Palaeoecology of postglacial treeline shifts in the northern Cascade Mountains, Canada

Postglacial changes in vegetation and chironomid communities at a subalpine lake in the Engelmann Spruce–Subalpine Fir zone in the northern Cascade Mountains, British Columbia, indicate patterns of treeline and climate fluctuation during the Holocene. In late-glacial sediments of Cabin Lake, pollen assemblages representative of alpine vegetation and cold-stenothermous chironomids indicate cold conditions prior to the Holocene. In the early Holocene (10,090 to 7000 14C yr BP) co-occurrence of spruce–fir parkland and a warm-adapted chironomid community indicates a warm and probably dry climate. In the mid-Holocene, inferred forest closure suggests that precipitation increased, and a mixture of warm- and cold-adapted chironomids indicates temperatures warmer than present, but cooler than in the early Holocene. This period between 7000 and 3200 14C yr BP represents a transitional climate in which temperature gradually declined, culminating in cool neoglacial conditions. This transitional interval may correspond with the `mesothermic periodʹ proposed for lowland sites in southern British Columbia. Palaeobotanical evidence suggests that moist subalpine forest began to establish around 4800 14C yr BP with minimum temperatures and maximum precipitation between 2435 and ca. 1700 14C yr BP, corresponding with neoglacial advances throughout the northern Cordillera. A cool late Holocene (3200 14C yr BP to present) is also supported by a further decline in warm-adapted chironomids. Comparisons with other study sites in the Pacific Northwest reveal that regional climatic changes were a major factor in driving biotic changes in this area.