frozen Lake CHAR record, and at ∼210 years in Dog Lake and Clayoquot Lake CHAR over the last 1000 years. Synchronous fire response at widely separated sites coupled with common periodicities support a proposed link between fire events, drought and ∼206-year solar variations. These findings, along with general circulation model results at 50°N, support an atmospheric mechanism where century-scale solar forcing strengthens the subtropical Pacific High and increases the potential for drought and forest fire ignition in wet coastal forests.; Subdecadal sampling of lake sediment resolved CHAR peaks that compared well with modern stand-origin maps and fire-scar dates around Dog Lake, in the montane spruce zone of southeastern BC. CHAR peaks indicate frequent stand-destroying fires during the Medieval Solar Maximum between AD 900--1300. Large fires were also identified at ∼AD 1610 and 1800, supporting a link between century-scale solar maxima, drought and forest fire events at 50°N latitude. Accumulation rates of Chara globularis oospores were used to infer changing lake levels in Dog Lake. Low accumulation rates inferring lower water levels correspond to times of drought and frequent fire in western North America, while high lake levels correspond to wetter climate periods with little or no fire activity. (Abstract shortened by UMI.)...