FIRE, CLIMATE AND VEGETATION CHANGE IN THE HIGH-ELEVATION MOUNTAIN HEMLOCK RAINFORESTS OF SOUTHERN BRITISH COLUMBIA, CANADA

Fire is very rare in this forest type because fuels are wet or covered in snow for most of the year. Historic fires are associated with blocking high-pressure cells that promote late summer drought in the region. The rarity of fire and narrow fire window in mountain hemlock forests suggests that frequent disturbance in this ecosystem may indicate periods with prolonged summer drought. A reconstruction of Holocene fire history in mountain hemlock forests was done using a combination of high-resolution analysis of macroscopic charcoal from lake sediment cores and 102 accelerator mass spectrometry (AMS) ages on soil charcoal layers. AMS ages on soil charcoal provide independent evidence of local fire around a lake and support the interpretation of peaks in lake sediment charcoal as distinct fire events during the Holocene. Local fires occurred within 1-2 km of the lake and had fire intervals ranging from centuries to several millennia at some sites. Fire frequency inferred from charcoal peaks varied continuously throughout the Holocene suggesting that fire regimes are linked to climate via large-scale atmospheric circulation patterns. Fires were frequent between 11,000 to 8800 calendar years BP during the warm and dry early Holocene. The onset of humid conditions in the mid-Holocene, as rainforest taxa established in the region, produced a variable fire period until 3500 calendar years BP. A synchronous decrease in fire frequency from 3500 to 2400 calendar years BP corresponds with Neoglacial advances in the region and cool humid climate. A return of frequent fire between 2400 to 1300 calendar years BP suggests that prolonged summer drought occurred more often during this interval. The present-day fire regime was established after 1300 calendar years BP and is characterized by a trend of decreasing fire frequency from the Medieval Warm Period to the Little Ice Age.