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

Abstract

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.

Holocene records of Dendroctonus bark beetles in high elevation pine forests of Idaho and Montana, USA

“…Sites in North America that record evidence of a cooling event include Elk Lake in Minnesota (Dean et al, 2002), BC2 in British Columbia (Pisaric et al, 2003), and Bear Lake, ID and UT (Dean et al, 2006). Sediment samples collected from Deep Lake, Minnesota also indicate a period of cooling around the 8200 year event (8900-8300 cal yr BP), and support the hypothesis that the cooling event lasted more than one or two centuries in areas distant from the North Atlantic (Hu et al, 1999).…”

Methoxy-serratenes in a soil under conifers and their potential use as biomarkers of Pinaceae

“…Boucher et al (2009) applied the same approach to unravel the impact of logging on the composition and structure of the sub-boreal forest over the last 200 yr. Palynology provides additional information on the evolution of forestry, afforestation, treeline dynamics, forest clearance and, more generally, the evolution of continental landscapes under anthropogenic or climatic control (e.g. Pisaric et al, 2003;Stebich et al, 2005).…”

Late Quaternary Vegetation History of Northern North America Based on Pollen, Macrofossil, and Faunal Remains*

“…It advanced northward into forest tundra along the Mackenzie Valley (MacDonald, 1987a), northwestward into tundra through northern British Columbia (MacDonald, 1987b;Pisaric et al, 2003) and central Yukon (Cwynar, 1988;Keenan and Cwynar, 1992;Cwynar and Spear, 1995), reaching as far as central Alaska (Hu et al, 1993). White and black spruce both migrated early and white birch was a significant component at many sites.…”