CLIMATE AND VEGETATION IN NORTHEASTERN ALASKA 18,000 YR BP - PRESENT

Changing abundances of taxa in the pollen record of northeastern Alaska contain a climate signal and may be compared with GCM simulations of paleoclimates. Cold, dry conditions indicated by full-glacial pollen spectra are in broad agreement with model simulations. Successive expansion of Betula and Salix, and later, Populus, during the deglacial period (14,000-11,000 yr B.P.) are likely correlated with the crossing of temperature thresholds determined largely by the size and position of the Laurentide ice. A degree of asynchroneity between sites probably reflects differential site responses to regional effects plus inaccuracies of radiocarbon dating. The early Holocene was probably the warmest period of the present interglacial, as summer climate was influenced by a strong positive insolation anomaly. Picea glauca expanded in major river valleys ca. 9000 yr ago and slightly later at upland sites in the Brooks and Alaska ranges. At some sites a Juniperus subzone, which suggests particularly warm and dry conditions, occurs between ca. 10,000 and 8000 yr B.P. Early mid-Holocene expansions of Alnus (8500-7000 yr B.P.) and Picea mariana (7500-5000 yr B.P.) suggest the subsequent development of moister conditions, which have continued to the present. Temporal patterns in Picea treeline position during the Holocene are different in northern Alaska, northwestern Canada and central Canada. In Alaska, the treeline is currently at its furthest Holocene extent. In northwestern Canada the treeline has retreated since the mid-Holocene, and in central Canada a mid-Holocene advance and retreat occurred. Such differences between regions suggest a complex response of this taxon to climatic controls and underscore the need to interpret vegetation changes in relation to regional-scale climate phenomena. If the pollen chronology is calibrated with reference to U-Th dates, the relationship of vegetational events to the insolation maximum at ca. 10,000 yr B.P. is significantly altered. The changed chronology also affects the interpretation of pollen records in relation to GCM simulations, which are partly keyed to calendar years.