Holocene fire history of a coastal temperate rain forest, Vancouver Island, British Columbia, Canada.

摘要

The current lack of information about the temporal and landscape pattern of fires is a key source of uncertainty about natural disturbance regimes in coastal temperate rain forests. Two approaches were employed to examine fire history within a 700 ha low elevation area on the west coast of Vancouver Island: (1) point estimates of time-since-fire from tree ages and radiocarbon dates on soil charcoal, and (2) spatially aggregated estimates of fire occurrence from an 1800-year lake sediment record of charcoal accumulation. To aid with the first approach, the accuracy of the radiocarbon method for dating known fire events was evaluated. It was found that calibrated radiocarbon ages of soil charcoal consistently overestimated fire ages by 100–400 years due to the age of wood at the time of the fire. Estimates of time-since-fire at 83 sites ranged from 64 to ca. 12,220 cal. years before present. Approximately 20% of the sites have not burned for over 6000 years; these are on low fire-susceptibility landforms (i.e., north aspects and low terraces), which burned mainly in the early Holocene. In contrast, all sites on high-susceptibility landforms (i.e., south aspects) burned within the last 800 years. In the second approach, distinct peaks in charcoal in the lake sediment record corresponded with fires within 250 m of the lake. Fire intervals in this area increased from ca. 50 years (AD 200–900) to ca. 350 years (AD 1100–present), corresponding with regional climatic change. The decadal-scale fire frequency detected in the lake sediment record contrasts with the >2000-year time-since-fire detected in a large area near the lake. Interpreted together, these records suggest that fires repeatedly burn certain sites. Fire frequency on high-susceptibility landforms was probably sufficient during the late Holocene to support Douglas-fir, a species dependent on fire for regeneration. On low-susceptibility landforms, extremely low fire frequency probably allowed late-successional forest structure to persist throughout the late Holocene. The extremely long time-since-fire detected in a large portion of the study area supports the distinction of the coastal temperate rain forest as affected by a fundamentally different fire history than forests further inland.