POSTGLACIAL CHANGES IN CHIRONOMID COMMUNITIES AND INFERRED CLIMATE NEAR TREELINE AT MOUNT-STOYOMA, CASCADE MOUNTAINS, SOUTHWESTERN BRITISH-COLUMBIA, CANADA

Analysis of the distributions of chironomid (midge) and other dipteran subfossils from two high elevation lake sediment cores in the Cascade Mountains reveals changes in midge communities and inferred climate s ince the late-glacial. Cabin Lake and 3M Pond are located near treelin e in the subalpine Engelmann Spruce/Subalpine Fir biogeoclimatic zone of British Columbia. In Cabin Lake, chironomid head capsule assemblage s depict a typical late-glacial community, and three distinct Holocene communities. In Cabin Lake, the late-glacial community is composed of cold-stenothermous taxa dominated by Stictochironomus, Mesocricotopus , Heterotrissocladius, Parakiefferiella nigra, Protanypus and Paraclad ius, whereas warm water midges are absent or rare, indicating cold con ditions. A late-glacial chironomid community was not found in 3M Pond. In both lakes the early Holocene is dominated by a diverse warm-adapt ed assemblage, corresponding to the warm climatic conditions of the xe rothermic period. Cabin Lake's mid-Holocene zone records a decrease in relative abundance of the warm water types and is accompanied by an i ncrease in cold-stenotherms. At 3M Pond this period shows a dramatic l oss in diversity of warm-adapted taxa, as the temperate genus Dicroten dipes dominates. This zone corresponds to Hebda's (1995) mesothermic p eriod. Further cooling in the late Holocene (to modem conditions) is i nferred from continued reduction of warm water midges and persistence (at Cabin Lake) or appearance (at 3M Pond) of a cold-stenothermal comm unity. This late Holocene cooling is similar in timing to Neoglacial a dvances in the Coast, Cascade, and Rocky Mountains of southern British Columbia. Similarities in the timing of chironomid and vegetation com munity changes at these high elevation sites, along with the more rapi d response time of the Chironomidae, support the sensitivity of midges to postglacial climatic change at high elevation sites.