PALEOECOLOGY OF POSTGLACIAL TREELINE SHIFTS IN THE NORTHERN CASCADE MOUNTAINS, CANADA

Postglacial changes in vegetation and chironomid communities at a suba lpine 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 an d cold-stenothermous chironomids indicate cold conditions prior to the Holocene. In the early Holocene (10,090 to 7000 C-14 yr BP) co-occurr ence of spruce-fir parkland and a warm-adapted chironomid community in dicates a warm and probably dry climate. In the mid-Holocene, inferred forest closure suggests that precipitation increased, and a mixture o f warm- and cold-adapted chironomids indicates temperatures warmer tha n present, but cooler than in the early Holocene. This period between 7000 and 3200 C-14 yl. BP represents a transitional climate in which t emperature gradually declined, culminating in cool neoglacial conditio ns. This transitional interval may correspond with the 'mesothermic pe riod' proposed for lowland sites in southern British Columbia. Palaeob otanical evidence suggests that moist subalpine forest began to establ ish around 4800 C-14 yr BP with minimum temperatures and maximum preci pitation between 2435 and ca. 1700 C-14 yr BP, corresponding with neog lacial advances throughout the northern Cordillera. A cool late Holoce ne (3200 C-14 yr BP to present) is also supported by a further decline in warm-adapted chironomids. Comparisons with other study sites in th e Pacific Northwest reveal that regional climatic changes were a major factor in driving biotic changes in this area. (C) 1998 Elsevier Scie nce B.V. All rights reserved.