Holocene treeline dynamics in the mountains of northeastern British Columbia, Canada, inferred from fossil pollen and stomata

Changes in pollen and stomata assemblages in sediment cores recovered from tundra and foresttundra lakes in alpine regions of northeastern British Columbia reflect vegetation and inferred climatic change throughout the Holocene. Pollen and stomata records are presented from two lakes, BC2 located in the alpinetundra zone and Dead Spruce Lake at the present elevation of subalpine treeline. The pollen and stomata records from BC2 indicate that an ephemeral shrub and herb assemblage was rapidly replaced by an aspen (Populus)-spruce (Picea)-birch (Betula) woodland at ~ 10600 cal. yr BP. The occurrence of stomata suggests that treeline was at least 235 m higher than present from ~10 600 until ~7500 cal. yr BP and temperatures were at least 1.4°C warmer than at present. Analyses from Dead Spruce Lake indicate that the density of trees was also higher between ~9000 and 4800 cal. yr BP. High concentrations of macroscopic charcoal between ~9800 and 8000 cal. yr BP suggest that conditions were also drier at this time. Changes in the position of treeline during the early to mid-Holocene appear to track closely shifts in climate, while forest development at our forest-tundra site reflects a combination of both heightened summer insolation and increased fire activity. The fossil stomata record from BC2 suggests that a rapid cooling event at ~8200 cal. yr BP may have been the trigger that resulted in the downslope movement of treeline to its present elevation. Increased Picea percent ages, pollen-accumulation rates (PARs) and concentration of stomata between ~3600 and 2700 cal. yr BP provide evidence for a late-Holocene increase in forest-tundra density in response to warmer temperatures.